No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Climate change impeded the re-oligotrophication of the Saidenbach Reservoir: Re-oligotrophication of the Saidenbach Reservoir
Abstract
Regular ecological investigations usually on a fortnightly basis have been carried out at the Saidenbach Reservoir (SE Germany) and its tributaries since 1975. The data show a sudden decrease in the phosphorus import of over 60% in 1990, resulting in the abrupt change of the trophic state from eutrophic to weakly mesotrophic. Contrary to expectations, the average annual phytoplankton abundance did not decrease but almost doubled on average over the years after 1990. This was primarily due to mostly warmer winters after 1990 causing longer spring overturns, which minimized sedimentation losses, enabled better utilization of the phosphorus reserves, and resulted in higher yields of the diatom-dominated phytoplankton spring mass development. In the summers after 1990, the mass growth of the diatom Fragilaria crotonensis, which in the past used to dominate in this season and effectively transported phosphorus to the sediment by settling, was considerably reduced because of lower P supply and stronger stratification. However, the decrease in the biomass of Fragilaria was overcompensated by the increased abundance of cyanobacteria. They benefitted not only from declining phosphorus competition by Fragilaria but also from higher thermal stability and temperature in the epilimnion. Hence, even higher summer biomasses were observed despite considerably lowered phosphorus import. The analysis of the long-term dataset clearly illustrates the deciding impact of hydrophysical factors on the phytoplankton growth, also under nutrient deficient conditions. The altered mixing and stratification pattern caused by climate change did not only prevent the re-oligotrophication of the reservoir but even enhanced the phytoplankton production. It seems that global warming modifies the interplay between physical and nutrient limitation mechanisms and the limits and models used in the past to classify trophic-state levels have to be verified. The study shows the enormous significance and indispensability of uninterrupted ecological long-term datasets, including reliable data of the ecosystem's organismic structure, for research about the consequences of climate change.
... Die Veränderungen sind progressiv fortschreitend und lassen sich mit vorhandenen Messdaten klar nachvollziehen [3]. Die beschriebenen Auswirkungen beeinträchtigen die Wassergüte und zeigen Parallelen zur großräumigen anthropogen verursachten Eutrophierung der Gewässer in den 1970er und 1980er Jahren [4]. Von einer erhöhten Wassertemperatur, Schichtungsdauer und Schichtungsstabilität profitieren vor allem Cyanobakterien (Blaualgen), von denen einige toxisch sind [5]. ...
... Die Größenordnungen der erzielten Effekte entsprechen dabei im Wesentlichen den Auswirkungen der Klimaerwärmung in den letzten 30 Jahren. Dies zeigt sich sowohl bei den drei realen Talsperren als auch im Falle der idealisierten Tal [4,24]. Dabei sind für die Trinkwassergewinnung die Verlängerung der Sommerstagnation und der Anstieg der hypolimnischen Wassertemperatur besonders problematisch. ...
... In der Literatur beschriebenen Studien legen aber nahe, dass sich Änderungen von Schichtung und Temperatur auch auf die Wassergüte auswirken. Eine stärkere Schichtung kann zu anoxischen Bedingungen im Hypolimnion führen [25], zusätzlich führen Änderungen von Temperatur und Schichtung zu Verschiebungen in der Phytoplanktongemeinschaft [4,26]. ...
Die globale Klimaerwärmung führt in Talsperren zu einem Anstieg
der Wassertemperatur und einer Verlängerung der sommerlichen
Schichtung, wodurch einerseits biogeochemische Prozesse und damit die Wasserqualität beeinflusst werden. Andererseits kann die thermische Struktur von Talsperren durch Anpassung der Entnahmetiefe aktiv gesteuert werden. Anhand von Messdaten und mit Hilfe von hydrophysikalischen Modellen haben wir die Auswirkungen der Klimaerwärmung auf Temperatur und Schichtung von Talsperren sowie mögliche Anpassungsstrategien
untersucht. Die Wassertemperatur im Tiefenwasser und das Ende der sommerlichen Schichtung können durch das Entnahmeregime gesteuert werden. Die Simulationen zeigen, dass eine Steuerung der Entnahmetiefen den Klimaerwärmungseffekt der letzten 30 Jahre bisher teilweise kompensieren konnte.
... mussels 7 ), the exclusion of food-web top species (e.g. cyprinid fish 8 ) and the impact of fungal epidemics 9 and/ or atmospheric depositions 10 . ...
... One of the main factors believed to act on long-term phytoplankton dynamics is global warming, but its effects are not always as expected 11 . For example, phytoplankton biomass has been shown to decrease with global warming 12 , or in other instances its effects are counterbalanced by other proccesses such as reoligotrophication, resulting in resilient phytoplankton structures 10,13 . There are some studies ( Table 1) that deal with long-term biomass responses to local environments, including local warming as one of the drivers 5,8 . ...
... There are some studies ( Table 1) that deal with long-term biomass responses to local environments, including local warming as one of the drivers 5,8 . Others also try to link global warming and phytoplankton, using linear relationships of the latter with water temperature to prove this 10,13 . Many studies have looked for teleconnections 11 (i.e. ...
Faced with an environment of accelerated change, the long-term dynamics of biotic communities can be approached to build a consistent and causal picture of the communities' life. We have undertaken a 25-year monthly-sampling study on the phytoplankton of a meso-oligotrophic lake, paying attention to controlling factors of overall biomass (TB) and taxonomical group biomass (TGBs). Long-term series included decreased trends of TB and TGBs, and multi-scale periodicity. A decadal TB periodicity emerged related to nitrogen concentration and Cryptophytes. Annual periodicities were mainly related to air and water temperature controlling the abundance of Chlorophytes or Dinoflagellates. Intra-annual cycles could arise from autogenic processes. The analysis by periods revealed relevant dynamics (for example, Diatom periodicities), hidden in the analysis of the complete series. These results allow us to establish that: i) two organizational levels of phytoplankton change differently in time scales from months to decades; ii) controlling factors (climate, water physics and chemistry) act at different time scales and on different TGBs, and iii) different combinations of the "taxonomical group-control factor-trend and periodicity" set throughout the studied time explain total biomass dynamics. A holistic approach (multiple complementary analyses) is necessary to disentangle the different actors and relationships that explain non-stationary long-term phytoplankton dynamics.
... Phytoplankton biomass showed no response to reduced nutrient loadings in four of the study lakes (Damhussøen, Denmark; Bryrup, Denmark; Maggiore, Italy; V€ attern, Sweden), while an increase in biomass was even observed in three lakes (Võrtsj€ arv, Estonia; Peipsi, Estonia; Tystrup, Denmark). In addition, an increase in algal biomass after oligotrophication was evident in Lake Geneva, Switzerland (Anneville & Pelletier, 2000;Tadonleke, Lazzarotto, Anneville, & Druart, 2009), in Sweden's largest lake, Lake V€ anern (Weyhenmeyer & Broberg, 2014) and in Germany's Saidenbach Reservoir (Horn, Paul, Horn, Uhlmann, & R€ oske, 2015). These findings are in conflict with the assumption that biomass forms a linear relationship with phosphorus, and that is, the yield of biomass per unit phosphorus should be constant (representing the slope of this relationship) and independent of the absolute nutrient concentrations (Vollenweider, 1971). ...
... Diatoms suffer from higher sinking velocities compared to other taxa and are therefore very efficient in removing nutrients from the photic zone (Reynolds, 2006;Sommer, 1984). Hence, when diatom biomass decreases, less nutrients are removed from the epilimnion by sedimenting algal cells, and consequently, more nutrients stay available for the summer period (Benndorf, 1968;Frassl, Rothhaupt, & Rinke, 2014;Horn et al., 2015). A more even distribution of resources over the entire growing season results in biomass also being more evenly distributed. ...
... As a result of both traits, diatom cells sink out of the photic zone before they can be remineralised and therefore lead to high nutrient losses by sedimentation. Empirical evidence for the effect of sinking algae on phosphorus concentrations is, for example, also given by Benndorf (1968) and Horn et al. (2015). Due to the decreasing dominance of diatoms during oligotrophication, the P removal by sedimentation in the Rappbode Reservoir has most probably diminished. ...
To counteract the severe consequences of eutrophication on water quality and ecosystem health, nutrient inputs have been reduced in many lakes and reservoirs during the last decades. Contrary to expectations, in some lakes phytoplankton biomass did not decrease in response to oligotrophication (nutrient reduction). The underlying mechanisms preventing a decrease in biomass in these lakes are the subject of ongoing discussion.
We used a hitherto unpublished long‐term data set ranging from 1961 until 2016 from a German drinking water reservoir (Rappbode Reservoir) to investigate the underlying mechanisms preventing a decrease in biomass. Total phosphorus (TP) concentrations in the Rappbode Reservoir dropped abruptly in 1990 from 0.163 to 0.027 mg/L within three consecutive years, as a result of banning phosphate‐containing detergents. Despite substantial reductions in TP, total annual phytoplankton biomass did not decline in the long‐run, and therefore, the yield of total phytoplankton biomass per unit phosphorus largely increased.
Regression analysis revealed a positive association between the yield and potentially phagotrophic mixotrophs (R2 = .465, p < .001). We infer that by ingesting bacteria, mixotrophic species were capable of exploiting additional P sources that are not accessible to obligate autotrophic phytoplankton, eventually preventing a decrease in algal biomass after TP reductions.
