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Huisman J, van Oostveen P, Weissing FJ.. Critical depth and critical turbulence: two different mechanisms for the development of phytoplankton blooms. Limnol Oceanogr 44: 1781-1787
Limnology and Oceanography
Abstract
A turbulent diffusion model shows that there are two different mechanisms for the development of phytoplankton blooms. One of these mechanisms works in well-mixed environments and corresponds to the classical critical depth theory. The other mechanism is based on the rate of turbulent mixing. If turbulent mixing is less than a critical turbulence, phytoplankton growth rates exceed the vertical mixing rates, and a bloom develops irrespective of the depth of the upper water layer. These results demonstrate that phytoplankton blooms can develop in the absence of vertical water-column stratification.
... That is, if we consider a year starting from the summer solstice (i.e., 21st June), then spring bloom initiation occurs around 265th day (i.e., mid-March) and bloom lasts up to 365th day (end of June). Several hypotheses (for example, critical depth hypothesis (CDH) [4], critical turbulence hypothesis (CTH) [5], and disturbance-recovery hypothesis (DRH) [6][7][8]) were proposed to explain the bloom phenomena in the North Atlantic Ocean. According to the Critical depth hypothesis (CDH) [4], if the surface mixed layer depth is above a critical depth then the net phytoplankton production will be greater than the net respiration and bloom will happen. ...
... According to the Critical depth hypothesis (CDH) [4], if the surface mixed layer depth is above a critical depth then the net phytoplankton production will be greater than the net respiration and bloom will happen. Critical turbulence hypothesis (CTH) [5] suggests that if turbulent mixing rates in the surface layer are smaller than the growth rates, then the growth will outpace mixing losses and bloom will occur. According to the DRH, as the mixed layer depth deepens, contact rates between predators (zooplankton) and prey (phytoplankton) decrease, which in turn leads to phytoplankton bloom. ...
... Thus, in view of (A.4), it is evident that the solution is feasible. Now, observe from (A.3) that we have = * (see (5)) and 0 < * ≤ iff ...
... Irradiance and spectral composition are critical factors affecting primary production, and hence important in governing competitive outcomes in phytoplankton communities (Dubinsky & Stambler, 2009;Huisman et al., 1999;Stomp et al., 2007). ...
... Moreover, their accessory pigments (phycoerythrin and phycocyanin) enable cyanobacteria to capture light at longer wavelengths, which is less absorbed by cDOM than shorter-wavelength light (Erratt et al., 2021;Stomp et al., 2007). Although light requirements vary widely among species and strains of cyanobacteria, some taxa (e.g., Planktothrix and other filamentous Oscillatoriales) tolerate shade (Huisman et al., 1999;Reinl et al., 2021;Reynolds et al., 2002;Zapomělová et al., 2010), often forming a DCM below the thermocline in clearwater lakes, including Lake Stechlin (Kasprzak et al., 2017;Posch et al., 2012). These features would suggest that low light levels caused by browning are less detrimental to these cyanobacteria than to most other types of phototrophic phytoplankton. ...
... High turbulence in deeply mixed water columns prevents effective buoyancy regulation by gas vacuoles, implying an adverse influence of mixing on cyanobacteria in turbulent conditions (e.g., Huisman et al., 1999;Posch et al., 2012). Our long-term data from ...
Lakes worldwide are affected by multiple stressors, including climate change. This includes massive loading of both nutrients and humic substances to lakes during extreme weather events, which also may disrupt thermal stratification. Since multi-stressor effects vary widely in space and time, their combined ecological impacts remain difficult to predict. Therefore, we combined two consecutive large enclosure experiments with a comprehensive time-series and a broad-scale field survey to unravel the combined effects of storm-induced lake browning, nutrient enrichment and deep mixing on phytoplankton communities, focusing particularly on potentially toxic cyanobacterial blooms. The experimental results revealed that browning counteracted the stimulating effect of nutrients on phytoplankton and caused a shift from phototrophic cyanobacteria and chlorophytes to mixotrophic cryptophytes. Light limitation by browning was identified as the likely mechanism underlying this response. Deep-mixing increased microcystin concentrations in clear nutrient-enriched enclosures, caused by upwelling of a metalimnetic Planktothrix rubescens population. Monitoring data from a 25-66 year time-series of a eutrophic lake and from 588 northern European lakes corroborate the experimental results: Browning suppresses cyanobacteria in terms of both biovolume and proportion
68 of the total phytoplankton biovolume. Both the experimental and observational results indicated a lower total phosphorus threshold for cyanobacterial bloom development in clearwater lakes (10-20 µg P L-1) than in humic lakes (20-30 µg P L-1). This finding provides management guidance for lakes receiving more nutrients and humic substances due to more frequent extreme weather events.
... Although studied for 70 years (Sverdrup, 1953), the optimal conditions that trigger the initiation of phytoplankton growing period (IPGP) in ocean waters in early spring are not well understood (Sathyendranath et al., 2015). Three main theories are proposed to date: the critical depth hypothesis (Sverdrup, 1953), the critical turbulence hypothesis (Huisman et al., 1999), and the disturbance-recovery hypothesis (Banse, 1994;Behrenfeld, 2010;Behrenfeld et al., 2013). For Sverdrup (1953), phytoplankton blooms occur when the surface mixed layer shoals to a depth shallower than the critical depth, according to light conditions. ...
... For Sverdrup (1953), phytoplankton blooms occur when the surface mixed layer shoals to a depth shallower than the critical depth, according to light conditions. While Huisman et al. (1999) agreed with Sverdrup, they proposed that relaxation of turbulent mixing allows the bloom to develop if it occurs below a critical turbulence rate. Behrenfeld (2010) observed blooms in the absence of spring mixed layer shoaling and declared that the initiation of bloom is controlled by a balance between phytoplankton growth and grazing rate and suggested a seasonal control of this balance by physical processes. ...
... The main theories to explain the initiation of phytoplankton blooms (Sverdrup, 1953;Huisman et al., 1999;Banse, 1994) are not relevant in the context of shallow and wellmixed coastal waters under the influence of river plumes. ...
Decadal time series of chlorophyll a concentrations sampled at high and low frequencies are explored to study climate-induced impacts on the processes inducing interannual variations in the initiation of the phytoplankton growing period (IPGP) in early spring. We specifically detail the IPGP in two contrasting coastal temperate ecosystems under the influence of rivers highly rich in nutrients: the Bay of Brest and the Bay of Vilaine. In both coastal ecosystems, we observed a large interannual variation in the IPGP influenced by sea temperature, river inputs, light availability (modulated by solar radiation and water turbidity), and turbulent mixing generated by tidal currents, wind stress, and river runoff. We show that the IPGP is delayed by around 30 d in 2019 in comparison with 2010. In situ observations and a one-dimensional vertical model coupling hydrodynamics, biogeochemistry, and sediment dynamics show that the IPGP generally does not depend on one specific environmental factor but on the interaction between several environmental factors. In these two bays, we demonstrate that the IPGP is mainly caused by sea surface temperature and available light conditions, mostly controlled by the turbidity of the system before first blooms. While both bays are hydrodynamically contrasted, the processes that modulate the IPGP are similar. In both bays, the IPGP can be delayed by cold spells and flood events at the end of winter, provided that these extreme events last several days.
... The seminal work by Riley [5] was perhaps one of the first theoretical studies addressing the mathematical modeling of blooms and proposing a mechanistic explanation for their occurrence. Among the many different subsequent developments, I would like to cite, here, those by Huisman and collaborators (see, e.g., [6,7,8]), which provided a more complete picture in terms of the relevant physical parameters, particularly addressing the role of turbulence intensity, and which have been instructive as a term of comparison for the work performed in this thesis. ...
... The history of phytoplankton light limited growth models is quite rich, starting from the seminal work of Sverdrup [58], it gained much impetus through the introduction of specific studies on the importance of light limitation for phytoplankton dynamics [52,54], reaching the status of a well established methodology in the modern days [7,6,50,56]. According to Huisman [59] a variety of competition models lead to the following prediction: if one resource is the main limiting factor for the survival of multiple species, the species that is able to survive at the lowest resource density equilibrium value (R * ) competitively excludes all other species. ...
... From then to the current days a lot has been done in the efforts of explaining the behavior of plankton blooms. Here we describe the critical depth hypothesis, a theory based on a article [58] that achieved much impetus in oceanography and aquatic ecology [63,7]. The concept presented there was later thoroughly debated and so refinements and alternatives to the theory appeared, the critical turbulence hypothesis is one of these spawns that we will also discuss in this section. ...
Reaction transport systems emerge in many areas of research and applications as, e.g.: chemical reaction kinetics in fluids, plankton dynamics, infection spreading. While the dynamics of reacting species have been widely studied in homogeneous media, their understanding in heterogeneous environments, particularly relevant for ecological problems, is still limited. In this project we propose to elucidate, by means of numerical simulations of advection-reaction-diffusion systems, the complex interplay between environmental heterogeneity (due to resource limitations, such as light availability in the water column in the sea or in a lake) and transport by a fluid flow, which controls the appearance of phytoplankton blooms in stirred aquatic environments. The focus is on the role of spatially structured flows and turbulent mixing on growth and persistence of algal populations, as well as on the resulting detailed vertical/horizontal population distribution. To investigate these issues, turbulent flows arising from Large-Eddy-Simulation (LES) models of Navier-Stokes equation or kinematic models will be considered. Such an approach is expected to allow access to the details at different scales of the interacting mechanisms that determine phytoplankton dynamics. Due to the wide range of spatial and temporal scales involved, this represents a highly challenging task and it constitutes the main original aspect of the project. For the purpose of enhancing biological and physical realism, model complexity will be gradually increased. By analysing the temporal behaviour of the adopted model systems it will eventually be possible to explore the mechanisms controlling the onset of environmental conditions acting as precursors of algal blooms. This study is expected to provide insights on how to better account for the complexity of fluid motions, with respect to the population dynamics models typically used in fundamental and applied studies at scales larger than the spatial (turbulent) structures of the flow. In particular, we aim at characterizing the main effects due to turbulent fluctuations and, hence, at the possibility to account for a richer phenomenology. Results will then be useful to establish new constraints for improving food web models in marine ecology. Ameliorating the possibility to predict and, potentially, control phytoplankton blooms also has a major societal impact, e.g. for the management of the anthropogenically induced eutrophication (enhanced algal growth due to excess nutrients) of freshwater and coastal marine ecosystems.
... Two alternative models have been suggested explaining algal blooms under non-stratified conditions. Huisman et al. (1999) developed the critical turbulence model, where phytoplankton blooms form under combinations of low turbulence and high algal growth rates., while the dilution-recoupling hypothesis (Behrenfeld, 2010) focuses on the balance between algal growth and grazing. Stronger grazing pressure may inhibit phytoplankton biomass accumulation in a shallow mixed layer suggesting maximum of algal biomass accumulation prior to maximum stratification, while algae are more diluted. ...
... This would allow to identify trends and changes in biological properties and relate those with environmental drivers in long term time series. Although barcoding indicated increased photosynthetic activity in February, algal biomass remained low in March, which we attribute to a combination of vertical export rate and a high mixing exceeding euphotic layer production until April (critical turbulence hypothesis, Huisman et al., 1999). At this time, the net heat loss to the atmosphere with limited freshwater input caused deep convection, and algal production remained rather low. ...
... At this time, the net heat loss to the atmosphere with limited freshwater input caused deep convection, and algal production remained rather low. In April the reversed heat flux allowed the build-up of algal biomass in a less intensively but still fully mixed column (Huisman et al., 1999;Hegseth et al., 2019). The low zooplankton biomass during this time (Coguiec et al., 2021) indicates a minor control of algal growth by grazing. ...
The polar night has recently received increased attention as a surprisingly active biological season. Yet, polar night microbial ecology is a vastly understudied field. To identify the physical and biogeochemical parameters driving microbial activity over the dark season, we studied a sub-Arctic fjord system in northern Norway from autumn to early spring with detailed monthly sampling. We focused on the impact of mixing, terrestrial organic matter input and light on microbial ecosystem dynamics. Our study highlights strong differences in the key drivers between spring, autumn, and winter. The spring bloom started in March in a fully mixed water column, opposing the traditional critical depth hypothesis. Incident solar radiation was the key driver maximum Chlorophyll was reached in April. The onset of the autumn phytoplankton bloom was controlled by vertical mixing, causing nutrient upwelling and dilution of zooplankton grazers, which had their highest biomass during this time. According to the dilution-recoupling hypothesis grazer dilution reduced grazing stress and allowed the fall bloom formation. Mixing at that time was initiated by strong winds and reduced stratification as a consequence of freezing temperatures and lower freshwater runoff. During the light-limited polar night, the primary production was extremely low but bacteria continued growing on decaying algae, their exudates and also allochthonous organic matter. A melting event in January could have increased input of organic matter from land, supporting a mid-winter bacterial bloom. In conclusion, polar night biogeochemistry and microbial ecology was not only driven by light availability, but strongly affected by variability in reshwater discharge and allochthonous carbon input. With climate change freshwater discharge will increase in the Arctic, which will likely increase importance of the dynamics described in this study.