Long‐term epilimnetic phosphorus concentrations during the winter mixing period decreased to a greater degree than summer phosphorus concentrations. Apparently, TP losses over the season were less intense. Spring diatom biomass also markedly decreased after oligotrophication. In fact, spring diatom biomass was positively related to the TP loss over the season suggesting diatoms play an important role in P reduction. However, this intraannual P processing was not the primary factor when focusing on the average yearly yield, which remained to be fully explained by mixotrophs.
Our study demonstrates this ecosystem's ability to compensate for changes in resource availability through changes in phytoplankton community composition and functional strategies. We conclude that an increase in mixotrophy and the ability to make bacterial phosphorus available for phytoplankters were the main factors that allowed the phytoplankton community of the Rappbode Reservoir to adapt to lower nutrient levels without a loss in total biomass.
... In particular, the altered stability and duration of stratification might trigger changes in phytoplankton composition and dynamics [51]. Especially decreased stability, stratification duration, and lower temperatures can weaken the otherwise competitive advantage of cyanobacteria in dimictic systems [52,53]. Likewise, hypolimnetic oxygen might be stabilized at higher levels as the stability and duration of stratification is reduced [49,54]. ...
... We did not consider biological or chemical implications, but literature suggests that changes in stratification and temperature can affect these two. Increased stratification can lead to anoxic conditions in the hypolimnion [50], and changes in temperature and stratification can lead to shifts in the phytoplankton community [51,52]. Reservoir managers thus also need to consider possible consequences on water quality when adapting withdrawal strategies. ...
Background
Climate change induced a rise in surface water temperature and a prolongation of summer stratification in drinking water reservoirs. Stratification and temperature are important factors for drinking water production because they influence bio-geo-chemical processes and thus affect water quality. Most drinking water reservoirs have outlet structures that allow water to be withdrawn from different depths at variable rates. The thermal structure of these reservoirs can thus be managed actively by means of dynamic withdrawal schemes.
Results
We employed the hydro-physical General Lake Model to simulate the effects of different withdrawal strategies on temperatures and stratification in three German reservoirs. In particular, we assessed the potential of depth- and time-variable withdrawal to mitigate the impacts of climate change. We found that deep water temperatures (25 m below surface) and the end of summer stagnation are strongly controlled by the withdrawal regime. Specifically, the simulated impact of the withdrawal scheme was of the same order of magnitude as the observed impact of climate change over the last 30 years. However, the end of ice cover, the onset of summer stagnation, and near-surface temperatures (3 m depth) were rather insensitive to altered withdrawal strategies.
Conclusions
Our results suggest that an adaption of withdrawal depth and timing will partly compensate for the effects of climate change. Dynamic withdrawal should thus be considered as an integral part of future reservoir management strategies.
... The quality of water, and aquatic environments generally, are influenced by climate change, but these relationships are complicated in part due to variations of associated meteorological factors, including precipitation, temperature, radiation, and wind speed/direction. For example, changes in precipitation frequency and intensity can directly impact the input of nonpoint source pollutants to lakes by regulating surface runoff (Prowse et al., 2006;Karim and Mimura, 2008;Horn et al., 2015). Also, global warming impacts the thermal characteristics of lakes, changing water temperatures, which can extend stratification periods, decreasing the depths of mixed layers and thermoclines, which can the reduce convective mixing (Fan and Kao, 2008;Kraemer et al., 2015;Ma et al., 2015). ...
... O'Beirne et al. (2017) showed that global warming caused an increase in surface water temperatures and longer ice-free periods, generating longer seasonal stratification, resulting in a rapid increase in primary productivity in Lake Superior, USA. Climate change has facilitated salinization, eutrophication, shrinkage, and even the total desiccation of some lakes (Hayes et al., 2015;Horn et al., 2015;Wu et al., 2017), and so it is urgent to conduct research on the impacts of climate change on lacustrine environments. ...
Information on both the climate change and anthropogenic activities on lacustrine ecosystem is of crucial importance for understanding the current state and future development of lake systems. The sediment profiles of arsenic, mercury, other metals, and nutrients were used to investigate climate change and anthropogenic activities impacts on three lakes located on the Yunnan-Guizhou Plateau (Lake Chenghai, Qionghai) and Northeastern Plain region (Lake Jingpohu) of China. The enrichment factor (EF), geoaccumulation index (Igeo) and anthropogenic factor (AF) were used to assess the enrichment degree of metals. The results show that these lakes have been progressively eutrophied since the development of widespread industrialization and urbanization in these areas. The enrichment of heavy metals is generally not serious (EF < 1.5, Igeo < 0), except for Cd, Pb, and Hg in Lakes Chenghai and Qionghai. Correlation analysis shows that generally, the heavy metals characterized had significant correlations with nutrient concentrations (TOC, δ¹³C, TP), which implied the establishment of geochemical associations during transport, that they had similar anthropogenic sources (such as fertilizers), or both. Cluster analysis grouped nutrients, As, and most other metals (except Ca, Mg, Fe, Al), the annual average temperature, and annual precipitation into one category. Increases in both average annual air temperatures and total precipitation are likely influencing the input of heavy metals and nutrients to these lakes.
... The quality of water, and aquatic environments generally, are influenced by climate change, but these relationships are complicated in part due to variations of associated meteorological factors, including precipitation, temperature, radiation, and wind speed/direction. For example, changes in precipitation frequency and intensity can directly impact the input of nonpoint source pollutants to lakes by regulating surface runoff (Prowse et al., 2006;Karim and Mimura, 2008;Horn et al., 2015). Also, global warming impacts the thermal characteristics of lakes, changing water temperatures, which can extend stratification periods, decreasing the depths of mixed layers and thermoclines, which can the reduce convective mixing (Fan and Kao, 2008;Kraemer et al., 2015;Ma et al., 2015). ...
... O'Beirne et al. (2017) showed that global warming caused an increase in surface water temperatures and longer ice-free periods, generating longer seasonal stratification, resulting in a rapid increase in primary productivity in Lake Superior, USA. Climate change has facilitated salinization, eutrophication, shrinkage, and even the total desiccation of some lakes (Hayes et al., 2015;Horn et al., 2015;Wu et al., 2017), and so it is urgent to conduct research on the impacts of climate change on lacustrine environments. ...
... In other instances, its effects are counterbalanced by other processes, such as re-oligotrophication, resulting in resilient algal structures. (67). ...
... Lake warming indeed played a role, a recent study shows that internal loading and lake warming explains 68% and 32% of the blooms respectively (Kong et al., 2023). The self-amplifying mechanisms between high temperature and low redox conditions are multiple: (i) high temperature accelerate oxygen depletion and hence rapidly induce low redox conditions, (ii) high temperatures also amplify mineralization (Gudasz et al., 2010) and thus support nutrient recycling, (iii) high temperatures induce a shift towards cyanobacteria (Paerl and Huisman 2008) that have a high potential for bloom-formation and such a shift comes along with low phosphorus sedimentation (Horn et al., 2015). Once a lake becomes eutrophic, as what happened to Lake Barleber in 2016, there are also self-stabilizing mechanisms at play given that the higher algal biomasses lead to higher sediment oxygen demand and a phosphorus accumulation at the sediment surface. ...
Aluminum salts are widely used to immobilize phosphorus (P) in lakes suffering from internal loading. However, longevity of treatments varies among lakes; some lakes eutrophy faster than others. We conducted biogeochemical investigations of sediments of a closed artificial Lake Barleber, Germany that was successfully remediated with aluminum sulfate in 1986. The lake became mesotrophic for almost 30 years; a rather rapid re-eutrophication took place in 2016 leading to massive cyanobacterial blooms. We quantified internal loading from sediment and analyzed two environmental factors that might have contributed to the sudden shift in trophic state. Increase in lake P concentration started in 2016, reaching 0.3 mg L-1, and remained elevated into the spring of 2018. Reducible P fraction in the sediment was 37 - 58% of total P, indicating a high potential for mobilization of benthic P during anoxia. Estimated P release from sediments for 2017 was approximately 600 kg for the whole lake. This is consistent with sediment incubation results; higher temperature (20°C) and anoxia contributed to release of P (27.9 ± 7.1 mg m-2 d-1, 0.94 ± 0.23 mmol m-2 d-1) to the lake, triggering re-eutrophication. Loss of aluminum P adsorption capacity together with anoxia and high water temperatures (organic matter mineralization) are major drivers of re-eutrophication. Accordingly, treated lakes at some time require a repeated aluminum treatment for sustaining acceptable water quality and we recommend regular sediment monitoring in treated lakes. This is crucial given the effects of climate warming on duration of stratification in lakes which may result in the need for treatment of many lakes.
... Changes in lake physics also impact lake ecology at different levels from phytoplankton community composition (Rühland et al. 2015) to trophic interactions due to warming in sensitive periods like winter or onset of stratification (Wagner et al. 2013). Changes in lake mixing can lead to increases of phytoplankton biomass, even in lakes where the nutrient loading is decreasing (see, e.g., Horn et al. 2015;Swann et al. 2020;Mesman et al. 2021). This effect can be caused by increased internal nutrient recycling and shifts in the phytoplankton community (bottom-up) or by predator-prey interaction (top-down, Anneville et al. 2019). ...
Water temperature, ice cover, and lake stratification are important physical properties of lakes and reservoirs that control mixing as well as bio-geo-chemical processes and thus influence the water quality. We used an ensemble of vertical one-dimensional hydrodynamic lake models driven with regional climate projections to calculate water temperature, stratification, and ice cover under the A1B emission scenario for the German drinking water reservoir Lichtenberg. We used an analysis of variance method to estimate the contributions of the considered sources of uncertainty on the ensemble output. For all simulated variables, epistemic uncertainty, which is related to the model structure, is the dominant source throughout the simulation period. Nonetheless, the calculated trends are coherent among the five models and in line with historical observations. The ensemble predicts an increase in surface water temperature of 0.34 K per decade, a lengthening of the summer stratification of 3.2 days per decade, as well as decreased probabilities of the occurrence of ice cover and winter inverse stratification by 2100. These expected changes are likely to influence the water quality of the reservoir. Similar trends are to be expected in other reservoirs and lakes in comparable regions.
... Anticipated climate change will inevitably impact fish communities (this study, [6]) and exacerbate existing stressors [11,14,16,55,76], with cascading effects on ecosystem functioning and services like shifts in body size and biomass fluxes, decoupling of food web interactions or even partial collapses of food webs when predators decline [77][78][79][80]. Besides functioning, ecosystem services will as well face climate induced changes. ...
To improve the robustness of projections of freshwater fish distributions under climate change, species distribution models (SDMs) were calculated for six fish species in southwestern Germany with different ecological requirements along an upstream-downstream gradient in a multi-general circulation model (GCM) approach. Using the maximum entropy (Maxent) algorithm and a high number of occurrence records (N = 4684), species distributions were projected to future climate conditions derived from 13 GCMs across the most likely representative carbon pathways (RCP4.5 and 8.5) and two time spans (near future 2050, and far future 2070), resulting in 104 distribution maps per species that were then used for the statistical analysis of future trends. Climate change is likely to affect the distribution of four of the six fish species. The potential ranges of salmonids are predicted to decline by up to 92% (brown trout) and 75% (grayling). In contrast, habitat suitability for perch and roach is predicted to increase by up to 108% and 53%, respectively. Even when accounting for broad variation in GCMs and realistic RCPs, these results suggest climate change will drive a significant redistribution of fish habitat. Salmonid-dominated communities in headwaters seem more sensitive to climate change than the fish communities of downstream sections. Because headwaters are a prevailing element of the hydrographic network in southwestern Germany, such changes may result in large-scale regressions of valuable fish communities that currently occupy broad geographic niches.
... Beyond eutrophication, the introduction of new species, pollution with for instance pesticides or heavy metals, hydrological changes, such as damming or water-level regulations, and climate warming may significantly affect characteristics of aquatic habitats and species composition. Increased water temperatures and water-level rises can increase water-column stability, i.e. reduce mixing and prolong stratification with consequences for nutrient cycling and/or turbidity that, in particular, affect non-motile species (Nõges, 2005;Selbie et al., 2011;Horn et al., 2015). Alterations in cell/colony size or morphology are wellknown adaptations of phytoplankton to such changes in abiotic conditions (e.g. ...
In this multi-proxy palaeo study, the response of primary producers and consumers to anthropogenic disturbances in a large shallow lake was investigated. Overall, eutrophication and water-level changes (partly inferred by bisaccate pollen grains) are well-reflected in changes in sedimentation rates of pigments, diatom and clado-ceran species composition and other proxies (e.g. Rhabdocoela oocytes). In particular, the stepwise decrease in relative abundance of Pediastrum simplex compared to P. boryanum/P. duplex proved to be a good indicator for eutrophication intensity. Furthermore, reductions in the body size (i.e. coenobia diameter) of Pediastrum simplex were clearly related to phases of low visibility that were caused by lake damming in the 1950 s and by accelerating eutrophication in the 1990 s. During the latter period, strong algal growth led to turbid conditions, which are indicated by the co-occurrence of the diatoms Fragilaria crotonensis, Aulacoseira ambigua and A. granulata, the green algae Staurastrum sp. as well as the cladoceran Anthalona cf. milleri. This study highlights that subfossil remains , which are often found in, for instance, standard cladoceran analysis (green algae such as Pediastrum spp. and Staurastrum spp. or bisaccate pollen grains), can add valuable information to the reconstruction of past environmental conditions and biotic response. Such better and more holistic understanding of freshwater ecosystems is crucial to meet future challenges in a rapidly changing world.
... However, the species dominating these blooms, and thus their dynamics, may differ from the more eutrophic species modeled here. Deep drinking water reservoirs usually experience typical spring diatom blooms ( Horn et al., 2015 ;Wentzky et al., 2019 ). Altogether, we infer that our results apply in general to temperate, eutrophic, shal-low to moderately deep lakes that can experience mild winters or intermittent ice cover. ...
In temperate lakes, it is generally assumed that light rather than temperature constrains phytoplankton growth in winter. Rapid winter warming and increasing observations of winter blooms warrant more investigation of these controls. We investigated the mechanisms regulating a massive winter diatom bloom in a temperate lake. High frequency data and process-based lake modeling demonstrated that phytoplank-ton growth in winter was dually controlled by light and temperature, rather than by light alone. Water temperature played a further indirect role in initiating the bloom through ice-thaw, which increased light exposure. The bloom was ultimately terminated by silicon limitation and sedimentation. These mechanisms differ from those typically responsible for spring diatom blooms and contributed to the high peak biomass. Our findings show that phytoplankton growth in winter is more sensitive to temperature, and consequently to climate change, than previously assumed. This has implications for nutrient cycling and seasonal succession of lake phytoplankton communities. The present study exemplifies the strength in integrating data analysis with different tem poral resolutions and lake modeling. The new lake ecological model serves as an effective tool in analyzing and predicting winter phytoplankton dynamics for temperate lakes.
... cyanobacteria) over fast sinking diatoms and a shift in phytoplankton communities can be expected (Wentzky et al., 2018). Interestingly, Horn et al. (2015) found such a shift from diatoms towards cyanobacteria over the last decades within another German drinking water reservoir. These authors also considered climate warming and the associated changes in thermal structure as the main drivers of this change. ...
The thermal structure in reservoirs affects the development of aquatic ecosystems, and can be substantially influenced by climate change and management strategies. We applied a two-dimensional hydrodynamic model to explore the response of the thermal structure in Germany's largest drinking water reservoir, Rappbode Reservoir, to future climate projections and different water withdrawal strategies. We used projections for representative concentration pathways (RCP) 2.6, 6.0 and 8.5 from an ensemble of 4 different global climate models. Simulation results showed that epilimnetic water temperatures in the reservoir strongly increased under all three climate scenarios. Hypolimnetic temperatures remained rather constant under RCP 2.6 and RCP 6.0 but increased markedly under RCP 8.5. Under the intense warming in RCP 8.5, hypolimnion temperatures were projected to rise from 5 °C to 8 °C by the end of the century. Stratification in the reservoir was projected to be more stable under RCP 6.0 and RCP 8.5, but did not show significant changes under RCP 2.6. Similar results were found with respect to the light intensity within the mixed-layer. Moreover, the results suggested that surface withdrawal can be an effective adaptation strategy under strong climate warming (RCP 8.5) to reduce surface warming and avoid hypolimnetic warming. This study documents how global scale climate projections can be translated into site-specific climate impacts to derive adaptation strategies for reservoir operation. Moreover, our results illustrate that the most intense warming scenario, i.e. RCP 8.5, demands far-reaching climate adaptation while the mitigation scenario (RCP 2.6) does not require adaptation of reservoir management before 2100.
... For example, efforts to reduce external phosphorus loading have often resulted in lower chlorophyll-a concentrations, deeper Secchi depths and shifts in community structure; all indicative of an improvement in water quality (Jeppesen et al. 2005). Nevertheless, over the last decade, we have witnessed a trend towards an expansion and proliferation of undesirable phytoplankton species in some aquatic habitats (Sukenik et al. 2012, Horn et al. 2015, Salmaso et al. 2015, Morabito et al. 2018, Zohary et al. 2019, even for the lakes that had undergone re-oligotrophication (Ernst et al. 2009, Michalak et al. 2013. ...
... For example, efforts to reduce external phosphorus loading have often resulted in lower chlorophyll-a concentrations, deeper Secchi depths and shifts in community structure; all indicative of an improvement in water quality (Jeppesen et al. 2005). Nevertheless, over the last decade, we have witnessed a trend towards an expansion and proliferation of undesirable phytoplankton species in some aquatic habitats (Sukenik et al. 2012, Horn et al. 2015, Salmaso et al. 2015, Morabito et al. 2018, Zohary et al. 2019, even for the lakes that had undergone re-oligotrophication (Ernst et al. 2009, Michalak et al. 2013. ...
Increases in phytoplankton biomass have been widely observed over the past decades, even in lakes experiencing nutrient reduction. However, the mechanisms giving rise to this trend remain unclear. Here, we unveil the potential mechanisms through quantifying the relative contribution of bottom–up versus top–down control in determining biomasses of phytoplankton assemblages in Lake Geneva. Specifically, we apply nonlinear time series analysis, convergent cross mapping (CCM), to decipher the degree of bottom–up versus top–down control among phytoplankton assemblages via quantifying 1) causal links between environmental factors and various phytoplankton assemblages and 2) the relative importance of bottom–up, top–down, and environmental effects. We show that the recent increase in total phytoplankton biomass, albeit with phosphorus reduction, was mainly caused by a particular phytoplankton assemblage which was better adapted to the re‐oligotrophicated environment characterized by relatively low phosphorus concentrations and warm water temperature, and poorly controlled by zooplankton grazing. Our findings suggest that zooplankton act as a critical driver of phytoplankton biomass and strongly impact the dynamics of recovery from eutrophication. Therefore, our phytoplankton assemblage approach in combination with causal identification of top–down versus bottom–up controls provides insights into the reason why phytoplankton biomass may increase in lakes undergoing phosphorus reduction.