... In catchments with snow cover, spring high flows may provide an important source of nutrients for the phytoplankton community (Hrycik et al., 2021). Lastly, following turbulent water conditions in deep lakes, the onset of stratification is often a prerequisite for the spring phytoplankton bloom (Huisman et al., 1999;Peeters et al., 2007b). Despite this, these events are rarely studied together in a single lake, and the separate projected trends in timing are seldom compared to each other within the same study site. ...
... Earlier spring discharge into Lake Erken also provides an earlier supply of nutrients, but in this lake the majority of spring discharge occurs prior to ice-off and the spring chlorophyll peak. In Lake Erken, spring phytoplankton growth is not reliant on stratification due to the limited mean depth of the lake, and the spring chlorophyll peak (dominated by diatoms) tends to occur prior to onset of stratification ( Fig. 2; Weyhenmeyer et al., 1999;Moras et al., 2019) -an order of events which is commonly reversed in deeper lakes where stratification is required to overcome light limitation (Huisman et al., 1999;Gronchi et al., 2021). Altogether, the earlier chlorophyll peak therefore seems to be mostly attributable to the increased availability of light Table 1). ...
Lakes experience shifts in the timing of physical and biogeochemical events as a result of climate warming, and relative changes in the timing of events may have important ecological consequences. Spring, in particular, is a period in which many key processes that regulate the ecology and biogeochemistry of lakes occur and also a time that may experience significant changes under the influence of global warming. In this study, we used a coupled catchment–lake model forced by future climate projections to evaluate changes in the timing of spring discharge, ice-off, the spring phytoplankton peak, and the onset of stratification in a temperate mesotrophic lake. Although the model explained only part of the variation in these events, the overall patterns were simulated with little bias. All four events showed a clear trend towards earlier occurrence under climate warming, with ice cover tending to disappear at the end of the century in the most extreme climate scenario. Moreover, relative shifts in the timing of these springtime events also occurred, with the onset of stratification tending to advance more slowly than the other events and the spring phytoplankton peak and ice-off advancing faster in the most extreme climate scenario. The outcomes of this study stress the impact of climate change on the phenology of events in lakes and especially the relative shifts in timing during spring. This can have profound effects on food web dynamics as well as other regulatory processes and influence the lake for the remainder of the growing season.
... Among these factors, vertical stabilization of the water column has been identified as the primary factor triggering initiation of the phytoplankton spring bloom. However, there remains controversy regarding whether depth-associated parameters such as the mixed layer depth (MLD) play a critical role or whether the strength of vertical mixing is the key determinant of bloom initiation (Sverdrup, 1953;Huisman et al., 1999;Behrenfeld et al., 2013;Rumyantseva et al., 2019). To elucidate the mechanism underlying phytoplankton spring bloom initiation in the Yellow Sea, timeseries data were collected regarding phytoplankton biomass, nutrient concentrations, conductivity-temperature-depth (CTD) sensor readings, and environmental factors that influence vertical mixing (e.g., wind speed, wave height, and surface water temperature). ...
... As surface phytoplankton are associated with stratification, and most edges linked with Cryptophyceae are associated with vertical mixing, the surface phytoplankton bloom appears to have been initiated by the onset of vertical mixing. This finding is consistent with the concepts underlying the critical turbulence hypothesis (Huisman et al., 1999). ...
The spring phytoplankton bloom is a critical event in temperate oceans typically associated with the highest productivity levels throughout the year. To investigate the bloom process in the Yellow Sea, daily data on physical, chemical, and phytoplankton taxonomic group biomass, calculated via the chemotaxonomic approach, were collected from late March or early April to late May between 2018 and 2020 at the Socheongcho Ocean Research Station. During early spring (late March to mid-April), phytoplankton biomass increased, accompanied by a decrease in nutrient levels, with Bacillariophyceae and Cryptophyceae being the dominant groups. As water temperature increased, a pycnocline began to develop in late April, leading to a peak of the phytoplankton bloom dominated by chlorophytes and Cryptophyceae. Network analysis suggested that this phytoplankton bloom was caused by the onset of vertical stratification induced by increased sea surface temperature. The chlorophyte peak induced phosphate limitation above the pycnocline, resulting in succession to Prymnesiophyceae and Dinophyceae. Following pycnocline formation, phytoplankton biomass below the pycnocline was dominated by Bacillariophyceae and Cryptophyceae, with decreasing or fluctuating trends depending on phosphate concentration. Apart from these general patterns, 2019 and 2020 both had distinctive traits. The 2019 data revealed lower phosphate concentrations than the other 2 years, leading to a smaller chlorophyte peak at the surface compared to 2018 and extreme phosphate limitation above the pycnocline. This limitation resulted in decreased biomass of late successional groups, including Prymnesiophyceae and Dinophyceae. Pycnocline formation was delayed in year 2020, and stratification was significantly weaker compared to the previous 2 years. Due to the pycnocline delay, the surface chlorophyte peak did not develop and no succession to late successional groups was observed. Instead, high levels of Bacillariophyceae and Cryptophyceae biomass were observed throughout the water column with no surface bloom. Thus, among various environmental factors, increasing surface water temperature and phosphate concentrations play pivotal roles in shaping phytoplankton bloom dynamics. Distinct yearly variation points to the broader impacts of climate shifts, emphasizing the need for continued marine monitoring.
... In catchments with snow cover, spring high flows are common due to snowmelt, and these may provide an important source of nutrients for the phytoplankton community (Hrycik et al., 2021). Lastly, following turbulent water conditions in deep lakes, the onset of stratification is often a prerequisite for the spring phytoplankton bloom (Huisman et al., 1999; Peeters et al., https://doi.). Despite this, these processes are rarely studied together in a single lake, and the separate projected trends in timing 50 ...
... Earlier spring discharge into Lake Erken also provides an earlier supply of nutrients, but in this lake, the majority of spring discharge occurs prior to iceoff and the spring chlorophyll peak. In Lake Erken, the spring chlorophyll peak tends to occur prior to onset of stratification, 215 an order of events which is commonly reversed in deeper lakes where stratification is required to overcome light limitation (Huisman et al., 1999;Gronchi et al., 2021). Altogether, the earlier chlorophyll peak therefore seems to be mostly attributable to the increased availability of light due to earlier ice-off, causing growth to commence earlier. ...
Lakes experience shifts in the timing of processes as a result of climate warming, and especially relative changes in the timing of events may have important ecological consequences. Spring in particular is a period in which many key processes that regulate the ecology and biogeochemistry of lakes occur, and also a time which may experience significant changes under influence of global warming. In this study, we used a coupled catchment-lake model forced by future climate projections to evaluate changes in the timing of spring discharge, ice-off, the spring phytoplankton peak, and the onset of stratification, in a mesotrophic, temperate lake. All these events showed a clear trend towards earlier occurrence with climate warming, with ice cover tending to disappear at the end of the century in the most extreme climate scenario. Moreover, relative shifts in the timing of these springtime events also occurred, with the onset of stratification tending to advance slower than the other events, and the spring phytoplankton peak and ice-off advancing faster in the most extreme climate scenario. The outcomes of this study stress the impact of climate change on the phenology of processes in lakes and especially the relative shifts in timing during spring. This can have profound effects on food-web dynamics as well as other regulatory processes, and influence the lake for the remainder of the growing season.
... The depth of the mixed layer (Z mix ) is determined by the balance between the rate at which turbulence is produced due to wind action and convection, and the buoyancy created by surface heat flux towards the waterbody (Imberger, 1985;Imboden & Wuest, 1995;Spigel & Imberger, 1987). The depth of the mixed layer affects light availability, nutrient supply, grazing pressure and sedimentation losses of phytoplankton (Diehl, 2002;Huisman et al., 1999;Winder & Sommer, 2012). The shallowing of Z mix relative to the depth of the euphotic layer (Z eu )-the depth at which the photosynthetically active radiation (PAR) attenuates to 1% of the surface values (Wetzel, 2001)-enhances light conditions for phytoplankton in the mixed layer (Huisman et al., 1999). ...
... The depth of the mixed layer affects light availability, nutrient supply, grazing pressure and sedimentation losses of phytoplankton (Diehl, 2002;Huisman et al., 1999;Winder & Sommer, 2012). The shallowing of Z mix relative to the depth of the euphotic layer (Z eu )-the depth at which the photosynthetically active radiation (PAR) attenuates to 1% of the surface values (Wetzel, 2001)-enhances light conditions for phytoplankton in the mixed layer (Huisman et al., 1999). ...
The depth of the mixed layer is a major determinant of nutrient and light availability for phytoplankton in stratified waterbodies. Ongoing climate change influences surface waters through meteorological forcing, which modifies the physical structure of fresh waters including the mixed layer, but effects on phytoplankton biomass are poorly known.
To determine the responses of phytoplankton biomass to the depth of the mixed layer, light availability and associated meteorological forcing, we followed daily changes in weather and water column properties in a boreal lake over the first half of a summer stratification period.
Phytoplankton biomass increased with the deepening of the mixed layer associated with high wind speeds and low air temperature relative to the temperature of the mixed layer ( T air − T mix < 0), whereas heatwave conditions—shallow mixed layer driven by high T air − T mix value and low wind speed—reduced the biomass.
Improving light availability from low to moderate light conditions increased the phytoplankton biomass, while the highest light availability was associated with low phytoplankton biomass.
Our study demonstrates that the climatic impact‐drivers wind speed and T air − T mix are major drivers of mixed layer depth, which controlled phytoplankton biomass during the early summer stratification period. Our study suggests that increasing air temperature relative to water temperature and declining wind speeds have potential to lead to reduced phytoplankton biomass due to a shallower mixed layer during the first half of the stratification period in non‐eutrophic lakes with sufficient light availability.
... Note that we will assume I in = 0 in correspondence with the surface obstacles. The biological parameter values used in our study, representative of realistic situations [26], are reported in Table 1. They correspond to average measured values, with growth parameters obtained from freshwater phytoplankton species, and κ bg from clear lakes and coastal areas [20]. ...
... The horizontal and vertical domain sizes will be respectively set to L x = 60 m and L z = 30 m. Considering the chosen biological parameters (Table 1), a mixed layer depth of 30 m is smaller than the critical depth (about 50 m). Hence, in the absence of surface light heterogeneity and of a fluid flow, with the values of diffusivity explored, 1 cm 2 s −1 ≤ D ≤ 20 cm 2 s −1 , which appear quite realistic for the ocean [6,19,26], a bloom occurs. The presence of obstacles considerably reduces production, due to the absence of light below them. ...
Plankton dynamics depend in a complex manner on a variety of physical phenomena, according to both experimental and numerical data. In particular, experimental field studies have highlighted the relation between phytoplankton survival and turbulent upwelling and downwelling from thermal convection. Recent numerical works have also shown the importance of accounting for advective transport by persistent structures in simulation models. In nutrient-rich polar marine environments phytoplankton blooms are critically limited by light availability under ice-covered waters. Such heterogeneity of the light intensity distribution, in association with a large-scale coherent fluid flow, can give rise to nontrivial growth dynamics. In this work we extend a previous advection-reaction-diffusion model of phytoplankton light-limited vertical dynamics in the presence of convective transport. Specifically, we consider horizontally heterogeneous light conditions through the use of two regions with different production regimes, modelling the absence (presence) of light under (in between) obstacles. Such a model is intended as an idealized representation of nonuniformly ice-covered polar waters. By means of numerical simulations, we find that the main role of advective transport is to hinder phytoplankton growth, but also that such effect depends on the positions of the obstacles with respect to the upwelling and downwelling flow regions. Furthermore, we show that the sinking speed due to the density difference between phytoplankton organisms and water, while small, plays an important role, which depends on how it adds to the flow. These results indicate that advective transport can have a crucial impact on the survival conditions of sinking phytoplankton species in polar environments.
... Previous studies (e.g., Chiswell, 2011;Franks, 2014;Huisman et al., 1999;Lévy, 2015) have shown the importance of using the active mixed layer depth (AMLD), as opposed to MLD calculated from temperature gradients, when dealing with the phytoplankton annual cycle. The AMLD takes into account the active mixing in the water column. ...
... The AMLD was set to be the minimum depth in which Eddy diffusivity was larger than the threshold. According to Huisman et al. (1999), vertical eddy diffusivity can range between 10 −2 m 2 /s (very turbulent water) and 10 −5 m 2 /s (stratified conditions). As no significant difference was found between the two thresholds in terms of the depth of the AMLD, we used the value of 10 −2 m 2 /s to calculate the AMLD. ...
... Note that we will assume I in = 0 in correspondence with the surface obstacles. The biological parameter values used in our study, representative of realistic situations [26], are reported in Table 1. They correspond to average measured values, with growth parameters obtained from freshwater phytoplankton species, and κ bg from clear lakes and coastal areas [20]. ...
... The horizontal and vertical domain sizes will be respectively set to L x = 60 m and L z = 30 m. Considering the chosen biological parameters (Table 1), a mixed layer depth of 30 m is smaller than the critical depth (about 50 m). Hence, in the absence of surface light heterogeneity and of a fluid flow, with the values of diffusivity explored, 1 cm 2 s −1 ≤ D ≤ 20 cm 2 s −1 , which appear quite realistic for the ocean [6,19,26], a bloom occurs. The presence of obstacles considerably reduces production, due to the absence of light below them. ...
Plankton dynamics depend in a complex manner on a variety of physical phenomena, accord- ing to both experimental and numerical data. In particular, experimental field studies have highlighted the relation between phytoplankton survival and turbulent upwelling and downwelling from thermal con- vection. Recent numerical works have also shown the importance of accounting for advective transport by persistent structures in simulation models. In nutrient-rich polar marine environments phytoplankton blooms are critically limited by light availability under ice-covered waters. Such heterogeneity of the light intensity distribution, in association with a large-scale coherent fluid flow, can give rise to nontrivial growth dynamics. In this work we extend a previous advection-reaction-diffusion model of phytoplankton light- limited vertical dynamics in the presence of convective transport. Specifically, we consider horizontally heterogeneous light conditions through the use of two regions with different production regimes, modelling the absence (presence) of light under (in between) obstacles. Such a model is intended as an idealized representation of nonuniformly ice-covered polar waters. By means of numerical simulations, we find that the main role of advective transport is to hinder phytoplankton growth, but also that such effect depends on the positions of the obstacles with respect to the upwelling and downwelling flow regions. Furthermore, we show that the sinking speed due to the density difference between phytoplankton organisms and water, while small, plays an important role, which depends on how it adds to the flow. These results indicate that advective transport can have a crucial impact on the survival conditions of sinking phytoplankton species in polar environments.
... However, despite the high-nutrient standing stock, phytoplankton biomass in the northern strait is relatively low during boreal winter and early spring (Zhang, 2001), although episodes of anomalously high Chl-a can occur in winter (Wang et al., 2016;Zhang & Huang, 2000). There are two primary reasons for the relatively low Chl-a in the TWS, namely, low water temperature (Zhang & Huang, 2000) and intensive turbulent mixing due to strong NE monsoon winds (Huang & Oey, 2015;Huisman et al., 1999, Taylor & Ferrari, 2011a. The critical depth theory suggests that a bloom begins when the mixed-layer becomes shallower than a critical depth (Sverdrup, 1953), whereas the critical turbulence theory suggests that a bloom develops when turbulent mixing is less than a critical turbulence level (Huisman et al., 1999(Huisman et al., , 2004. ...
... There are two primary reasons for the relatively low Chl-a in the TWS, namely, low water temperature (Zhang & Huang, 2000) and intensive turbulent mixing due to strong NE monsoon winds (Huang & Oey, 2015;Huisman et al., 1999, Taylor & Ferrari, 2011a. The critical depth theory suggests that a bloom begins when the mixed-layer becomes shallower than a critical depth (Sverdrup, 1953), whereas the critical turbulence theory suggests that a bloom develops when turbulent mixing is less than a critical turbulence level (Huisman et al., 1999(Huisman et al., , 2004. Wang et al. (2016) suggested that the relaxation of the NE wind can trigger off-coast phytoplankton blooms based on a coupled physical-biological model. ...
This study applied cruise, model, and satellite data to analyze the off‐coast phytoplankton blooming during the late fall to early spring monsoon period in the Taiwan Strait when northeasterly wind prevails. Based on the composite and self‐organizing map analyses, the three data sets consistently show high chlorophyll‐a concentration near the along‐shore front during the down‐front northeasterly wind relaxation period while lower concentration when relatively strong wind is persistent. Meanwhile, the off‐coast blooming always coincides with intense near‐surface stratification when the northeasterly wind relaxes. Diagnoses of balanced Richardson number, Ertel potential vorticity and instability energy budget from high‐resolution cruise observations and model results demonstrate that vigorous submesoscale symmetric and baroclinic instabilities can develop near the along‐shore front under the down‐front NE wind. Diagnoses of modeled buoyancy and chlorophyll‐a budget equations further suggest the submesoscale instabilities lead to rapid near‐surface restratification and offshore stretching of the along‐shore front within the upper 10‐m of the mixed layer when the down‐front NE wind relaxes, favoring the surface 10‐m phytoplankton growth. As comparison, contribution of the larger‐scale advection related with geostrophic adjustment and Ekman transport to the chlorophyll‐a increment reached beyond the middle layer of ∼20‐m depth.
... These concentrations are insufficient for phytoplankton biomass to reach the high values and sustain continuous massive development of algae in spring. Winter-spring phytoplankton intensive growth begins in early Febru- ary at the nitrate concentrations of approximately four μM and lasts four months in temperate Atlantic Ocean regions [34]. ...
... Providing the latter is lower than (below) certain critical value, phytoplankton growth rate exceeds a vertical mixing rate, while bloom unfolds regardless of the depth of the upper mixed water layer. The findings reveal that phytoplankton may bloom in the absence of vertical density stratification [34]. Therefore, a rate of turbulent diffusion is among the important parameters, controlling vital functions of phytoplankton community. ...
... Several hypotheses have provided potential mechanisms to explain the formation of SPB on shelves, e.g., critical depth, critical turbulence, and disturbance recovery (Behrenfeld and Boss, 2018;Gran and Braarud, 1935;Huisman et al., 1999;Sverdrup, 1953). In the hypothesis of critical depth, SPB might initiate when the upper mixed layer is shallower than the critical depth at which phytoplankton growth balances its respiration losses (Gran and Braarud, 1935;Sverdrup, 1953). ...
... In the hypothesis of critical depth, SPB might initiate when the upper mixed layer is shallower than the critical depth at which phytoplankton growth balances its respiration losses (Gran and Braarud, 1935;Sverdrup, 1953). In the hypothesis of critical turbulence, SPB often occurs when turbulent diffusivity is less than critical turbulence, which is that the relaxation of turbulent mixing allows phytoplankton to overcome turbulent dilution and aggregate in upper waters to obtain sufficient light (Huisman et al., 1999;Taylor and Ferrari, 2011a). Further, the hypothesis of disturbance recovery mentions that environmental "disturbances" of various forms, e.g., mixed layer light levels and nutrient availability, enable to accelerate phytoplankton division and provide biological potential for their blooms (Behrenfeld and Boss, 2018). ...
Major seasonal quasi-stationary fronts on shelves play an important role in regulating the spatiotemporal variations in the phytoplankton community. However, knowledge of their effects on the timing and magnitude of spring phytoplankton bloom (SPB) remains limited. Here, based on decadal satellite data (2003–2020), we examine the climatological relationship between the Shandong coastal front (SCF) and SPB in the Yellow Sea. The results show that the onset of SPB occurs either in March (∼56% of the seasons examined) or in April (44%). The peak of SPB most often occurs in April (∼56% of the seasons examined) or is advanced to March (16%) or delayed to May (28%), and that the peak ranges from 1.04 to 2.54 mg Chl-a m⁻³. The onset of SPB matches with lower turbulence, particularly when the rate of generation of turbulent kinetic energy (TKERT) reaches zero. A higher magnitude of bloom is associated with a greater change in front and lower TKERT. The in situ observations along the SCF transects in the Yellow Sea indicate that weakened SCF in spring associated with a shallower mixing layer enhances the transport of nutrients from the coast to the shelf waters. Weakened frontal structure and atmospheric forcing in spring can further increase the water stability and decrease turbulence in the upper waters. The variation in hydrodynamic conditions allows shelf phytoplankton to stay longer in the upper waters with sufficient light and nutrients and consequently generate a Chl-a peak. The results suggest that the seasonal changes in front intensity and structure and turbulence are important prerequisites for initiating SPB on the shelf, and that further determines the magnitude of SPB.
... The more traditional school of thought assumes that the spring bloom is triggered when the winter light limitation relaxes to a point that allows µ to surpass a critical threshold Boss, 2014, 2018). To this pure bottom-up view belong for instance the famous Critical Depth Hypothesis (Gran and Braarud, 1935;Sverdrup, 1953) and Critical Turbulence Hypothesis (Huisman et al., 1999). An alternative framework focuses on processes that lead to positive r by disrupting the equilibrium between phytoplankton division and loss processes, especially grazing and virus infections, and this disruption can occur even when light is still limiting. ...
... Overall, we showed that the spring bloom onset in a generally well-mixed coastal location of the North Sea supports the Disturbance Recovery Hypothesis (DRH). Nevertheless, the mechanisms described in other competing hypotheses such as the Critical Depth Hypothesis (Gran and Braarud, 1935;Sverdrup, 1953) or the Critical Turbulence Hypothesis ( Huisman et al., 1999) might contribute to the spring bloom development (Lindemann and St. John, 2014;Chiswell et al., 2015). For instance, a water column stratification due to the surface heating or a relaxation of the turbulent mixing caused by weak or calm winds can lead to fast (albeit temporary) increases in both light availability and division rates (Morison et al., 2019Mojica et al., 2021), as described for oceanic waters by Mignot et al. (2018) and Yang et al. (2020). ...
The spring phytoplankton bloom is a key event in temperate and polar seas, yet the mechanisms that trigger it remain under debate. Some hypotheses claim that the spring bloom onset occurs when light is no longer limiting, allowing phytoplankton division rates to surpass a critical threshold. In contrast, the Disturbance Recovery Hypothesis (DRH) proposes that the onset responds to an imbalance between phytoplankton growth and loss processes, allowing phytoplankton biomass to start accumulating, and this can occur even when light is still limiting. Although several studies have shown that the DRH can explain the spring bloom onset in oceanic waters, it is less certain whether and how it also applies to coastal areas. To address this question at a coastal location in the Scottish North Sea, we combined 21 years (1997–2017) of weekly in situ chlorophyll and environmental data with meteorological information. Additionally, we also analyzed phytoplankton cell counts estimated using microscopy (2000–2017) and flow cytometry (2015–2017). The onset of phytoplankton biomass accumulation occurred around the same date each year, 16 ± 11 d (mean ± SD) after the winter solstice, when light limitation for growth was strongest. Also, negative and positive biomass accumulation rates (r) occurred respectively before and after the winter solstice at similar light levels. The seasonal change from negative to positive r was mainly driven by the rate of change in light availability rather than light itself. Our results support the validity of the DRH for the studied coastal region and suggest its applicability to other coastal areas.
... Previous studies (e.g., Chiswell, 2011;Franks, 2014;Huisman et al., 1999;Lévy, 2015) have shown the importance of using the active mixed layer depth (AMLD), as opposed to MLD calculated from temperature gradients, when dealing with the phytoplankton annual cycle. The AMLD takes into account the active mixing in the water column. ...
... The AMLD was set to be the minimum depth in which Eddy diffusivity was larger than the threshold. According to Huisman et al. (1999), vertical eddy diffusivity can range between 10 −2 m 2 /s (very turbulent water) and 10 −5 m 2 /s (stratified conditions). As no significant difference was found between the two thresholds in terms of the depth of the AMLD, we used the value of 10 −2 m 2 /s to calculate the AMLD. ...
Phytoplankton bloom in the Gulf of Elat/Aqaba (the Gulf) was studied before, mainly using one‐dimensional models and observations from the northern Gulf. Thus, the spatial variability within the Gulf and the contribution of physical processes such as horizontal advection to the bloom have not yet been studied. Moreover, various factors such as the effect of light limitation on phytoplankton growth in the Gulf are still debated. Here, we used a three‐dimensional coupled physical‐ecological model for the Gulf to study the mechanisms for phytoplankton bloom throughout the Gulf. We found the southern surface bloom to be higher than the northern surface bloom. In contrast, southern integrated bloom is lower than the northern integrated bloom. These differences result from spatial variations in the mixed layer depth, which is much deeper in the northern Gulf compared with the south. Moreover, horizontal advection controls phytoplankton integrated biomass during the northern bloom, a process often neglected when dealing with phytoplankton blooms. Finally, we found that light limits growth in the northern integrated bloom. The results from the northern Gulf are compared to the North Atlantic bloom, while those from the southern Gulf are compared to the bloom in subtropical oceans due to similarities in mixing depth and consequently in nutrient versus light limitation on phytoplankton growth.
... For example, we do not include variation in the thickness of the surface and bottom mixed layers. Within the surface layer, phytoplankton are vertically mixed and exposed to a variety of light conditions as a consequence of light attenuation (Huisman et al., 1999;Obata et al., 1996;Mahadevan et al., 2012;Sverdrup, 1953), which may be important in determining the ecosystem composition, specifically nearshore where surface and bottom mixed layer merge. In the absence of along-shore pressure gradients, upwelling is mostly constrained to the bottom boundary layer (Jacox & Edwards, 2011;Lentz & Chapman, 2004). ...
Eastern boundary upwelling systems (EBUSs) are among the most productive regions in the ocean because deep, nutrient‐rich waters are brought up to the surface. Previous studies have identified winds, mesoscale eddies and offshore nutrient distributions as key influences on the net primary production in EBUSs. However uncertainties remain regarding their roles in setting cross‐shore primary productivity and ecosystem diversity. Here, we use a quasi‐two‐dimensional (2D) model that combines ocean circulation with a spectrum of planktonic sizes to investigate the impact of winds, eddies, and offshore nutrient distributions in shaping EBUS ecosystems. A key finding is that variations in the strength of the wind stress and the nutrient concentration in the upwelled waters control the distribution and characteristics of the planktonic ecosystem. Specifically, a strengthening of the wind stress maximum, driving upwelling, increases the average planktonic size in the coastal upwelling zone, whereas the planktonic ecosystem is relatively insensitive to variations in the wind stress curl. Likewise, a deepening nutricline shifts the location of phytoplankton blooms shore‐ward, shoals the deep chlorophyll maximum offshore, and supports larger phytoplankton across the entire domain. Additionally, increased eddy stirring of nutrients suppresses coastal primary productivity via “eddy quenching,” whereas increased eddy restratification has relatively little impact on the coastal nutrient supply. These findings identify the wind stress maximum, isopycnal eddy diffusion, and nutricline depth as particularly influential on the coastal ecosystem, suggesting that variations in these quantities could help explain the observed differences between EBUSs, and influence the responses of EBUS ecosystems to climate shifts.