... Among them are plankton dynamics (Horn & Uhlmann, 1995); limnophysical processes and climate change impact (Horn et al., 2011;Wagner et al., 2013;Horn et al., 2015); the function of sediments (Maaßen et al., 2003); and management issues (Paul et al., 2013). Additionally, as a result of his work, fundamental eutrophication studies could be carried out at the reservoirs (Uhlmann & Albrecht, 1968). ...
The limnology community has lost a scientist who was highly esteemed
both nationally and internationally, as well as a great university
lecturer. It is with deep sadness that we must report that Professor
Dietrich Uhlmann, passed away in Dresden on 22 October 2018, after
a short, severe illness.
DietrichUhlmann was born on 4 August 1930 in Chemnitz,Germany.
He studied zoology and botany at the University of Leipzig, earning his
Ph.D. in 1957 for his focus on the biological self-purification of domestic
sewage in ponds. Beginning in 1959, he taught and researched as an
Oberassistent (a position similar to that of Associate Professor) in the
Department of Drinking Water, Process Water andWastewater Biology
at theZoological Institute,which he also temporarily headed.At the age of
34, he habilitated (i.e., received the qualification for teaching as professor)
with results from his investigation of plankton dynamics in extremely
nutrient-rich shallow ponds. In 1967, Dr.Uhlmann becamea full professor
of Hydrobiology at the DresdenUniversityofTechnology. In1987, he was
awarded the Einar Naumann-August Thienemann Memorial Medal (de
Limnologia optime merito) for his fundamental work on the limnology of
wastewater ponds and reservoirs, the biology and control of eutrophic
and polluted ecosystems, and other areas of applied limnology. Many of
his most important scientific papers have been published in the
Internationale Revue der gesamten Hydrobiologie (now International Review
of Hydrobiology). Between 1983 and 1993, he served as this journal's
editor-in-chief. Even after his retirement in 1995, Professor Uhlmann
remained active as a researcher at the Dresden University of Technology
and the Saxon Academy of Sciences.
... In many central European catchments, P concentrations increased from the 1960s to 1980s and then declined reflecting the changes in human activities and regional socio-economic conditions (e.g., [27][28][29][30][31]). However, several studies indicate that in-lake P concentrations and trophic conditions reflect not only the P-load changes but also are influenced by processes in the lake ecosystem that can either strengthen [32,33] or weaken [34,35] the decreasing P-load trends in the inflow. Climate change is a factor that can act both at the level of P export from the catchment and at the level of processes in the lake ecosystem. ...
European freshwater ecosystems have undergone significant human-induced and environmentally-driven variations in nutrient export from catchments throughout the past five decades, mainly in connection with changes in land-use, agricultural practice, waste water production and treatment, and climatic conditions. We analysed the relations among concentration of total phosphorus (TP) in the Slapy Reservoir (a middle reservoir of the Vltava River Cascade, Czechia), and socio-economic and climatic factors from 1963 to 2015. The study was based on a time series analysis, using conventional statistical tools, and the identification of breaking points, using a segmented regression. Results indicated clear long-term trends and seasonal patterns of TP, with annual average TP increasing up until 1991 and decreasing from 1992 to 2015. Trends in annual, winter and spring average TP concentrations reflected a shift in development of sewerage and sanitary infrastructure, agricultural application of fertilizers, and livestock production in the early 1990s that was associated with changes from the planned to the market economy. No trends were observed for average TP in autumn. The summer average TP has fluctuated with increased amplitude since 1991 in connection with recent climate warming, changes in thermal stratification stability, increased water flow irregularities, and short-circuiting of TP-rich inflow during high flow events. The climate-change-induced processes confound the generally declining trend in lake-water TP concentration and can result in eutrophication despite decreased phosphorus loads from the catchment. Our findings indicate the need of further reduction of phosphorus sources to meet ecological quality standards of the EU Water Framework Directive because the climate change may lead to a greater susceptibility of the aquatic ecosystem to the supply of nutrients.
... Its trophic state changed from mesotrophic to eutrophic in the 1970s, remained eutrophic in the 1980s and quickly returned to the mesotrophic state due to a strong reduction of the phosphorous import since 1990 after German Reunification. Supported by the Saxony Academy of Science and hosted at the Ecological Station Neunzehnhain of the Technical University of Dresden, Heidemarie Horn analyzed the phytoplankton weekly from 1975 to 1985 and fortnightly until 2011, culminating in 1259 sampling dates (for details see Horn 2003;Horn et al. 2011Horn et al. , 2015. As in LC, samples were counted with an inverted microscope. ...
The shape of trait distributions may inform about the selective forces that structure ecological communities. Here, we present a new moment-based approach to classify the shape of observed biomass-weighted trait distributions into normal, peaked, skewed, or bimodal that facilitates spatio-temporal and cross-system comparisons. Our observed phytoplankton trait distributions exhibited substantial variance and were mostly skewed or bimodal rather than normal. Additionally, mean, variance, skewness und kurtosis were strongly correlated. This is in conflict with trait-based aggregate models that often assume normally distributed trait values and small variances. Given these discrepancies between our data and general model assumptions we used the observed trait distributions to test how well different aggregate models with first- or second-order approximations and different types of moment closure predict the biomass, mean trait, and trait variance dynamics using weakly or moderately nonlinear fitness functions. For weakly non-linear fitness functions aggregate models with a second-order approximation and a data-based moment closure that relied on the observed correlations between skewness and mean, and kurtosis and variance predicted biomass and often also mean trait changes fairly well and better than models with first-order approximations or a normal-based moment closure. In contrast, none of the models reflected the changes of the trait variances reliably. Aggregate model performance was often also poor for moderately nonlinear fitness functions. This questions a general applicability of the normal-based approach, in particular for predicting variance dynamics determining the speed of trait changes and maintenance of biodiversity. We evaluate in detail how and why better approximations can be obtained.
Eutrophication severely impacts water quality, impeding tourism, recreation, and rendering it unfit for consumption, swimming, and fishing. To address this challenge, innovative methods for nutrient inactivation in lakes and reservoirs are imperative in restoring surface water. In this context, we investigated whether the technology based on ion exchange (IX) resins can decrease the concentrations of nitrate-nitrogen (NO3-N) and phosphate-phosphorus (PO4-P) in the water in the vicinity of the anion exchange resin installation located in the backwater area of a reservoir. The Anion Exchange Resin Installation will be referred to as the Water Quality Improvement Installation (WQII) in this article. We also aimed to identify the factors that affect water quality in the water in the vicinity of the WQII area. Our research area was the Turawa eutrophic reservoir located in south-western Poland (Central Europe). Our results indicate that the WQII effectively lowers the concentrations of NO3-N and PO4-P in the tested water. Environmental factors have negligible influence on NO3-N and PO4-P concentrations in the water in the vicinity of the WQII. An inversely proportional relationship between the removal of NO3-N and PO4-P indicates that the configuration of the regeneration system of each column is appropriate. The WQII effectively decreases NO3-N and PO4-P concentrations during periods of cyanobacterial blooms in the backwater area of the reservoir. This novel WQII utilizing IX technology, emerges as a promising solution for remediating eutrophic water aligning with the Water Framework Directive, assuming the achievement of good water status of water bodies.
Global warming is shifting the thermal dynamics of lakes, with resulting climatic variability heavily affecting their mixing dynamics. We present a dual ensemble workflow coupling climate models with lake models. We used a large set of simulations across multiple domains, multi‐scenario, and multi GCM‐ RCM combinations from CORDEX data. We forced a set of multiple hydrodynamic lake models by these multiple climate simulations to explore climate change impacts on lakes. We also quantified the contributions from the different models to the overall uncertainty. We employed this workflow to investigate the effects of climate change on Lake Sevan (Armenia). We predicted for the end of the 21st century, under RCP 8.5, a sharp increase in surface temperature (4.3±0.7K) and substantial bottom warming (1.7±0.7K) , longer stratification periods (+55 days) and disappearance of ice cover leading to a shift in mixing regime. Increased insufficient cooling during warmer winters points to the vulnerability of Lake Sevan to climate change. Our workflow leverages the strengths of multiple models at several levels of the model chain to provide a more robust projection and at the same time a better uncertainty estimate that accounts for the contributions of the different model levels to overall uncertainty. Although for specific variables, for example, summer bottom temperature, single lake models may perform better, the full ensemble provides a robust estimate of thermal dynamics that has a high transferability so that our workflow can be a blueprint for climate impact studies in other systems.
Phytoplankton are key components in aquatic ecosystems and therefore an important target of monitoring analyses. Microscopical counting, although providing most detailed results, is time and labor intensive and requires highly skilled analysts. In situ spectral fluorescence measurements provide a much faster analysis with a higher spatiotemporal resolution. A one-season survey of phytoplankton assemblages was performed in order to compare the results of the spectrofluorometric measurements to classical microscopical determination in different sections of a drinking water reservoir. The investigations were performed with the spectrofluorometer FluoroProbe (FP) by bbe Moldaenke GmbH (Kiel, Germany), which is designed to discriminate among diatoms, green algae, Cryptophyta and cyanobacteria. The results of phytoplankton quantification as revealed by total chlorophyll a (Chla) measurements with the FP and total biovolumes determined by microscopy showed a good correlation.The accordance between the two approaches was best for diatoms and much lower for the other spectral groups. The proportion of green algae was generally overestimated by FP measurements in comparison to biovolumes. Contrary, the percentage of cyanobacteria was often underestimated by FP compared to microscopical analyses. A clear underestimation of cyanobacteria by FP measurements even at high abundances of Microcystis sp. was observed in two samples. No influence of species composition on the congruence between microscopical analyses and FP measurements was detected.