... Cependant, des effets contrastés ont été reporté à la suite de précipitations (Chorus et al., 2021), notamment à la suite d'orages pouvant entraîner une diminution de la biomasse phytoplanctonique, lié à une diminution du temps de résidence de l'eau au sein de l'écosystème et donc une dilution de la masse d'eau (Padisák et al., 1999). De plus, les fortes précipitations peuvent modifier l'intensité lumineuse au sein de la colonne d'eau : soit suite à un mélange plus profond de la colonne d'eau suite à l'augmentation de la vitesse du vent et la réduction de la température de l'air survenant avec une pluie, exposant ainsi le phytoplancton à des niveaux de lumière plus faibles (Huisman et al., 1999) ; soit par l'apport accru de matière organique par le ruissellement des bassins versants lors des fortes précipitations, ce qui peut entraîner une augmentation de la turbidité et une diminution de la profondeur euphotique (Chorus et al., 2021;Stockwell et al., 2020). Dans tous les cas, la diminution en luminosité constituera une limite pour la croissance phytoplanctonique. ...
... Due to the short growth cycles and rapid reproduction rates of marine phytoplankton, their cell abundance and species structure respond swiftly and sensitively to environmental changes (Smayda, 1997;Behrenfeld et al., 1997;Huisman et al., 2013). Therefore, weekly high-frequency sampling can better capture short-term marine ecological changes (Huisman et al., 1999), which is crucial for analyzing the relationship between marine phytoplankton and environmental factors, as well as for studying trends in marine ecosystem changes. Moreover, the rich data obtained from high-frequency sampling provides more detailed information on model training, which enhances the accuracy of model predictions, thereby better forecasting future changes in marine ecology (Racault, 2012;Plank et al., 2008 The Random Forest (RF) algorithm is a powerful ensemble learning technique that is widely applied to various tasks, such as classification and regression. ...
... Filamentous cyanobacteria tended to dominate the surface layer of the reservoir water column (0-10 m) during the thermal stratification's stabilisation and weakening phases (period_III and period_IV) (Fig. 3d). In the context of global warming and the escalating intensity and duration of thermal stratification [22], coupled with the ongoing proliferation of harmful cyanobacteria [6,15], these observations raise significant concerns, prompting heightened vigilance regarding the potential production of odour compounds and increased likelihood of odour events. ...
2-Methylisoborneol (2-MIB) and geosmin are compounds released by algae that significantly degrade reservoir water quality, posing a threat to both the safety of drinking water and the quality of aquatic products sourced from these environments. However, few studies have explored how enhanced thermal stratification affects the occurrence and regulation of odorants in large drinking water reservoirs. Through systematic monitoring and investigation of Xin'anjiang Reservoir, we found that enhanced thermal stratification promotes filamentous cyanobacteria, particularly Leptolyngbya sp., as the primary contributor to 2-MIB production within the 1-10 m layer of the water column. The highest 2-MIB concentration, 92.5 ng/L, was recorded in the riverine region, which was 2.54 and 14.52 times higher than that in the transitional and central parts of the reservoir, respectively. Temperature indirectly impacted algal growth and odorant production by modulating TN/TP ratios. Geosmin concentration responded rapidly to relatively low TN/TP ratios (< 25). Our findings suggest that phosphorus control in estuaries should be enhanced during thermal stratification period. In summary, our study provides valuable insights to inform pragmatic water intake strategies and the distribution and release of odorants caused by thermal stratification. This is particularly relevant in the context of future global warming and extremely high temperatures during the warm season.
... Previous research provided three spring phytoplankton bloom patterns, which may be applied independently to the study period (Hegseth et al., 2019;Hegseth and Tverberg, 2013a). The timing of spring bloom can be explained by well-known hypothesis such as critical depth and critical turbulence (Huisman et al., 1999) although it is still in debate (Behrenfeld, 2010), which explain the phytoplankton bloom from large scale well-mixed environments and relatively small scale turbulence mixing. Dormant spores may serve as a seed population for spring blooms, and this theory was later validated in several regions (Garrison, 1981). ...
... proposed by Huisman et al. (1999). This re-evaluation of Sverdrup's original hypothesis underscores that phytoplankton position in the water column is not always determined by the mixed layer depth (as defined by the seasonal thermocline). ...
Marine phytoplankton are responsible for approximately half of the photosynthetic production of organic matter (OM) and oxygen in Earth. The composition and reactivity of phytoplankton- derived OM influences two of the main C-sequestration mechanisms of the ocean: the biological carbon pump and the microbial carbon pump. Phytoplankton-derived OM can be classified as particulate (POM) or dissolved (DOM) and these size-fractions are subject to diverse production, consumption and transport processes involving biotic and abiotic interactions. Understanding how these processes influence OM composition and reactivity is essential to accurately describe the role of phytoplankton ecology in the marine Carbon cycle and ultimately in the regulation of Earth climate. This thesis aims, precisely, to better understand the controls over these processes. To do so, we combined fluorescence spectroscopy and elemental analysis of POM and DOM with multiple biotic and abiotic parameters during the development and decay of phytoplankton proliferations in micro- and mesocosm experiments and under natural conditions. The microcosm degradation experiment revealed that POM derived from diatom-dominated proliferations is degraded at a much slower rate than that of POM produced by a mixed phytoplankton community. In addition, accumulation of DOM of apparent recalcitrant nature was observed during the processing of diatom-derived POM. The analysis of four phytoplankton proliferations in Antarctic waters revealed that protein-like fluorescent OM was contributed by dissolved and particulate materials. The abundance and composition of phytoplankton and their interactions with viruses and grazers were identified as the main controls over the quantity and fractionation of protein-like fluorescent OM. By contrast, humic-like substances were mostly in the dissolved fraction, and their composition was related to photochemical degradation and microbial transformation. The mesocosm experiment showed that the balance between production and degradation of protein-like fluorescent DOM was controlled by the nitrogen availability of the planktonic community. Whereas the humic-like fluorescent DOM composition was influenced by photochemical processes and production of specific humic-like substances by autotrophic and heterotrophic prokaryotes, the taxonomic composition of eukaryotic phytoplankton did not have a profound influence over the fluorescent DOM composition. Overall, this thesis shows that the composition of plankton assemblages, and the interactions between organisms and between organisms and environmental conditions influence the composition and reactivity of phytoplankton-derived OM, ultimately determining its fate and role in the marine Carbon cycle.
... The mixing layer depth was computed as the maximum vertical gradient of Chl (MLD bio ) following Lacour et al. 12 . The underlying concept is that Chl, as a proxy of phytoplankton concentration, is homogeneous over the whole mixing layer if turbulent mixing overcomes vertical variations in phytoplankton net growth rate 50 . MLD bio should capture the high-frequency variability of the mixing layer at time scales typical of phytoplankton growth 51 . ...
At high latitudes, the biological carbon pump, which exports organic matter from the surface ocean to the interior, has been attributed to the gravitational sinking of particulate organic carbon. Conspicuous deficits in ocean carbon budgets challenge this as a sole particle export pathway. Recent model estimates revealed that particle injection pumps have a comparable downward flux of particulate organic carbon to the biological gravitational pump, but with different seasonality. To date, logistical constraints have prevented con-comitant and extensive observations of these mechanisms. Here, using year-round robotic observations and recent advances in bio-optical signal analysis, we concurrently investigated the functioning of two particle injection pumps, the mixed layer and eddy subduction pumps, and the gravitational pump in Southern Ocean waters. By comparing three annual cycles in contrasting physical and biogeochemical environments, we show how physical forcing, phytoplankton phenology and particle characteristics influence the magnitude and seasonality of these export pathways, with implications for carbon sequestration efficiency over the annual cycle.
... This is formally known as the critical depth hypothesis (Sverdrup 1953), which predicts that blooms occur when the MLD shoals to become less than a critical value at which gross primary production and respiration are balanced. In addition to this hypothesis, other mechanisms have been proposed to explain the onset of the spring bloom, the most relevant of which are the critical turbulence hypothesis (Huisman et al. 1999), which states that a phytoplankton bloom can occur independent of the MLD as long as the local turbulence is low enough to keep the cells in the illuminated surface layer, and the disturbance−recovery hypothesis (Behrenfeld 2010, Behrenfeld & Boss 2014, according to which the phytoplankton blooms start in winter when the ocean surface is cooling and mixing is strong. These conditions would be favourable for the accumulation of phytoplankton due to reduced encounter rates with grazers as a consequence of dilution. ...
A coupled physical (Regional Ocean Modeling System, ROMS)-biogeochemical model (N2PZD2) at a horizontal resolution of 3.5 km was implemented for N and NW Iberia, an area of high productivity associated with upwelling. The physical model has been the object of previous studies and has proven its capability to perform well in reproducing the main oceanographic features in the area (upwelling, river plumes, slope currents, fronts, filaments), which is fundamental to properly representing the variability and distribution of the biogeochemical variables. The biogeochemical model was set up to account for the main nutrient inputs in the area: upwelling and rivers. Upwelling input required proper characterization of the nutrient content variability of the Eastern North Atlantic Central Water, which was achieved by using a temperature-nitrate relationship obtained from observations to impose nitrate at the open boundaries. The resulting biophysical model accurately reproduced the timing and interannual variability of the spring bloom compared with satellite chlorophyll (chl a) observations. A comparison with the Instituto Español de Oceanografía’s in situ spring-monitoring Pelacus cruises (which include plankton) revealed that the model was able to reproduce the variability at shorter scales (days) and demonstrated its ability to complement the observational data and reveal the variability in the area around the spring transition. In this respect, both the model and observations showed that productivity on this narrow shelf is affected by seasonal upwelling that results from the interplay of wind, river plumes and light intensity, all varying at interannual, seasonal and event scales.
... Obata et al., 1996). In other cases, however, this relation did not work (Lucas et al. 1998) and the application of the classical formula for estimating Z cr was criticized (Smetacek and Passow 1990;Platt et al. 1991;Huisman et al. 1999;Backhaus et al. 2003;Behrenfeld 2010). It was suggested that the critical depth formula should consider that: (1) the dependence of P on light is not a simple linear function (Platt et al. 1991); and (2) the community loss rate, R, as a function of depth is not constant and should include also loss terms for sinking export, grazing, viral infection, parasitism, physical loss and a reverse dependence on P (Smetacek and Passow 1990; ...
... The abundance, distribution, and diversity of phytoplankton depends upon the heterogeneity of many habitat characteristics (Reynolds 1984;Klausmeier and Litchman 2001;Clegg et al. 2007) and the major axis of spatial heterogeneity for phytoplankton is in the vertical dimension (Mellard et al. 2011). Physical conditions of the water such as water depth, intensity of mixing, and thermal regime impact phytoplankton (Huisman et al. 1999(Huisman et al. , 2006O'Brien et al. 2003). These create gradients in other important habitat characteristics such as water temperature, average light climate, and nutrient availability (Longhi and Beisner 2009). ...
... It is also clear that even a well-mixed ocean is neither isotropic nor unstructured (d'Ovidio et al., 2010). Spatial partitioning can thus occur at many different scales and ecological equilibrium is often prevented by external perturbations (Litchman et al., 2009) and internal dynamics (Huisman et al., 1999) such that competitive exclusion can be indefinitely postponed. ...
Marine microbial communities are extremely complex and diverse. The number of locally coexisting species often vastly exceeds the number of identifiable niches, and taxonomic composition often appears decoupled from local environmental conditions. This is contrary to the view that environmental conditions should select for a few locally well‐adapted species. Here we use an individual‐based eco‐evolutionary model to show that virtually unlimited taxonomic diversity can be supported in highly evolving assemblages, even in the absence of niche separation. With a steady stream of heritable changes to phenotype, competitive exclusion may be weakened, allowing sustained coexistence of nearly neutral phenotypes with highly divergent lineages. This behaviour is robust even to abrupt environmental perturbations that might be expected to cause strong selection pressure and an associated loss of diversity. We, therefore, suggest that rapid evolution and individual‐level variability are key drivers of species coexistence and maintenance of microbial biodiversity.
... Because of the complexity of the ML, it is difficult to understand the effect of its behavior on the transport of suspended particles. In particular, the behavior of ML is influenced by factors such as stratification or mixing as a result of differential wind intensities, which can generate considerable changes in the conditions of the ML (Huisman et al., 1999;Ghosal and Mandre, 2003;Ross, 2006;Ryabov et al., 2010;Delhez and Deleersnijder, 2010). ...
The sinking speed of coccoliths, considered as small particles, mainly depends on turbulence processes in the upper layer of the ocean. In deeper layers, the size, shape, buoyancy, and drag force experienced between the contact surface and the fluid in which particles are immersed are fundamental determinants of the sinking speed. For spherical bodies, the sinking speed is explained by the Stokes formula. For other geometries, it can be explained by the shape resistance factor of an equivalent sphere with the same density and volume as the original body. To model the sinking trajectories of coccoliths in the upper layer, a Lagrangian turbulence model was applied. To estimate how the size and shape of some coccoliths influence their sinking speed and trajectory in deeper layers, experimental videos were taken of sinking coccoliths made at amplified scales in a graduated water tank and analyzed. In the upper layer, the sinking speed only depends on the intensity of turbulence. Below this layer, the shape resistance factor reduced the sinking speed with respect to that of the equivalent sphere by an average of 50%. The Lagrangian model shows that, in the turbulent mixing layer, the distribution of particles is homogeneous, but once the particles fall below the laminar layer, the distribution takes on the form of a thin lens with a central thickness that is proportional to the sinking speed. The particles then flatten out to a thin strip as they continue along their path to the ocean bottom.
... For example, the critical depth hypothesis predicts that bloom initiation onlys begins when mixed layer light levels exceed the threshold of respiration rates, allowing phytoplankton biomass to increase if division rates exceed other losses (Behrenfeld andBoss 2014, 2018). The critical turbulence hypothesis assumes the same threshold as the critical depth hypothesis but focuses on division rates in the actively mixing turbulent layer (Huisman et al. 1999;Taylor and Ferrari 2011). The dilution-recovery hypothesis suggests a more general balance between phytoplankton division and loss rates (often driven by grazing) allowing blooms to develop when division rates are declining so long as loss rates decline below division rates, for example, with dilution via mixed layer deepening (Evans et al. 1985;Behrenfeld 2010). ...
The North Atlantic phytoplankton bloom depends on a confluence of environmental factors that drive transient periods of exponential phytoplankton growth and interannual variability in bloom magnitude. I analyze interannual bloom variability in the North Atlantic via extreme value theory where the generalized extreme value distribution (GEVD) is fitted spatially to annual maxima of satellite‐measured surface chlorophyll. I find excellent agreement between the observed distribution of interannual bloom maxima and those predicted from the GEVD. The spatial distribution of fitted GEVD parameters closely follows basin bathymetry where the largest extremes and heaviest distribution tails are found on the continental shelves and slopes. Trend analyses suggest weak evidence for changes in GEVD parameters, despite regional trends in mean chlorophyll levels and sea surface temperature. These results provide a framework to quantify interannual bloom variability and call for further work examining how extreme blooms propagate through food webs and contribute to carbon export.
We present a nonlocal reaction‐diffusion‐advection system that models the predator–prey relationship between zooplankton and phytoplankton species in a eutrophic vertical water column. The invasion dynamics of zooplankton are analyzed in terms of the spontaneous death rates and buoyant/sinking velocities of both phytoplankton and zooplankton. Our analysis reveals that the zooplankton species can successfully invade and coexist with the phytoplankton only under conditions of low spontaneous death rates and matching buoyant/sinking velocities with phytoplankton. Additionally, we derived asymptotic profiles for the unique positive steady state of this system when one of the sinking or buoyant velocities of either phytoplankton or zooplankton approaches infinity, while the other velocity remains fixed. These findings highlight the significant role of advection due to buoyancy in shaping the dynamics of plankton ecosystems.
A precise understanding of the mechanisms causing phytoplankton blooms in reservoirs is still lacking, especially in large riverine reservoirs. To better understand these blooms, the role of the complex hydrodynamics caused by dam operation must be quantified. Here we examine how synergistic hydrodynamic processes, rather than individual metrics, trigger blooms in Xiangxi Bay, a typical tributary bay of the Three Gorges Reservoir, China. We used a 3D ecological-hydrodynamic model, which integrated hydrodynamics with the abiotic factors that limit phytoplankton growth to simulate one whole year (2010). By implementing a scaling criterion, we quantified the contribution of local phytoplankton growth and hydrodynamic processes, including advection transport and vertical mixing, on bloom dynamics. Results indicated vertical mixing was the main process inhibiting blooms in colder months (from October to February) but horizontal advection, which flushed and diluted blooms, was dominant in warmer months (from May to July) when stratification was intense and nutrients were replete. Accordingly, blooms occurred when both vertical mixing and horizontal advection were low. We suggested a potential dam operation strategy to mitigate blooms during stratification, which involves withdrawing the warm surface water from upstream reservoirs to increase horizontal flows in the surface layer. Extending the application of critical turbulence model, our study shows how vertical mixing and horizontal advection rate interact with phytoplankton growth rate to drive blooms in highly dynamic riverine systems.
The intensity of eddy diffusivity and the spatial average of water velocity at the depths of the water column in oceans and lakes play a fundamental role in phytoplankton production and phytoplankton and zooplankton biomass, and community composition. The critical depth and intensity of turbulent mixing within the water column profoundly affect phytoplankton biomass, which depends on the sinking characteristic of planktonic algal species. We propose an Nutrient-Phytoplankton-Zooplankton (NPZ) model in 3D space with light and nutrient-limited growth in a micro-scale ecological study. To incorporate micro-scale observation of phytoplankton intermittency in bloom mechanism in stationary as well as oceanic turbulent flows, a moment closure method has been applied in this study. Experimental observations imply that an increase in turbulence is sometimes ecologically advantageous for non-motile planktonic algae. How do we ensure whether there will be a bloom cycle or whether there can be any bloom at all when the existing phytoplankton group is buoyant, heavier, motile, or non-motile? To address these questions, we have explored the effects of critical depth, the intensity of eddy diffusivity, spatial average of water velocity, on the concentration as well as horizontal and vertical distribution of phytoplankton and zooplankton biomass using a mathematical model and moment closure technique. We quantify a critical threshold value of eddy diffusivity and the spatial average of water velocity and observe the corresponding changes in the phytoplankton bloom dynamics. Our results highlight the importance of eddy diffusivity and the spatial average of water velocity on seasonal bloom dynamics and also mimic different real-life bloom scenarios in Mikawa Bay (Japan), Tokyo Bay (Japan), Arakawa River (Japan), the Baltic Sea, the North Atlantic Ocean, Gulf Alaska, the North Arabian Sea, the Cantabrian Sea, Lake Nieuwe Meer (Netherlands) and several shallower lakes.
Understanding the dynamic interplay between physical and biological processes is a major challenge in ocean-related studies, especially to develop predictive capabilities and while addressing the climate change impacts. Biological and physical dynamics in the oceans are coupled, and primary producers being the most important element in an ecosystem, the subject is vastly explored, in terms of the chaotic interactions between various elements of the ecosystem in different spatiotemporal scales. Fluid (ocean) properties are a key factor interacting with plankton behaviour, driving the biological processes and their spatiotemporal patterns. The present chapter is on the role of density gradient in determining the vertical profile of chlorophyll-a in a warm/stratified region, the north-eastern Arabian Sea (NEAS), during the winter-spring season. The dynamics of the recurring bloom (green Noctiluca), one of the important regional ecosystem issues, is explained for initial, peak, and withdrawal stages based on in situ observations. The Bio-Argo and conductivity-temperature-depth (CTD) profiler-based analysis for 2003–2019 shows the early onset of the spring bloom and intensification in the subsurface chlorophyll maxima (SCM) in the NEAS since the recent past. The empirical orthogonal teleconnection (EOT) is effectively utilised to explain the surface-subsurface interaction in maintaining the upper layer production pattern and the adaptive strategies of the phytoplankton in respect of the buoyancy control.
We conducted continuous mooring observation from autumn to winter of fiscal year 2020 to elucidate the mechanism of red tide development in the inner Ariake Sea, a very turbid shallow coastal water in Japan. The red tide dominated by Skeletonema spp. (mainly Skeletonema dohnii) developed at first neap tide after the annual minimum water temperature. Red tides at similar times of the year have been frequently observed here. Formation of two physical environments favorable for phytoplankton proliferation played a trigger role. One was stabilization of water column due to net heat flux transition through the sea surface from cooling to heating in mid-winter. Another was deepening of euphotic layer up to or exceeding water depth at the neap tide. Since the inner Ariake Sea has the small heat capacity due to its shallowness, the air and water temperature fluctuated almost in tandem, and reached their respective lowest values with a short time lag. The sea-surface heat flux, a main factor governing water temperature fluctuations, was dominated by latent heat and showed the highest correlation with the difference between atmospheric and sea-surface specific humidity. After mid-January, the atmosphere stabilized as the air temperature exceeded the water temperature, and the sea-surface cooling due to the latent heat weakened. With the heat flux change from negative to positive, the water column was stabilized. Then, winter bloom occurred during the neap tide when the compensation depth became deep with the decrease in suspended sediment concentration.
Phytoplankton production as indicated by chlorophyll a concentrations in Lake Karāpiro, the last hydro power station lake on the Waikato River, shows high seasonal variability but a long term trend of decrease, despite an abundance of nutrients (N and P) in the river. Early studies pre-2005 of the river system’s eight hydro dams identified that only Lakes Ohakuri and Arapuni thermally stratified in summer, and it was hypothesised that thermal stratification could enhance phytoplankton productivity through increased retention time. During. a more recent study (2013–2014), Lake Karāpiro was also found to be thermally stratified with the thermocline around the depth of the power station intake. In this study we found that thermal stratification enhanced phytoplankton growth by confining them to the well-lit epilimnion. However, growth was limited as the nutrient supply became depleted and zooplankton grazing was found to adversely impact phytoplankton biomass, with highest grazing pressure in spring and lowest in late summer to autumn, consistent with an increase in chlorophyll a concentration. We found that wind stress along the lake axis can induce an internal seiche, which causes a substantial stepwise change in water quality downstream as the thermocline sweeps up and down across the power station intake.
Le plancton marin est constitué d'organismes microscopiques allant des virus jusqu'aux petit métazoaires en passant par les bactéries et les protistes. Il est transporté passivement par les courants, prospère sur l'ensemble des océans et a plusieurs rôles cruciaux au sein du système Terre. Le phytoplancton permet par la photosynthèse la production primaire de matière organique dans les océans soutenant l'ensemble des réseaux trophiques océaniques. Le plancton participe aussi à la pompe à carbone biologique, mécanisme par lequel la matière organique sédimente vers les fonds marins et y est stockée. Il est aujourd'hui primordial d'évaluer et projeter la réponse du plancton au changement climatique provoqué par la combustion des énergies fossiles. Dans cette thèse, j'étudie cette question sous le prisme de la biogéographie, discipline s'intéressant à la distribution des organismes dans et en interaction avec leur milieu à travers le temps et l'espace. Dans une première partie, j'étudie à l'aide des données omiques des expéditions Tara Océans et des modèles climatiques la distribution du plancton dans les océans et sa réponse au changement climatique. Il est mis en évidence un partitionnement des océans en provinces génomiques dépendant de la taille des organismes. Ces provinces sont mises en relation avec les paramètres physico-chimiques à l'aide de techniques de machine learning et extrapolées à l'ensemble des océans. Un ensemble de « génomes signatures » pour chaque province est aussi mis en évidence. Une importante réorganisation des provinces en réponses au changement climatique est projetée sur environ 50% des océans d'ici la fin du siècle. D'importants changements en composition du plancton sont projetés et mis en lien avec une diminution de 4% de la pompe à carbone biologique ce qui aurait un effet aggravant sur le changement climatique. Dans une seconde partie, j'étudie les changements de l'expression des gènes du plancton eucaryote le long de la transition entre l'océan Atlantique Nord et le bassin Arctique. Parmi les variables physiques, la température est la variable expliquant le mieux les changements transcriptionnels. L'analyse fonctionnelle des gènes corrélant avec le gradient fort de température révèle une stratégie commune d'acclimatation des algues eucaryotes. Cette stratégie inclut d'importants changements dans la machinerie d'expression génétique mais aussi la surexpression d'un ensemble de fonctions liées à l'acclimatation au froid. A l'échelle des communautés, des processus liés à la maintenance du pool protéique et de la machinerie transcriptionnelle semblent plus actifs dans l'arctique. Dans une troisième partie, je développe un modèle mécanistique basé individu de communauté de phytoplancton structurée par la taille et advectée dans un champ de vitesse imitant la circulation océanique Nord Atlantique. Le modèle représente les principaux groupes de phytoplancton: les cyanobactéries, les algues eucaryotes et les diatomées. Les caractéristiques écologiques de bases résultant de simulations sont présentées : α-diversité, biogéographie et biomasse. Une attention est apportée à l'importance du nombre de particules modélisées et son impact sur la diversité maximale représentée ainsi que l'influence des courants sur les niches écologiques des organismes modélisés. Cette thèse aborde des sujets et enjeux fondamentaux de l'écologie contemporaine et son apport s'inscrit dans la révolution omique du début du XXIème siècle. Une biogéographie génomique du plancton est proposée et sa réponse au réchauffement climatique est explorée. L'analyse omique in situ de la physiologie des algues unicellulaires du plancton à l'échelle de la communauté permet de mettre en évidence leurs capacités d'acclimatation directement dans leur milieu de vie. Enfin, le modèle basé individu du phytoplancton permet d'explorer théoriquement de nombreuses questions écologiques à mettre en lien dans le futur avec l'analyse des données omiques.