Freshwater Algae of North America: Ecology and Classification, Second Edition is an authoritative and practical treatise on the classification, biodiversity, and ecology of all known genera of freshwater algae from North America. The book provides essential taxonomic and ecological information about one of the most diverse and ubiquitous groups of organisms on earth. This single volume brings together experts on all the groups of algae that occur in fresh waters (also soils, snow, and extreme inland environments). In the decade since the first edition, there has been an explosion of new information on the classification, ecology, and biogeography of many groups of algae, with the use of molecular techniques and renewed interest in biological diversity. Accordingly, this new edition covers updated classification information of most algal groups and the reassignment of many genera and species, as well as new research on harmful algal blooms.
1930 Climate change may have profound eff ects on phosphorus (P) transport in streams and on lake eutrophication. Phosphorus loading from land to streams is expected to increase in northern temperate coastal regions due to higher winter rainfall and to a decline in warm temperate and arid climates. Model results suggest a 3.3 to 16.5% increase within the next 100 yr in the P loading of Danish streams depending on soil type and region. In lakes, higher eutrophication can be expected, reinforced by temperature-mediated higher P release from the sediment. Furthermore, a shift in fi sh community structure toward small and abundant plankti-benthivorous fi sh enhances predator control of zooplankton, resulting in higher phytoplankton biomass. Data from Danish lakes indicate increased chlorophyll a and phytoplankton biomass, higher dominance of dinophytes and cyanobacteria (most notably of nitrogen fi xing forms), but lower abundance of diatoms and chrysophytes, reduced size of copepods and cladocerans, and a tendency to reduced zooplankton biomass and zooplankton:phytoplankton biomass ratio when lakes warm. Higher P concentrations are also seen in warm arid lakes despite reduced external loading due to increased evapotranspiration and reduced infl ow. Th erefore, the critical loading for good ecological state in lakes has to be lowered in a future warmer climate. Th is calls for adaptation measures, which in the northern temperate zone should include improved P cycling in agriculture, reduced loading from point sources, and (re)-establishment of wetlands and riparian buff er zones. In the arid Southern Europe, restrictions on human use of water are also needed, not least on irrigation. O n average, global surface temperatures have increased by about 0.74°C over the past 100 yr (Trenberth et al., 2007), with the majority of the increase (0.55°C) occurring over the past 30 yr. We may expect marked changes to occur in the global climate during this century (IPCC, 2007). Increasingly reliable regional climate projections are available for many regions of the world, but fewer projections are available for many developing countries than for the developed world (Christensen et al., 2007). Th e warming generally increases the spatial variability of precipitation with reduced rainfall in the subtropics and increases at higher latitudes and in parts of the tropics. Th e changes in temperature and rainfall lead to changes in agricul-tural land use and management, including changes in soil cultivation and in the rates and timing of fertilization (Howden et al., 2007). Th ese changes have cascading eff ects on the P cycling, directly and indirectly, that aff ect the aquatic environment. Th e direct eff ects are related to the increased temperatures, increased rainfall intensity, and changes in winter rainfall that are expected to enhance the P loading to freshwaters in the temperate zone (IPCC, 2007) and the Arctic (Arctic Climate Impact Assessment, 2002) and to reduce the loading, but not the concentrations, in streams and freshwater lakes in the Mediterranean region. However, a few quantitative studies are avail-able (Chang, 2004; Andersen et al., 2006). Th e indirect eff ects are related to changes in the choice of crops, crop rotations, use of catch crops, and agricultural practices, including tillage and fertilization. In northern temperate areas, new heat-demanding, warm-season crops (e.g., maize and sunfl ower) will replace many of the present grain cereals and oilseed crops (Olesen and Bindi, 2002). At the same time, changes occur in planting and harvesting times (Olesen, 2005) and in fertilization rates and strategies (Olesen et al., 2007). Crop rotation must be adapted to changes in crop choices, in crop maturing, and in the need to control weeds, pests, and diseases. Th is will aff ect the amount of P released to freshwaters and its seasonal pattern. More-Abbreviation: TP, total phosphorus.
A controversial precept of aquatic ecology,asserts that low ratios of nitrogen to phosphorus,(N:P) lead to noxious and sometimes,toxic blooms,of Cyanobacteria. Cyanobacteria dominance,is a major risk to human,and ecosys- tem health. The stoichiometric control of Cyanobacteria therefore has become,central to freshwater resource manage- ment. This controversial concept is based on observed,Cyanobacteria dominance,in lakes with low N:P and the results of lab and field experiments. Here we analyze data from 99 of the temperate,zone’s most,studied lakes and show,that this model,is flawed. We show,that the risk of water quality degradation,by Cyanobacteria blooms,is more,strongly correlated with variation in total P, total N, or standing algae biomass than the ratio of N:P. Risks associated with Cyanobacteria are therefore less associated with N:P ratios than a simple increase in nutrient concentrations and algal biomass. Résumé : Un précepte controversé de l’écologie aquatique,veut que des faibles rapports azote:phosphore,(N:P)
Phytoplankton dominance (as biomass) by heterocystous cyanobacteria, nonheterocystous cyanobacteria, and chlorophytes was studied along a trophic gradient (0.011-2.2 mg P.L(-1)) by analyzing regularly collected semiquantitative data from 178 shallow Danish lakes (mean depth <3 m) and quantitative data from 32 lakes. Heterocystous cyanobacteria were dominant at low total P (TP) (<0.25 mg P.L(-1)) and nonheterocystous cyanobacteria at intermediate TP (0.25-0.8 mg P.L(-1)), while chlorophytes often were dominant at high TP (>1 mg P.L(-1)). In contrast with many earlier findings, heterocystous cyanobacteria were not dominant at low total N (TN):TP or low inorganic N concentrations; chlorophytes were dominant at extremely high pH, and the shift from cyanobacterial to chlorophyte dominance could not be explained by a change in the photic zone to mixing zone ratio. We suggest that chlorophyte dominance in hypertrophic shallow lakes is attributable to continuous input of nutrients and carbon from the sediment and external sources. This renders the fast-growing chlorophytes a superior competitor compared with the relatively slow-growing cyanobacteria, even when inorganic nutrient concentration is low and pH high. New predictive models relating phytoplankton dominance to TP in shallow lakes were developed, as former models failed to predict our observations satisfactorily.
The relative biomass of blue-green algae in freshwater (total dissolved solids < 500 mg∙L−1) Alberta lakes was consistently underestimated by two recent empirical models based on total nitrogen (TN), total phosphorus (TP), Secchi disc depth (SD), and depth of the mixed layer (Zm). We regrouped the data used in these empirical models to eliminate the potential biases introduced by including data from lakes not in dynamic equilibrium and generated a new model based on TP. This new model accounted for 11% more of the variation in relative blue-green algal biomass than the original model based on TN, TP, SD, and Zm and 21% more than the model based on TN to TP ratios and SD to Zm ratios. This new model was also a much better predictor of the relative biomass of blue-green algae in Alberta lakes than the original models. In addition, for lakes in Alberta, TP was a much better predictor of total blue-green algal biomass than TN or the TN to TP ratio. Our analyses suggest that for large numbers of lakes, TP may be as good or better an indicator of relative and total biomass of blue-green algae than TN or TN to TP ratios.
Anthropogenic-induced changes in nutrient ratios have increased the susceptibility of large temperate lakes to several effects of rising air temperatures and the resulting heating of water bodies. First, warming leads to stronger thermal stratification, thus impeding natural complete water turnover (holomixis), which compensates for oxygen deficits in the deep zones. Second, increased water temperatures and nutrient concentrations can directly favour the growth of harmful algae. Thus, lake-restoration programmes have focused on reducing nutrients to limit toxic algal blooms. Here we present evidence that the ubiquitous harmful cyanobacterium Planktothrix rubescens has become the dominant species in a large lake during the past four decades, although the phosphorus content of the ecosystem decreased fivefold. However, the nitrogen input was not diminished concomitantly, favouring this non-N2-fixing cyanobacterium owing to increased N:P ratios. P. rubescens contains gas vesicles that allow for buoyancy to accumulate within the depth of optimal irradiance. As the toxic cyanobacterium has low consumption by predators, water turnover represents the main mechanism of seasonal population control. Thus, unidirectional lake-restoration measures in parallel with recurrent absence of holomixis owing to lake warming may lead to similar undesired effects that have formerly emerged from fertilization.
Significant changes in physical and biological systems are occurring on all continents and in most oceans, with a concentration of available data in Europe and North America. Most of these changes are in the direction expected with warming temperature. Here we show that these changes in natural systems since at least 1970 are occurring in regions of observed temperature increases, and that these temperature increases at continental scales cannot be explained by natural climate variations alone. Given the conclusions from the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report that most of the observed increase in global average temperatures since the mid-twentieth century is very likely to be due to the observed increase in anthropogenic greenhouse gas concentrations, and furthermore that it is likely that there has been significant anthropogenic warming over the past 50 years averaged over each continent except Antarctica, we conclude that anthropogenic climate change is having a significant impact on physical and biological systems globally and in some continents.