The spatial arrangement of biodiversity and annual plankton succession are key phenomena influencing ocean biogeochemistry (e.g. the biological pump) and the life cycle of many species. Biodiversity and phenological shifts induced by climate change might alter species succession and lead to trophic desynchronization and community reorganisation in space and time. The aim of this PhD is to improve our understanding of plankton biodiversity and phenology by identifying factors and processes that control them and by modelling the annual plankton succession in the context of global climate change. To do so, we used an approach based on the MacroEcological Theory on the Arrangement of Life (METAL) and observations collected by the Continuous Plankton Recorder (CPR) survey. To understand how annual plankton succession and species phenology will be altered in the context of global warming, it is important to identify what parameters and processes control these phenomena. Therefore, in the first part of this PhD, we describe the seasonal variations of major phytoplanktonic taxa in the North Sea and demonstrate that species' phenology results from the interaction between species' niche and the environment. We also show that diatoms with similar cell shape have also similar phenology and niches, i.e. that oblates (flattened cells) dominate the spring and autumn periods whereas prolates (elongated cells) dominate the summer period. We therefore establish a salient link between functional traits, the niche and the phenology. In the second part, we examine the spatio-temporal organisation of plankton biodiversity in the North Atlantic Ocean and show that this region is characterised by large spatial coenoclines (i.e. gradient of biocoenosis or community) induced by the niche-environment interaction. We also develop a new method, called a "species chromatogram", that gives a graphical summary of the niche by representing together abundance gradients along various environmental dimensions. This method can be used to characterise and display the multidimensional ecological niches of different species and also to compare the niches of different species by means of an index that quantify the degree of niche overlapping. We use this index to demonstrate that the plankton belonging to the same trophic guildshave sufficiently distinct niches to coexist is an impermanent and heterogeneous environment in space and time. Finally, we apply a model based on METAL and the projections of six Earth Systems Mdels involved in the Coupled Model Intercomparison Project Phase 6 (CMIP6) to project the past, present and future phenological changes of three phytoplanktonic groups, i.e. oblate and prolate diatoms and dinoflagellates, in three keys areas of the North Atlantic Ocean. We show that the phenology of oblates is likely to continue to shrink and their abundance to decline whereas the phenology of prolates and dinoflagellates will expand and their abundance rise. Our results suggest a climate-induced reorganisation of the phytoplanktonic community in space and time that will affect the rhythm of generation of endosomatic energy, which may have strong consequences on ecosystem functioning and services.
Temporal variability in plankton community structure and biomass is often driven by environmental fluctuations: nutrient supplies, light, stratification and temperature. But plankton time series also exhibit variability that is not strongly correlated with key physical variables and is distinctly nonlinear in nature. There is evidence, from both laboratory and modeling studies, that oscillations can arise from ecological interactions alone. In the open ocean, it is challenging to establish the roles and relative importance of environmental versus intrinsic processes in generating the observed ecological variability. To explore this competition, we employ a marine plankton model that supports two mechanisms of intrinsic ecological variability operating at distinct frequencies: predator‐prey interactions between zooplankton and phytoplankton, with timescales of weeks, and resource competition that occurs with multiple nutrients phytoplankton species, with timescales of years. The model is forced by imposing variable nutrient input rates. Representing typical open ocean situations, with periods ranging from subseasonal to multi‐annual. We find that intrinsically‐driven variability generally persists in the presence of extrinsic forcing, and that the interaction between the two can produce variability at frequencies that are not characteristic of either source. The intrinsic frequencies are found to be even more energetic when the extrinsic variability is augmented with stochastic noise. We conclude that interactions between intrinsic and extrinsic sources of variability may contribute to the wide range of observed frequencies in phytoplankton time series, and may explain why it is often difficult to relate planktonic variation to environmental variation alone.
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
Marine phytoplankton possess a key position in multiple ecological, biological, and environmental processes; therefore, constructing a systematic and comprehensive mechanistic understanding of the leading mechanisms and key biophysical drivers of phytoplankton dynamics is crucial for evaluating biophysical forcings and ecological responses. Here, we develop a process-based model to improve the characterization of controlling mechanisms and assess the relative importance of nutrients, light, temperature, and oceanic mixing for explaining chlorophyll-a variability in the western North Pacific Ocean. The model is calibrated using 20-year observational data of chlorophyll-a concentrations from 1993 to 2012 in a Bayesian framework to provide data-driven inference and a rigorous uncertainty quantification of important biophysical rates. The model explains approximately 61% of the variability in chlorophyll-a. Our results, supported by cross-validation, show that the chlorophyll-a concentration is much more responsive to temperature variability than nutrients, light, or mixing. By synthesizing the effects of multiple nutrients (i.e., nitrogen, phosphorus, silica, and iron) on phytoplankton dynamics, we find that iron and nitrogen largely determine the chlorophyll-a variability. Sensitivity analysis shows that future warming conditions with +3°C change in temperature will impede phytoplankton production by 24% due to an intensified phytoplankton mortality impact. Our findings highlight the potential of “deductive-and-inductive” modeling approaches combining embedded biophysical mechanisms and probabilistic uncertainty quantification to effectively integrate and leverage sporadic ocean monitoring efforts and ultimately improve the marine water quality predictions.
The cover image is based on the Original Article A trophic cascade triggers blooms of Asterionella formosa in subtropical eutrophic Lake Taihu, China, by Xia Liu et al. https://doi.org/10.1111/fwb.13986.
1. The importance of trophic interactions for determining the distributions, abundances,and taxonomic compositions of organisms in ecosystems has long been studied and debated. Here we test the effect of a trophic cascade on diatom (Asterionella formosa) blooms in subtropical, eutrophic Lake Taihu, China.
2. A long-term data series (2005–2015) on planktivorous fish, zooplankton and diatoms has been analysed. Structural equation modelling is used to test our hypotheses about the influences of top-down and bottom-up forces on A. formosa.
3. Since 2009, a spring bloom of A. formosa has occurred in the lake, coinciding with reduction of Daphnia galeata biomass and of total cladoceran biomass following a marked increase in the stock of planktivorous fish (bighead carp and silver carp). Light, phosphorus (total and soluble reactive phosphate) and silica did not mact as limiting factors for the growth of A. formosa.
4. Structural equation modelling analysis showed that top-down
effects of cladoceran on A. formosa biomass were more important than bottom-up effect (wind speed and soluble reactive phosphate). Although A. formosa was negatively correlated with total nitrogen, total with other inorganic nitrogen (NO3 and NH4) did not restrict the proliferation of diatom blooms after 2007. These results suggested that the substantial reduction of the Daphnia population caused a diatom bloom through a trophic cascade by planktivorous fish.
5. Our study provides new insight into the effects of trophic interactions on diatom bloom formation in natural freshwater ecosystems.
Strong relationships between size and other traits have long motivated studies of the size structure and dynamics of planktonic food webs. Size structured ecosystem models (SSEMs) are often used to represent the behavior of these ecosystems, with organism size as a first order approximation of the axis of biological diversity. Previous studies using SSEMs have reported the emergence of localized “peaks” in the size spectrum, a phenomenon that will be referred to in this study as “quantization”. However, SSEMs that are used routinely in Earth System Models (ESMs), they tend to be too coarsely discretized to resolve quantization. Observational studies of plankton biomass have also shown qualitatively similar patterns, with localized peaks along the size spectrum. The conditions under which quantization occurs and the ecosystem parameters that control the locations of the biomass “peaks” along the size spectrum have not been systematically explored. This study serves to simultaneously advance our understanding of the constraints on quantization in size-structured ecosystems, and to suggest an approach to discretizing SSEMs that leverages quantization to select a greatly reduced number of size classes. A size-structured model of the pelagic food web, similar to those implemented in global models, is used to investigate the sensitivity of biomass peaks to predator–prey interactions, and nutrient forcing. This study shows that the location of biomass peaks along the size spectrum is strongly controlled by the size selectivity of predation, and the location of biomass peaks along the size spectrum is less sensitive to variations in nutrient supply, external ecosystem forcing, and vertical heterogeneity. Taking advantage of a robust localization of biomass peaks, the dynamics of a continuous planktonic size spectrum to be represented using a few selected size classes, corresponding to locations of the peaks along the size spectrum. These findings offer an insight on how to approach discretization of size structured ecosystem model in Earth system models.
In this paper we use numerical models of coupled biological-hydrodynamic processes to search for general principles of bloom regulation in estuarine waters. We address three questions: What are the dynamics of stratification in coastal systems as influenced by variable freshwater input and tidal stirring? How does phytoplankton growth respond to these dynamics? Can the classical Sverdrup Critical Depth Model (SCDM) be used to predict the timing of bloom events in shallow coastal domains such as estuaries? We present results of simulation experiments which assume that vertical transport and net phytoplankton growth rates are horizontally homogeneous. In the present approach the temporally and spatially varying turbulent diffusivities for various stratification scenarios are calculated using a hydrodynamic code that includes the Mellor-Yamada 2.5 turbulence closure model. These diffusivities are then used in a time- and depth-dependent advection-diffusion equation, incorporating sources and sinks, for the phytoplankton biomass. Our modeling results show that, whereas persistent stratification greatly increases the probability of a bloom, semidiurnal periodic stratification does not increase the likelihood of a phytoplankton bloom over that of a constantly unstratified water column. Thus, for phytoplankton blooms, the physical regime of periodic stratification is closer to complete mixing than to persistent stratification. Furthermore, the details of persistent stratification are important: surface layer depth, thickness of the pycnocline, vertical density difference, and tidal current speed all weigh heavily in producing conditions which promote the onset of phytoplankton blooms. Our model results for shallow tidal systems do not conform to the classical concepts of stratification and blooms in deep pelagic systems. First, earlier studies (Riley, 1942, for example) suggest a monotonic increase in surface layer production as the surface layer shallows. Our model results suggest, however, a nonmonotonic relationship between phytoplankton population growth and surface layer depth, which results from a balance between several ''competing'' processes, including the interaction of sinking with turbulent mixing and average net growth occurring within the surface layer. Second, we show that the traditional SCDM must be refined for application to energetic shallow systems or for systems in which surface layer mixing is not strong enough to counteract the sinking loss of phytoplankton. This need for refinement arises because of the leakage of phytoplankton from the surface layer by turbulent diffusion and sinking, processes not considered in the classical SCDM. Our model shows that, even for low sinking rates and small turbulent diffusivities, a significant percentage of the phytoplankton biomass produced in the surface layer can be lost by these processes.
Interactions between nutrient limitation and light limitation are fundamental for the dynamics and structure of phytoplankton communities. We investigate a model that predicts the outcome of competition for nutrients and light in a mixed water column on the basis of monoculture characteristics. Growth in monoculture leads to a steady state. The nutrient availability and light penetration in this steady state characterize the minimal resource requirements of a species. These minimal requirements not only determine monoculture growth but also the outcome of competition. We show that competition for nutrients and light can be investigated by means of a graphical isocline approach. In contrast to earlier resource-based approaches, our model predicts that it is not only the ratio of nutrient supply to light supply that matters for the outcome of competition but also their absolute supply rates. It is even possible that a species that competes successfully when there is a high or low light supply is displaced when the light supply is intermediate. Factors such as mixing depth and background turbidity also affect the composition of phytoplankton communities. Still. our model predicts that at most two species can stably coexist in a mixed water column. Hence, the spatial heterogeneity imposed by a light gradient is nor sufficient to solve Hutchinson's paradox of the plankton.
This article is in Free Access Publication and may be downloaded using the “Download Full Text PDF” link at right.
A 3 wk cruise was conducted to investigate how the dynamics of nutrients and plankton biomass and production are coupled with the Fraser River discharge and a wind event in the Strait of Georgia estuary (B.C., Canada). The spring bloom was underway in late March and early April, 1991, in the Strait of Georgia estuary. The magnitude of the bloom was greater near the river mouth, indicating an earlier onset of the spring bloom there. A week-long wind event (wind speed >4 m s(-1)) occurred during April 3-10. The spring bloom was interrupted, with phytoplankton biomass and production being reduced and NO3 in the surface mixing layer increasing at the end of the wind event. Five days after the wind event (on April 15), NO3 concentrations were lower than they had been at the end of the wind event, indicating a utilization of NO3 during April 10-14. However, the utilized NO3 did not show up in phytoplankton biomass and production, which were lower than they had been at the end (April 9) of the wind event. During the next 4 d, April 15-18, phytoplankton biomass and production gradually increased, and NO3 concentrations in the water column decreased slowly, indicating a slow recovery of the spring bloom. Zooplankton data indicated that grazing pressure had prevented rapid accumulation of phytoplankton biomass and rapid utilization of NO3 after the wind event and during these 4 d. As a result, NH4 was generated at a rate faster than it was utilized by phytoplankton and hence, its concentrations remained at higher levels during April 15-18 than during the wind event. Also, total nitrogen in the water column decreased alter the wind event. This study presents the first set of data on daily scales to demonstrate how biological variables are coupled with physical variables in vertical profiles in the Strait of Georgia estuary and to reveal how a wind event affected the spring bloom and consequently the phasing between phytoplankton and zooplankton in the region.
The concept of critical depth, as formulated by Sverdrup (J. Cons. perm.
int. Explor. Mer. 18, 287 (1953)) to explain the onset of the spring
bloom of phytoplankton, is discussed. Simplifying assumptions (use of a
linear photosynthesis--light curve, use of daily averaged irradiance),
adopted by Sverdrup in his computation of the growth term, are removed.