The link between compensatory dynamics and regime shifts is not well understood. We analyse a regime shift in phytoplankton in a large lake with respect to: (1) environmental forcing and (2) the type of dynamics (compensatory or synchronous) between phytoplankton groups. The regime shift in phytoplankton was related to gradual changes in nutrient levels, but unrelated to an almost concurrent shift in climatic conditions. The relationship between total phytoplankton biomass and phosphorus concentrations was sigmoid. Trajectories of phytoplankton biomass and community dynamics suggest that eutrophication effects can successfully be reversed when management efforts decrease nutrient loading to a level sufficiently low to overcome community resilience. The regime shift was associated with a loss of biomass compensation and compensatory dynamics among the phytoplankton groups. This suggests that the type of interactions is important for a better understanding of the existence and shape of nonlinear responses of phytoplankton biomass to environmental change.
Internal loading as phosphorus (P) released from anoxic sediment surfaces often represents the main summer P load to lakes and reservoirs and can have an immense effect on their water quality. Many difficulties in internal load assessment exist, however, including ignoring internal load altogether, ambiguity about the origin of sediment released P and inexact definitions. Most of these problems are due to the difficulty in distinguishing internal from external P sources, which is particularly challenging in polymictic lakes. To prevent misconceptions and facilitate its evaluation, internal load in stratified and polymictic lakes should be expressed in a similar way to external loads: as annual, gross and areal load of total phosphorus (TP). Possible approaches to internal load quantification are: in situ determination from hypolimnetic P increases, mass balance approaches, and estimates from anoxic active area and P release. Further suggestions to facilitate the study of internal loading include: (a) the differentiation between polymictic and stratified lakes, sections of lakes, and time periods when evaluating indicators and impact of internal load; (b) the separation of internal load (upward flux) from sedimentation (downward flux) of external and internal loads, and (c) the consideration of the downward flux of both external (Lext, mg/m/yr) and internal (Lint, mg/m/yr) loads by a retention model (Rsed) when predicting lake TP averages in a mass balance model of the form (qs = annual areal water load in m/yr):
Dominance by cyanobacteria hampers human use of lakes and reservoirs worldwide. Previous studies indicate that excessive nutrient loading and warmer conditions promote dominance by cyanobacteria, but evidence from global scale field data has so far been scarce. Our analysis, based on a study of 143 lakes along a latitudinal transect ranging from subarctic Europe to southern South America, shows that although warmer climates do not result in higher overall phytoplankton biomass, the percentage of the total phytoplankton biovolume attributable to cyanobacteria increases steeply with temperature. Our results also reveal that the percent cyanobacteria is greater in lakes with high rates of light absorption. This points to a positive feedback because restriction of light availability is often a consequence of high phytoplankton biovolume, which in turn may be driven by nutrient loading. Our results indicate a synergistic effect of nutrients and climate. The implications are that in a future warmer climate, nutrient concentrations may have to be reduced substantially from present values in many lakes if cyanobacterial dominance is to be controlled.
Phytoplankton can become limited by the availability of nutrients when light and temperature are adequate and loss rates are not excessive. The current paradigms for nutrient limitations in freshwater, estuarine, and marine environments are quite different. A review of the experimental and observational data used to infer P or N limitation of phytoplankton growth indicates that P limitation in freshwater environments can be demonstrated rigorously at several hierarchical levels of system complexity, from algal cultures to whole lakes. A similarly rigorous demonstration of N limitation has not been achieved for marine waters. Therefore, we conclude that the extent and severity of N limitation in the marine environment remain an open question. Culture studies have established that internal cellular concentrations of nutrients determine phytoplankton growth rates, and these studies have shown that it is often difficult to relate growth rates to external concentrations, especially in natural situations. This should lead to a greater reliance on the composition of particulate matter and biomass‐based physiological rates to infer nutrient limitation. Such measurements have demonstrated their utility in a wide variety of freshwater and marine environments, and, most importantly, they can be applied to systems that are difficult to manipulate experimentally or budget accurately. Dissolved nutrient concentrations are most useful in determining nutrient loading rates of aquatic ecosystems. The relative proportions of nutrients supplied to phytoplankton can be a strong selective force shaping phytoplankton communities and affecting the biomass yield per unit of limiting nutrient.
Phytoplankton and environmental variables have been monitored in the large Swedish lakes Malaren, Hjalmaren, Vattern and Vanern since the 1960s. Measures to reduce phosphorus input and industrial waste products were taken during the 1970s. The phosphorus loading was then reduced by 90-95% resulting in a halving of the phosphorus concentrations in the most affected basins. The phytoplankton community reacted rapidly with decreased biomasses of cyanobacteria in summer as well as decreased biomasses of spring diatoms and cryptophycean flagellates. Other reactions were a contracted period of waterbloom, an increased taxon richness, an increased evenness in the biomass over the growth season, and a change in the species size structure within the phytoplankton community. Furthermore, the species richness in the large lakes is compared in relation to lake characteristics. A presentation of the occurrence of toxic cyanobacteria in the lakes is also given. Maximum-minimum values of 13-0.1 mug microcystin L(-1) are established in connection with water-blooms in Hjalmaren and Malaren. The use of phytoplankton as a monitoring variable to detect water-quality changes is outlined and assessment criteria are presented.
Reservoirs defined in agreement with ICOLD have in comparison with lakes higher watershed area / waterbody area ratios, shorter theoretical retention times, different basin morphology with deepest point at the dam and mostly deep outlets. This is manifested in higher pollution loads, temperature stratification highly dependent on retention time, longitudinal zonation with maxims of primary production in the transition zone and high phosphorus and other pollutants retention capacity.
Reservoirs defined in agreement with ICOLD have in comparison with lakes higher watershed area / waterbody area ratios, shorter theoretical retention times, different basin morphology with deepest point at the dam and mostly deep outlets. This is manifested in higher pollution loads, temperature stratification highly dependent on retention time, longitudinal zonation with maxima of primary production in the transition zone and high phosphorus and other pollutants retention capacity.
An overview is given as to the impacts of morphometry and operation on the trophic character of reservoirs, the conditions for phytoplankton growth and the significance of the littoral and the floating-leaved vegetation. Land-water interactions and the availability of allochthonic organic substances and of nutrient are dealt with in some detail. In the context of successional stages, the consequences of converting a riverine into a lake environment are discussed with emphasis upon phytoplankton growth, inundation of terrestrial vegetation, development of the zoobenthos and zooplankton and on the food web. Some final remarks are given as to siltation and senescence of reservoirs and to the question whether a reservoir may be considered to be a self-supporting ecosystem.
Observations have been made during five years on the fluctuations in the numbers of Asterionella formosa and other plankton diatoms, and in the concentrations of dissolved silica, nitrates and phosphates in four bodies of water in the English Lake District. This account refers mainly to the period from winter to summer. The close of the spring period of increase of Asterionella is not directly due to light or temperature. Loss of cells by floods is more or less compensated for by the replenishment of the lake with nutrients in the inflow water. Grazing by animals has no appreciable effect on the fluctuations in numbers. Fungal parasitism may affect the course of the spring increase in numbers, but is rarely the cause of its end. Very rarely, depression of the numbers of Asterionella by parasitism may lead to other diatoms becoming dominant and utilizing the available nutrients (e.g. silica). The close of the spring period of increasing numbers of Asterionella is frequently due to depletion of the available silica. Confirmation is deduced for Pearsall's (1932) view that diatoms cannot multiply to any marked extent when the concentration of silica is less than 0.5 mg. per litre. The supply of available silica does not provide a complete explanation of the fluctuations in diatom numbers in all the lakes considered or at all times of year. The mean silica content of the cells of Asterionella formosa is 140μg. per million. Confirmation has been obtained for the view of Einsele & Grim (1938) that the amount of silica per unit area of cell is constant under all conditions. The nitrogen content in natural and cultural populations varied from 6 to 12μg. per million cells (mean value 8μg.). The phosphorus content, by contrast with silica and nitrogen, varied widely in natural and cultural populations (0.06-4.2μg. per million cells). The cells of Asterionella can store phosphorus in excess of immediate requirements and when the concentration in the water is 1μg. per litre or less. Asterionella can utilize calcium at a concentration of 400μg. per litre, a lower concentration than occurs in any lake in the English Lake District. Reasons are given for the view that the supplies of nitrogen, phosphorus, carbon and calcium appear to be sufficient to support larger populations of Asterionella than are observed during the period under review. Little or nothing is known concerning the importance of any other substances in the lake waters. Limitation of growth due to lack of silica shows a difference from that due to lack of light. The present observations and conclusions are discussed in relation to those for other lakes made by the author and other workers. It is emphasized that every algal species and lake must be considered separately.