An exact expression is found for the critical depth in terms of a
generalized, biomass-specific, loss term. The loss term is calculated as
the sum of contributions from algal respiration and excretion, grazing
by micro- and macro-zooplankton, and sedimentation. Critical depth is
then estimated using figures typical of the North Atlantic, and a
sensitivity analysis is done. The respiratory costs of algal growth and
metabolism dominate the generalized loss term. It is pointed out that
the Sverdrup critical depth criterion is a necessary, but not
sufficient, condition for the initiation of phytoplankton blooms. As a
diagnostic tool, its value is therefore limited. It tells only whether
net growth is possible: it tells nothing about how rapidly the
phytoplankton will increase. But from the information used to calculate
critical depth, it is possible to deduce the rate of increase in algal
biomass (or, equivalently, the net production) for the mixed layer. As
long as the growth rate is positive, the Sverdrup criterion is
respected. This leads to a characteristic timescale for the development
of a bloom. When the timescale is short relative to the time between
mixing by storms, a bloom can be expected to occur.
According to recent competition theory, the population dynamics of phy- toplankton species in monoculture can be used to make a priori predictions of the dynamics and outcome of competition for light. The species with lowest ''critical light intensity'' should be the superior light competitor. To test this theory, we ran monoculture experiments and competition experiments with two green algae (Chlorella vulgaris and Scenedesmus protuberans) and two cyanobacteria (Aphanizomenon flos-aquae and a Microcystis strain) in light-limited continuous cultures. We used the monoculture experiments to estimate the critical light intensities of the species. Scenedesmus had by far the highest critical light intensity. The critical light intensities of Chlorella, Aphanizomenon,and Microcystis were rather similar. According to observation, Aphanizomenonhad a slightly lower critical light intensity than Chlorella and Microcystis. However, according to a model fit to the mono- culture experiments, Chlorella had a slightly lower critical light intensity than Microcystis, which in turn had a slightly lower critical light intensity than Aphanizomenon.These subtle differences between observed and fitted critical light intensities could be attributed to differences in the light absorption spectra of the species. The competition experiments were all consistent with the competitive ordering of the species according to the fitted critical light intensities: Chlorella displaced all three other species, Microcystis displaced both Aphanizomenon and Scenedesmus, and Aphanizomenon only displaced Scenedesmus. Not only the final outcomes, but also the time courses of competition predicted by the theory, were in excellent agreement with the experimental results for nearly all species combi- nations.
A reformulation of Sverdrup's critical-depth calculation, using recent optical and physiological information, is developed and applied to data from the Southern Ocean. Comparisons between calculated critical depths (Z{sub c}) and mixed-layer depths (Z{sub m}) indicate that both the marginal ice zone and the open waters of the antarctic Circumpolar Current provide favorable irradiance-mixing regimes for the initiation and early development of phytoplankton blooms in summer (i.e.) Z{sub c} > Z{sub m} when phytoplankton biomass is low and the water clear; that when ice-edge blooms develop, Z{sub c} shoals to depths about equal to Z{sub m}, implying the phytoplankton standing stocks in ice-edge blooms may be self-limiting as a result of reduced penetration of irradiance; and that the highest chlorophyll levels that can be sustained in summer in open waters not stabilized by meltwater are â¼1.0 μg literâ»Â¹ in the Weddell and Scotia Seas and may be less in areas that experience stronger winds.
Phytoplankton biomass and production in lakes tend to be increased by phosphorus input and decreased by grazing or high levels of colored, dissolved organic carbon (DOC). We estimated and compared the effects of these three factors by using data from three lakes that were manipulated during 1991-1995, and data from a reference lake. Multivariate probability distributions of chlorophyll or primary production, as predicted by P input rate, DOC, and grazer length, were fit to the data. All three factors had substantial effects on chlorophyll, primary production, and their variability. Comparable reductions in the mean and variance of chlorophyll and primary production were achieved by reducing P input rate from 5 to 0.5 mg me2 d-l, increasing DOC from 5 to 17 mg C liter I, or increasing mean crustacean length from 0.2 to 0.85 mm. The negative effect of mean crustacean length (an index of size-selective predation) results from grazing by herbivorous zooplankton. The negative effect of DOC on primary producers could be explained by shading. The results suggest that natural variation in colored DOC concentrations is a major cause of variation in primary production.
An investigation is made of the global relationship between seasonal variations of the surface mixed-layer depths derived from monthly climatological hydrographic data and seasonal variations of the surface pigments from monthly satellite ocean color data. At middle and high latitudes of the western North Pacific and the North Atlantic, shallowing of the mixed-layer depth from winter to spring largely explains basin-scale features of the spring bloom of phytoplankton in terms of Sverdrup's critical depth theory. In these areas the spring bloom occurs from middle to high latitudes along with the increase of insolation from winter to spring. In the eastern North Pacific and the Southern Ocean the absence of a spring bloom is difficult to explain using the critical depth theory because Sverdrup's parameters are treated as constants, which in nature vary with physiological and ecological conditions. At northern latitudes the termination of fall bloom corresponds to a deepening in the mixed layer beyond the critical depth. Sverdrup's critical depth theory is found useful as a first step in examining the general pattern of phytoplankton seasonality.
A decade of observation in South San Francisco Bay demonstrates that estuarine phytoplankton biomass fluctuates at the time scale of days to weeks, and that much of this variability is associated with fluctuations in tidal energy. During the spring seasons of every year from 1980–1990, episodic blooms occurred in which phytoplankton biomass rose from a baseline of 2–4 mg chlorophyll a m–3, peaked at 20–40 mg chlorophyll a m–3, and then returned to baseline values, all within several weeks. Each episode of biomass increase occurred during neap tides, and each bloom decline coincided with spring tides. This suggests that daily variations in the rate of vertical mixing by tidal stirring might control phytoplankton bloom dynamics in some estuaries.
Simulation experiments with a numerical model of phytoplankton population dynamics support this hypothesis. The model incorporates biological processes (light-dependent growth, zooplankton grazing, benthic grazing) and physical processes (sinking, vertical mixing) as controls on the biomass distribution of phytoplankton in a 10-m water column. Numerical simulations indicate that phytoplankton dynamics are highly sensitive to the rate of vertical mixing (parameterized as an eddy diffusivity Kz), such that biomass increases rapidly at small Kz (5 m² d–1), but not at large Kz (50 m² d–1). Cyclic variation of Kz between 5 and 50 over a 14-d period (simulated neap-spring cycle) yields simulation results that are similar to bloom events observed in this estuary.
Coastal ocean waters tend to have very different patterns of phytoplankton biomass variability from the open ocean, and the connections between physical variability and phytoplankton bloom dynamics are less well established for these shallow systems. Predictions of biological responses to physical variability in these environments is inherently difficult because the recurrent seasonal patterns of mixing are complicated by aperiodic fluctuations in river discharge and the high-frequency components of tidal variability. We might expect, then, less predictable and more complex bloom dynamics in these shallow coastal systems compared with the open ocean. Given this complex and dynamic physical environment, can we develop a quantitative framework to define the physical regimes necessary for bloom inception, and can we identify the important mechanisms of physical-biological coupling that lead to the initiation and termination of blooms in estuaries and shallow coastal waters? Numerical modeling provides one approach to address these questions. Here we present results of simulation experiments with a refined version of Cloern's (1991) model in which mixing processes are treated more realistically to reflect the dynamic nature of turbulence generation in estuaries. We investigated several simple models for the turbulent mixing coefficient. We found that the addition of diurnal tidal variation to Cloern's model greatly reduces biomass growth indicating that variations of mixing on the time scale of hours are crucial. Furthermore, we found that for conditions representative of South San Francisco Bay, numerical simulations only allowed for bloom development when the water column was stratified and when minimal mixing was prescribed in the upper layer. Stratification, however, itself is not sufficient to ensure that a bloom will develop: minimal wind stirring is a further prerequisite to bloom development in shallow turbid estuaries with abundant populations of benthic suspension feeders.
This book is a comprehensive treatment of major advances made in the past decade in understanding of the interactions between polar oceans and the local atmosphere and ocean system. Included are 38 papers discussing the circulation, dynamics and convective processes occurring in the polar oceans; its carbon cycle chemistry and biology; the paleooceanography and paleoclimate of the polar regions; the interaction between the polar ocean and the global climate, and a variety of strategies for detection of climate change in polar regions, predominantly Arctic.
: With the eutrophication of many freshwaters and coastal environments, phytoplankton blooms have become a common phe-nomenon. This article uses a reaction-diffusion model to investigate the implications of mixing processes for the dynamics and species composition of phytoplankton blooms. The model identifies four key parameters for bloom development: incident light intensity, back-ground turbidity, water column depth, and turbulent mixing rates. The model predicts that the turbulent mixing rate is a major deter-minant of the species composition of phytoplankton blooms. In well-mixed environments, the species with lowest "critical light intensity" should become dominant. But at low mixing rates, the species with lowest critical light intensity is displaced if other species obtain a better position in the light gradient. Instead of a gradual change in species composition, the model predicts steep transitions between the dominance regions of the various species. The model predicts a low species diversity: phytoplankton blooms in eutrophic environ-ments should be dominated by one or a few species only. The model predictions are consistent with laboratory competition experiments and many existing field data. We recommend examining competition in phytoplankton blooms under well-controlled laboratory condi-tions, and we derive scaling rules that facilitate translation from the laboratory to the field.
Self-shading of light by algae growing in a column of water plays an important role in the dynamics of algal blooms. Thus without self-shading the algal concentration would increase more rapidly, making the nutrient limitation too strong. Apart from the practical importance of self-shading, its inherent nonlinearity in the growth dynamics leads to an interesting mathematical problem, which warrants detailed analytical investigation. Our mathematical model for the self-shading effect includes vertical diffusion, algal settling, gross production, and collective losses of algae. Steady-state solutions of the model equation are investigated in detail by the phase plane method, and their stability examined. Finally we discuss the vertical profile of algal concentration.
THE spring phytoplankton bloom in temperate and boreal waters represents a pulsed source of organic carbon that is important to ecosystem productivity1 and carbon flux2 in the world ocean. It is widely accepted that the seasonal development of a thermocline, in combination with increasing solar elevation in spring, is requisite for the development of the bloom in shelf and open ocean environments3-7. Here we report results for the offshore waters of the Gulf of Maine which suggest that the spring bloom can precede the onset of vertical water column stability, and may even be a contributing factor in the development of the thermocline. Deep penetration of light in relatively clear, late-winter waters, and weak, or absent, wind-driven vertical mixing, appear to support cell growth rates that overcome the vertical excursion rates in the neutrally stable water column, leading to a bloom. Phytoplankton forms typical of a spring bloom, including gelatinous colonies and chains, may contribute to the cells' ab
1-D and 3-D models of plankton production in the Barents Sea are described and a few simulations presented. The 1-D model has two compartments for phytoplankton (diatoms and P. pouchelii), three for limiting nutrients (nitrate, ammonia and silicic acid), and one compartment called “sinking phytoplankton”. This model is coupled to a submodel of the important herbivores in the area and calculates the vertical distribution in a water column. Simulations with the 3-D model indicate a total annual primary production of 90-120g C m?2 yr?1 in Atlantic Water and 20-50g C m?2 yr?1 in Arctic Water, depending on the persistence of the ice cover during the summer. The 3-D model takes current velocities, vertical mixing, ice cover, and temperature from a 3-D hydrodynamical model. Input data are atmospheric wind, solar radiation, and sensible as well as latent heat flux for the year 1983. The model produces a dynamic picture of the spatial distribution of phytoplankton throughout the spring and summer. Integrated primary production from March to July indicates that the most productive area is Spitsbcrgenbanken and the western entrance to the Barents Sea. i.e. on the northern slope of Tromsøflaket.
The current status of the Sverdrup theory for the initiation of plankton blooms is examined. A prescription is given for the computation of the Sverdrup critical depth, using recently-published algorithms for mixed-layer primary production and a generalised loss term. Using no further information, the intrinsic rate of increase of phytoplankton biomass in the mixed layer can also be found. This rate, compared against the local frequency of storm occurrence, provides an alternative criterion for the initiation of blooms. The Eulerian (bulk property) methods used to derive these results are contrasted with the Lagrangian Ensemble method. The Lagrangian approach provides one avenue to the elaboration of the Sverdrup criterion to include the effect of processes with characteristic timescales small compared to one day. The incidence of blooms in the apparent absence of vertical stratification is reviewed: it is concluded that these observations do not undermine the basic logic of the Sverdrup theory. However, they do provoke interest in a re-examination of the feedbacks between the physical and biological dynamics in the mixed layer: an example is given. Finally, suggestions are made for further work in this subject area.
The new edition of this widely respected text provides comprehensive and up-to-date coverage of the effects of biological-physical interactions in the oceans from the microscopic to the global scale. considers the influence of physical forcing on biological processes in a wide range of marine habitats including coastal estuaries, shelf-break fronts, major ocean gyres, coral reefs, coastal upwelling areas, and the equatorial upwelling system investigates recent significant developments in this rapidly advancing field includes new research suggesting that long-term variability in the global atmospheric circulation affects the circulation of ocean basins, which in turn brings about major changes in fish stocks. This discovery opens up the exciting possibility of being able to predict major changes in global fish stocks written in an accessible, lucid style, this textbook is essential reading for upper-level undergraduates and graduate students studying marine ecology and biological oceanography.
Preface 1. What is phytoplankton? 2. Mechanisms of suspension 3. Spatial and temporal distribution of phytoplankton 4. Photosynthetic activity of phytoplankton 5. Nutrients 6. Growth and survival 7. Loss process 8. Periodicity and change in phytoplankton composition Glossary and symbols References Index to lakes and rivers Index to genera/species General index.
A growing body of laboratory, field, and theoretical work suggests that the dynamics of harmful algal blooms and their impacts on other organisms are frequently controlled not only by physiological responses to local environmental conditions as modified by trophic interactions, but also by a series of interactions between biological and physical processes occurring over an extremely broad range of temporal and spatial scales. All too frequently, major gaps in our ability to identify, measure, and model the underlying biological and physical processes and their interactions over the appropiate temporal and spatial scales have prevented the quantitative assessment of the importance of these factors in causing past blooms and the development of predictive models of bloom dynamics and impacts. For these reasons, we have combined fluid continuity equations with a conservation equation for population dynamics to quantify how biological and physical processes and their interactions affect the population dynamics of harmful algae and their potential impact on other organisms. Applications of the resulting numerical and conceptual models to toxic algal blooms in upwelling systems and pycnocline layers suggest not only that bloom dynamics and impacts are sensitive to biological-physical interactions occurring at multiple scales, but also that such interactions may be critical components of the life-history strategies of these organisms.
Profiles of temperature-gradient microstructure are used to define the size and location of mixing regions, the intensity of turbulence, and the potential exposure of phytoplankton to fluctuating irradiance in a shallow, turbid, productive lake. The part of the water column which was mixing tended to be subdivided into two regions with different dynamics, one in which the turbulence was active and one in which it was constrained by buoyancy. Generally the upper layer, which ranged from 0.3 to 1.5 m deep, was actively mixing. Energy dissipation rates were on the order of lo- 7 m2 s-~, vertical eddy diffusivities ranged from 1O-3 to 1 Op5 m2 s- I, and overturns mixed on a time scale of minutes. Phytoplankton could become well mixed before turbulent transport within overturns ceased and, while the wind persisted, were likely to experience continuous fluctuations in irradiance. In one of the largest overturns, phytoplankton could circulate between the 90% light level and the 5% light level in 3-4 min. Where buoyancy affected turbulence, energy dissipation rates ranged from 1O-9 to 1O-7 m2 ss3 and vertical eddy diffusivities from 1O-7 to 1 O-4 m2 s- ' . Mixing times based on these diffusivities exceeded r/N, the time scale for turbulent transport, indicating overturns would mix only partially. Phytoplankton could still experience large fluctuations in irradiance, but the fluctuations probably were not continuous.
A dense Synechococcus population has been observed in oceanic offshore waters (400 km from the Mauritanian coast) within the upper 30-m-thick layer. With cell counts >3 X 10" cells ml I, about half (0.6) of the total Chl a concentration (1.25 mg m-l) was attributable to the cyanobacteria. The OF tical properties of this water body mark- edly differed from those of case 1 waters with the same Chl a concentration. The euphotic depth was diminished (by - IO m on average) and the spectral attenuation coefficients for downwelling irradiance were increased (by 40- 60% in the green and blue parts of the spectrum); the absorption band characteristic of phycoerythrin was easily detected on filtered material, as well as within the in-water light field and in the water-leaving upward radiant flux. The amplitude of the departures of the actual optical properties from thos: predicted for normal case I waters are described and then analyzed through radiative transfer computations. The peculiar Chl-specific absorption capabil- ities of Synechococcus (originating from the small size and the presence of phycobilins and zeaxanthin), rather than a significant increase in the bulk scattering properties, are at the origin of the deviations observed. The possibility of detecting such a particular algal bloom, through remote-sensing techniques based on the ocean reflectance, requires a high radiometric sensitivity and dedicated spectral channels. If detection is feasible, quantifying the cyanobacterial biomass from satellite information remains a difficult task. The variability in the relative proportions of phycobilins and Chl a for these organisms impedes a quantitative assessement of their contribution to the total Chl biomass.
We have utilized data from a recently developed three-dimensional velocity fluctuation meter to compute the dissipation of turbulent kinetic energies (TRE) and the intensity of turbulent mixing in horizontal and vertical planes in the pelagic, epilimnic water of Lake Kinneret, Israel. These characteristics of wind-induced turbulent movement have been monitored from January 1992 through December 1996. The turbulence parameters were strongly correlated to wind energy inputs, calculated daily as 5 day cumulative inputs. There have been dramatic changes in the annual and seasonal development of phytoplankton, together with unusually high levels of primary production in this lake since 1994. We observed different patterns of vertical and horizontal turbulent movement and of TKE dissipation rates during the years when 'unusual' phytoplankton development occurred (1994-1996) compared to 'normal' years (1992, 1993). The first appearance of the filamentous cyanobacterium Aphanizomenon in this lake in August-September 1994 coincided with a period of markedly lower rates of TKE dispersion and a shift from vertical to horizontal dominance of the turbulent eddy spins. The absence of a regular winter-spring bloom of the dinoflagellate, Peridinium, in 1996 occurred when dissipation rates of TKE were extremely high, while record high amounts of dinoflagellates (1994, 1995) appeared when dissipation rates were very low. Correlations were shown between phytoplankton parameters (chlorophyll, primary production and the ratio of primary production to chlorophyll) and both the dissipation rate of TKE and the intensity of water turbulent mixing in the vertical plane. We suggest that the changes in the 'turbulence climate' of Lake Kinneret were an important factor in determining shifts in phytoplankton succession and the population composition of the algal assemblage.
In this paper a general model for growth and competition in a light gradient is developed. The model is based on a few qualitative assumptions: (i) biomass is continuously distributed over depth; (ii) the light gradient is one-dimensional and uni-directional; (iii) photosynthesis is positively related to the local light intensity; and (iv) biomass growth is governed by the carbon balance. By introducing the concept of "quantum return", it is shown that growth can be quantified directly in terms of the light gradient. In monoculture, growth leads to a globally stable equilibrium, at which the light intensity at the bottom of the light gradient is reduced to a "critical light intensity" I*out, I*out is not affected by the background turbidity but negatively related to the light supply. When all species are similarly distributed over the light gradient, the outcome of competition can be inferred from this monoculture characteristic: the species with lowest I*out will competitively exclude all other species. In contrast, spatial differentiation of the competitors may lead to a completely different situation: several species may co-exist, and the species with lowest I*out may be competitively displaced by species with a better position in the light gradient.
The dependence of phytoplankton photosynthesis on light intensity may be altered by the range and frequency of variations in light intensity recentlv experienced by the organisms. A major source of the fluctuations in light intensity experienced by phytoplankton in the upper ocean is vertical motion. We estimate time and space scales for \Tertical displacements of phytoplankton caused by turbulent mixing, internal waves, Langmuir circulations, and double diffusive processes. In the surface layer, depending on windspeed, current shear and stratification, we find that time scales for cycling of phytoplankton by turbulent eddies and mixing vary from about 0.5 h to hundreds of hours for vertical displacements of the order of 10 m. In the seasonal thermocline, turbulent diffusive time scales for displacements as small as several meters are weeks to months, whereas similar displacements by internal waves occur over periods of several minutes to several hours, according to the strength of the density stratification, and are then dominant. Langmuir cells seem to scale as the large turbulent eddies and need not be treated separately, and double diffusive processes seem to be of minor importance. The formulation used here of a vertical turbulent diffusion coefficient K, as a function of observable quantities-e the rate of dissipation of turbulent kinetic energy, and N the local buoyancy frequency-should also be
The dynamics of phytoplankton blooms were studied during spring 1992 in two typical coastal ecosystems of Western Europe, which differ in the influence of river discharges on the vertical stratification of the water column. The Bay of Brest is a semi-enclosed ecosystem, which is connected with the adjacent ocean and entered by two nutrient-rich rivers. The Western English Channel is an open ocean situation and the studied area was remote from any significant riverine influence during spring. Both areas are macrotidal environments and are usually considered as well-mixed. The beginning of the annual diatom bloom is delayed until late May in the Channel, in comparison with the Bay of Brest where the bloom starts by early April. Water column stability induced by freshwater runoff, and local topography are responsible for the earlier start of the phytoplankton bloom in the Bay of Brest. In both areas, the spring period is marked by a succession of diatom blooms which is strongly dependent upon the spring–neap tidal cycle. However, blooms develop under opposite mixing regimes: they occur during neap tides in the Bay of Brest and during spring tides in the Channel. In the Channel where light is not a limiting factor, increased mixing during spring tides enables nutrient replenishment from the water–sediment interface and phytoplankton responds immediately after nutrients have been renewed in the water column. In the more turbid waters of the Bay of Brest, relaxation of vertical mixing during neap tides is required before phytoplankton is able to utilize nutrients originating from freshwater inputs orin situregeneration later in the season.
The effect of simultaneous N2 fixation and light limitation on the growth of two strains of Anabaena sp. Bory de St. Vincent and Aphanizomenon flos-aquae (L.) Ralfs was investigated using continuous cultures. Under severely light-limited conditions, Aphanizomenon showed a broader absorption spectrum (due to the presence of phycoerythrin), a higher maximum efficiency of photosynthesis, a higher steady-state N2 fixation activity and a higher growth affinity for light than did Anabaena. On the other hand, under light saturation, Anabaena showed a higher maximum rate of O2 production and a higher maximum specific growth rate than Aphanizomenon. These monoculture results characterize Anabaena and Aphanizomenon, in relative terms, as a ‘sun’ and a ‘shade’ species respectively, and are in accordance with field observations. The difference between the two species in their acclimatory response is discussed in terms of a species-specific alteration of the PSI∶PSII stoichiometry. Besides the species-specific modulation of the accessory pigments, such an acclimation would provide a biochemical basis for the observed physiological differences. The monoculture results were used to differentiate the niches of the two species and suggested that Aphanizomenon would competitively displace Anabaena under N2-fixing, light-limited conditions. However, when both species were grown together, Anabaena became dominant and seemed to be the superior competitor for light. In order to explain this finding, the possible effects of release of allelopathic compounds, or dynamic aspects of light supply, are discussed.
1-D and 3-D models of plankton production in the Barents Sea are described and a few simulations presented. The 1-D model has two compartments for phytoplankton (diatoms and P. pouchelii), three for limiting nutrients (nitrate, ammonia and silicic acid), and one compartment called “sinking phytoplankton”. This model is coupled to a submodel of the important herbivores in the area and calculates the vertical distribution in a water column. Simulations with the 3-D model indicate a total annual primary production of 90-120g C m−2 yr−1 in Atlantic Water and 20-50g C m−2 yr−1 in Arctic Water, depending on the persistence of the ice cover during the summer.The 3-D model takes current velocities, vertical mixing, ice cover, and temperature from a 3-D hydrodynamical model. Input data are atmospheric wind, solar radiation, and sensible as well as latent heat flux for the year 1983. The model produces a dynamic picture of the spatial distribution of phytoplankton throughout the spring and summer. Integrated primary production from March to July indicates that the most productive area is Spitsbcrgenbanken and the western entrance to the Barents Sea. i.e. on the northern slope of Tromsøflaket.
1. Artificial mixing in the hypertrophic Lake Nieuwe Meer was successful in preventing blooms of the cyanobacterium Microcystis. During the 2 years of artificial, deep mixing the number of colonies of Microcystis per litre and also per m2 was lower than in the two preceding control years. Hardly any nuisance scums of Microcystis occurred in the lake.
2. The phytoplankton shifted from a cyanobacteria-dominated community in summer to a mixed community of flagellates, green algae and diatoms. Reduced sedimentation losses in the mixed lake, probably in combination with a lower pH, favoured non-buoyant algae, while the entrainment of cyanobacteria in the turbulent flow nullified their advantage of buoyancy.
3. The chlorophyll concentrations were much lower in the mixed lake, but the euphotic depth did not show clear differences between the years. The chlorophyll content integrated through depth (m–2) increased in the artificially mixed lake.
4. The deep lake normally stratified in summer, but artificial mixing of the lake in 1993 resulted in a homogeneous temperature and oxygen distribution with depth. In spring 1994, the mixing was applied intermittently with a reduction of 75% of the energy costs, while the mixing was still sufficient to prevent stratification.
5. Determination of the buoyancy state of the colonies on a sunny and calm day showed that the buoyancy loss was low close to the bubble plumes, and high at some distance from these plumes. This suggests that Microcystis could escape the mixing at some distance from the plumes, and could synthesize more carbohydrates during its stay in the upper illuminated layer of the lake than the deep mixed colonies close to the bubble plumes. Determination of the buoyancy state appeared to be a good and simple method to investigate the extent of entrainment of colonies in the turbulent flow.
We consider a nonlinear diffusion equation proposed by Shigesada and Okubo which describes phytoplankton growth dynamics with a selfs-hading effect.
We show that the following alternative holds: Either (i) the trivial stationary solution which vanishes everywhere is a unique stationary solution and is globally stable, or (ii) the trivial solution is unstable and there exists a unique positive stationary solution which is globally stable. A criterion for the existence of positive stationary solutions is stated in terms of three parameters included in the equation.
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