Summary1. Long‐term data on the meteorology, hydrology, physicochemistry and plankton of a reservoir and its tributaries in SE Germany run from 1976 until now. This dimictic reservoir changed from mesotrophic to eutrophic in the 1970s, remained eutrophic in the 1980s and returned to the mesotrophic state after a sharp reduction in P loading in 1990.2. Phytoplankton biomass reaches an annual maximum in spring and consists almost entirely of diatoms. While Asterionella formosa was dominant until 1990, Aulacoseira subarctica became more frequent at the end of the 1990s and was particularly abundant in years with short winters.3. Statistical analyses suggested that these changes were triggered primarily by the mild winters that were frequent after 1988. Climate‐related hydrophysical variables and the initial biomass of the diatoms at the beginning of the year, considered as an ‘inoculum’, were identified as most important. These variables explained 39% of the total variance of the relative abundance, whereas the change in trophic conditions was responsible for about 20%.4. The absolute and relative abundance of A. subarctica was positively related to short ice cover, early ice‐out and a long‐lasting spring circulation. Owing to its physiological traits, and particularly its ability to survive under low‐light conditions, A. subarctica benefitted from short, mild winters. Under such conditions, it could sustain or establish a high initial biomass, whereas the concentrations of the other diatoms decreased over winter. However, this advantage may be lost if further warming causes an early onset of summer stratification. Because of its low population growth rate and requirement for high turbulence, A. subarctica needs long, cold springs to exploit the improved starting conditions and to become abundant.5. In contrast to A. subarctica, A. formosa required a substantial soluble reactive phosphorus supply to compete successfully. The eutrophic conditions until 1990 were the prerequisite for its mass growth under low‐light and low‐temperature conditions during the spring. After reduction in P concentration from 1990, A. formosa declined and other diatom species became more abundant.6. These other diatoms may be viewed as ‘stopgaps’ when conditions were not favourable for A. subarctica or A. formosa. Diatoma elongatum exploited brief circulation periods in years with low P loading. Synedra acus and Fragilaria crotonensis, because of their poor competitive ability at low light intensity, reached high density in the upper water column in the transitional period between spring circulation and summer stratification.7. Our study suggests that climate‐related variables have crucial impacts on the spring phytoplankton dynamics of deep stratified waterbodies. They can mask the consequences of changes in the trophic conditions and, corresponding to the functional traits of the different phytoplankton species, also decisively control their relative abundances. In this reservoir, the warmer winters and prolonged spring circulations did not only lead to high phytoplankton biomass (despite considerably reduced nutrient loads) but also cause a marked shift in the diatom assemblage during the spring bloom.
Mixed layer dynamics were studied during June to August in 2008 to 2012 in Ford Lake, Michigan, using real-time data acquisition from in situ water temperature, dissolved oxygen, and chlorophyll sensors plus a shore-side weather station. The purpose of the study was to examine the effectiveness of managed hypolimnetic discharge for inducing vertical mixing, preventing hypolimnetic anoxia, and reducing the incidences of cyanobacterial blooms. Maximum mixed layer thickness each day was deduced from vertical density profiles measured at 30-minute intervals plus estimated kinetic energy supplied by wind stress and convection. Surface heat flux and resulting convection was more important to mixing dynamics than was wind stress. Selective withdrawal of hypolimnetic water was practiced in 2008 to 2011 and resulted in enhanced vertical mixing, hypolimnetic oxygenation, and absence of nuisance cyanobacterial blooms. In 2012, drought conditions restricted upstream water availability to such an extent that selective hypolimnetic withdrawal could not be used that year. The result was more than a month-long period of stable thermal stratification during which hypolimnetic oxygen concentrations became depleted and phosphate was released from anoxic sediments. A subsequent cold weather event induced temporary destratification, and a bloom of Aphanizomenon quickly ensued, despite reduced external loading of P from the influent river. Results of this study point to mixing dynamics and internal nutrient loading as the master variables responsible for nuisance algal blooms in this system.
Biomass and structure of phytoplankton of a flushed, shallow lake were compared between a decade with hypertrophic conditions (1980-1990) and a period with 40-50 % reduced external nutrient load (1991-1997). Nutrient concentrations in the inflow declined less than loads because of reduced discharge. At the same time, the phytoplankton biovolume decreased by 33 % in spring (March-May) and 40 % in summer (June-September, average 1980-1990 versus 1991-1997). The total phytoplankton biomass declined despite heavy internal phosphorus load in summer, an absence of macrophytes and reduced biomass of daphnids. Nutrient-based carrying capacities of Muggelsee diminished from the 1980s to recent years, but were still rarely fully saturated. Filamentous cyanobacteria dominated the phytoplankton under hypertrophic conditions but were much less abundant in recent years. Diatoms were less affected by nutrient reduction and became the dominant algal group in the 1990s. Phytoplankton development depended more strongly on the start conditions than on the seasonal averages of nutrient concentrations. Earlier exhaustion of phosphorus by diatoms was the only proper explanation of the shorter or lacking spring development of cyanobacteria from 1991-1997. During summer, cyanobacteria grew at the same rates as in the previous decade, but attained lower maxima because of their lower starting biomass. Obviously, temporary phosphorus limitation in spring might suppress the development of species with low growth rates in such flushed systems during the whole season.
RNA pieces in the spliceosome, has a domain V counterpart, containing a 2-nucleotide bulge located 5 base pairs away from an AGC triad (10). Formation of an analogous metal-binding platform in this region of U6 (11) may explain the apparent ability of spliceosomal RNAs to retain catalytic activity in the complete absence of the many protein components that usually accompany splicing (12). A domain V-like element could have played a major role during the RNA world era of evolution , serving as the catalytic center for RNA cleavage, transesterification, and polymeriza-tion reactions. The new structure provides a powerful starting point for future investigations of group II introns and the spliceosome. The lack of electron density for domain VI, which is important for the first step of splicing in many group II introns, and the absence of exons from the structure preclude us from seeing how these elements dock onto the surface created by domains I to V. Thus, the structural details of substrate recognition and catalysis remain undefined. The nature of the conformational change known to separate the two steps of splicing (13) also remains unclear. Finally, it will be important for our understanding of group II intron self-splicing to capture the structures of the other intermediates along the splicing pathway and to pursue experiments that link features of these structures with functionally defined interactions.
A linked chain of causal factors—namely, winter air temperature, spring lake temperature, extent of the spring lake overturn, and extent of surface nutrient enrichment—had significant effects on the annual development of phytoplankton structure and biomass in Lake Garda, a large (49 3 10 9 m 3 ), deep (350 m) lake located south of the Alps. The relationship between the winter climate and the North Atlantic Oscillation (NAO) was unclear, probably because of the location of the lake, which was on the border between different centers of action of the NAO (the Mediterranean region and central and northern Europe). Soon after the major enrichment episodes, conjugatophytes and large diatoms developed with higher biovolumes during April, whereas from midsummer to midautumn, the cyanobacteria and, more irregularly, the conjugatophytes were more important. In two other deep southern subalpine lakes (Iseo and Lugano), the interannual variations in the extent of spring vertical mixing, nutrient replenishment, and phytoplankton development were closely related because of the common effects of winter climate on the subalpine region. The effect was proportionally more evident in the two meromictic and more eutrophic lakes, Iseo and Lugano, because of the higher nutrient content in their hypolimnia.
Cyanobacteria are the most ancient phytoplankton on the planet and form harmful algal blooms in freshwater, estuarine, and marine ecosystems. Recent research suggests that eutrophication and climate change are two processes that may promote the proliferation and expansion of cyanobacterial harmful algal blooms. In this review, we specifically examine the relationships between eutrophication, climate change and representative cyanobacteria genera from freshwater (Microcystis, Anabaena, Cylindrospermopsis), estuarine (Nodularia, Aphanizomenon), and marine ecosystems (Lyngbya, Synechococcus, Trichodesmium). Commonalities among cyanobacteria genera include being highly competitive for low concentrations of inorganic P (DIP) and the ability to acquire organic P compounds. Both diazotrophic (= nitrogen (N2) fixers) and non-diazotrophic cyanobacteria display great flexibility in the N sources they exploit to form blooms. Hence, while some cyanobacterial blooms are associated with eutrophication, several form blooms when concentrations of inorganic N and P are low. Cyanobacteria dominate phytoplankton assemblages under higher temperatures due to both physiological (e.g. more rapid growth) and physical factors (e.g. enhanced stratification), with individual species showing different temperature optima. Significantly less is known regarding how increasing carbon dioxide (CO2) concentrations will affect cyanobacteria, although some evidence suggests several genera of cyanobacteria are well-suited to bloom under low concentrations of CO2. While the interactive effects of future eutrophication and climate change on harmful cyanobacterial blooms are complex, much of the current knowledge suggests these processes are likely to enhance the magnitude and frequency of these events.
Since 1975, different patterns of the growth of Fragdaria and Cyanobacteria in the mid or late summer have been observed in the Saidenbach Reservoir. In most of the years, there was a mutual exclusion of mass growths of these two groups. High yields of Fragilaria caused low yields of blue-greens and vice versa. In the first years, Fragilaria was always the first to achieve a mass growth. followed by the Cyanobacteria. Then, in the last years, the blue-greens were succeeded by Fragilaria. Only in some years, there was a large and simultaneous growth of both groups. It has been shown, that the supply of silicon and phosphorus immediately before controlled the pattern of succession in the midsummer period. Mostly, the Si:P ratio was the regulating factor, but there were also years in which the absolute concentration level of these two nutrients were either non-limiting high or below the minimum resource concentration necessary to compensate for the lgsses. In these cases, the Si:P ratio lost its controlling function. Other influencing factors are discussed, in particular the effect of turbulent mixing. The special nutrient conditions in the midsummer were not only determined by the external load but also by the Si depletion during the spring mass development of diatoms other than Fragilaria, which mostly is physically controlled. Therefore, the succession pattern in the midsummer is sensitive to the meteorological conditions to a large extent.
Langzeitdaten von deutschen Talsperren zeigen eine Erhöhung der Temperaturen der oberen Wasserschichten vor allem im Winter, Frühling und Frühsommer. Die Trends sind regional ähnlich. Da sich geringe Temperaturerhöhungen auf die biotische und abiotische Struktur und somit auf die Wasserbeschaffenheit auswirken, ergibt sich für Trinkwassertalsperren die Notwendigkeit, die Bewirtschaftung an die Veränderungen anzupassen.
Long-term data from German drinking water reservoirs show tendencies of increasing temperatures in the upper water layers, especially in winter, spring and early summer. The trends are regionally similar. Changes in water temperature affect the biotic and abiotic structure and, thus, raw water quality. Therefore, management strategies of drinking water reservoirs need to be adapted.
In recent years, anomalous, large, summer Fragilaria crotonensis pulses have been reported from Pigeon Bay. The purpose of this study was to identify the factors which contribute to these pulses, based on experiments designed to investigate the effects of nutrients and the interaction of light and nutrients on growth, field monitoring, and a review of published studies. It was found that summer not only provides optimal water temperatures for F. crotonensis growth, but peak incident solar radiation and water transparency. It is hypothesized that F. crotonensis pulses during summer are dependent upon three major factors: 1) adequate supplies of nutrients (silica and phosphorus); 2) the presence of slight thermal stratification with concomitant wind velocities sufficient to keep cells circulating in the surface mixed layer; and, 3) the presence of low turbidities with concomitant irradiance sufficient for growth. Once such physicochemical conditions exist, it appears that F. crotonensis can grow rapidly (because it is adapted to low light and is a superior exploitative competitor for phosphorus and silica) and achieve densities as high as 950,000 cells L−1. Such pulses may be terminated by lack of wind (which allows cells to sink out of the photic zone), high wind velocities (which increase turbidity and decrease light penetration), or depletion of nutrients. Fragilaria crotonensis is probably not affected by zooplankton grazing because of its large size and chain-forming morphology. An order of magnitude increase in summer nitrogen:phosphorus ratios in Pigeon Bay since the late 1960s may have provided a more optimal environment for F. crotonensis growth in recent years.
Phosphate-limited growth of Oscillatoria redekei in semicontinuous culture has been studied under conditions of continuous illumination at 20 °C as well as in a 12/12 hours light-dark cycle at temperatures between 5 °C and 20 °C. The subsistence quota (q0) amounted to 0.052 μmol mm−3 under all conditions, when the phosphate quota was expressed on the basis of cell volume. The interaction between temperature and phosphate quota and its impact on growth rate are described by the following model:
Parameter values are topt=24.5 °C, tmin=0.95 °C, μmax =0.873 d−1. The maximum phosphate quota was found to depend on temperature and to increase along with declining temperature.
In the mesotrophic reservoir Saidenbach, diatoms are dominating the phytoplankton. The long-term mean percentage (1989 to 1996) was 77% of the total phytoplankton biovolume. Sedimentation is the major loss process for these mainly microplanktic algae. In the sediment trap they contribute, on average, 95% to the biovolume, i.e. they are overrepresented because all other phytoplankton taxa reach the bottom in a non-recognizable state or do not arrive there at all.The mean phytoplankton biovolume sedimenting during the period 1989 to 1996 was 3.5 cm3 m–2 d–1. With a time delay of two weeks the flux showed the same high variations (about 0.2 to 20 cm3 m–2 d–1) as the abundance in the pelagic. The diatom concentration in the open water has the highest influence on dynamics and amount of the phytoplankton flux, thus explaining 82% of its variance.In the annual average the calculated sedimentation rates for selected diatoms are always 2 to 2.5 times higher than the positive rates of change, whereas the grazing rates are about 10% of these positive rates only. The comparison of yearly biomass yields and losses shows similar results: The sedimentation of diatoms amounts to 1217 cm3 m–2 a–1 (mean of 8 years), i.e. 2.5 times higher than the positive yearly diatom net production of 494 cm3 m–2 a–1 observedin the reservoir for the same period. Provided there are no further essential loss components and the yearly gross production can be assumed to be the sum of annual sedimentation and grazing losses, some 13% only of the phytoplankton gross production (or 9% of diatom production) are grazed in one year, while 87% (or 91% of the diatoms) sediment.The long-term measurements show the great significance of the diatom sedimentation and confirm their function as a so-called lake internal tertiary treatment step (Grim, 1967). Under the prerequisites of a diatom water (i.e. P limitation with sufficient Si supply and adequate turbulence) high sedimentation losses guarantee the maintenance of a low trophic state. Moreover, they are the cause for the high buffering capacity, even for unusually high nutrient loads within restricted periods.
Am Beispiel waschmittelrelevanter Kenngrößen wie Tenside, Phosphor und Bor, aber auch allgemeiner Verschmutzungsparameter wie TOC, DOC, CSB und O2 wird ein differenziertes Bild der Belastungssituation wichtiger ostdeutscher Fließgewässer dargestellt. Durch Vergleich mit früheren, in der DDR durchgeführten Untersuchungen wird die Entwicklung der Gewässerbelastung dokumentiert, und es kann für verschiedene Fließgewässer nachgewiesen werden, daß bereits deutliche Verbesserungen in der Wasserqualität der Flüsse eingetreten sind. Vergleiche zwischen der Belastung ost- und westdeutscher Gewässer machen ferner Unterschiede deutlich, die offensichtlich auch auf verschieden große Einwohnerzahlen in den Gewässereinzugsgebieten sowie ungleiche hydrographische Verhältnisse zurückzuführen sind. Hierbei zeigen sich besonders in kleineren ostdeutschen Fließgewässern mit relativ großen Einzugsgebieten höhere Tensid- und Borkonzentrationen. Bei den anorganischen Waschmittelinhaltsstoffen hat sich seit der Einführung von Phosphatersatzstoffen in Waschmitteln im Jahr 1990 die Phosphatkonzentration in den meisten ostdeutschen Flüssen drastisch verringert.
The results of nutrient competition experiments performed for all the pairwise combinations of four species of freshwater algae often agreed with the predictions of a graphical model of resource competition. As predicted by theory, the species with the significantly lower resource requirement, as measured by R*, was the superior competitor when both species were limited by the same resource. Two species were observed to coexist only if either (1) each was limited by a different resource and met the theoretical criteria for coexistence or (2) the species were limited by the same resource and did not differ significantly in the their resource requirements. Single-species experiments were used to determine the functional dependence of the growth rate of each Lake Michigan diatom species on the concentration of limiting silicate or phosphate. The results, fit to the Monod (Michaelis-Menten) model, predicted that Fragilaria crotonensis and Asterionella formosa had identical silicate requirements, and that both were silicate competitors superior to Synedra filiformis and Tabellaria flocculosa. The phosphate requirements of the four species were very similar, with Synedra requiring significantly less phosphate than Tabellaria. Competition experiments demonstrated that Tabellaria was competitively displaced by the three other species for all silicate-to-phosphate ratios, as predicted. Asterionella and Fragilaria were apparently competitive equal, coexisting for all silicate-to-phosphate ratios used, suggesting evolution of similar resource requirements by these two commonly dominant species. Asterionella and Synedra coexisted for intermediate silicate-to-phosphate ratios, as did Synedra and Fragilaria.
The implementation of a conservation program since the early 1980s resulted in a reduction in phosphorus concentrations in Lake Geneva. However, in the 1990s, phytoplankton biomass increased again, almost reaching the high values recorded during the period of greatest P loading. The structural changes in the phytoplankton of Lake Geneva over the past 25 yr have been analyzed using a recently developed statistical method based on hierarchical clustering and Bayesian probabilities. This method has been used to identify phytoplankton assemblages and to map annual and interannual successional patterns simultaneously. Characteristic species were identified for each cluster after calculation of their relative species fidelity and specificity indices. Six distinct phytoplankton assemblages were identified, and although the way species are organized into communities remains unclear, the seasonal patterns of succession are consistent with the C-S-R adaptive strategies and are characteristic of temperate lakes. This pattern broadly recurred over the years, but was markedly influenced by both human activity and regional climatic changes: The warmer winters and springs recorded in Europe since 1988 led to an earlier clear-water phase. In the 1990s, the earlier and deeper depletion of dissolved inorganic phosphorus led to colonization in the summer by large species tolerant of low light levels and that could develop deeper in the water column, where phosphorus was still abundant. Their size made them less vulnerable to grazing losses, which favors their accumulation and lead to an unexpected high biomass in recent years.
We analyzed a 42-yr record of primary productivity in small, subalpine Castle Lake to determine how climatic variability might influence lake primary productivity. A Pacific Decadal Oscillation (PDO) polarity reversal in 1977 significantly affected winter air and summer water temperatures in Castle Lake. The timing of lake ice-out was explained by spring air temperature and winter total precipitation (r2=0.72) and significantly affected water temperature ( ). Primary productivity was negatively correlated with ice-out date and positively correlated with primary productivity during the previous year ( ). Alternatively, primary productivity was positively correlated with water temperature and primary productivity during the previous year (r2=0.49). Ammonium availability immediately after ice-out was significantly related to primary productivity from the previous and the current year, suggesting that nutrient availability is an important mechanism for the serial correlation. Daphnia and cyanobacteria biomass also increased during warmer years. Our results suggest that variability in air temperature and precipitation from global warming, PDO, and the El Niño Southern Oscillation (ENSO) influence primary productivity and plankton communities in North American dimictic lakes.
Phytoplankton can become limited by the availability of nutrients when light and temperature are adequate and loss rates are not excessive. The current paradigms for nutrient limitations in freshwater, estuarine, and marine environments are quite different. A review of the experimental and observational data used to infer P or N limitation of phytoplankton growth indicates that P limitation in freshwater environments can be demonstrated rigorously at several hierarchical levels of system complexity, from algal cultures to whole lakes. A similarly rigorous demonstration of N limitation has not been achieved for marine waters. Therefore, we conclude that the extent and severity of N limitation in the marine environment remain an open question. Culture studies have established that internal cellular concentrations of nutrients determine phytoplankton growth rates, and these studies have shown that it is often difficult to relate growth rates to external concentrations, especially in natural situations. This should lead to a greater reliance on the com- position of particulate matter and biomass-based physiological rates to infer nutrient limitation. Such measurements have demonstrated their utility in a wide variety of freshwater and marine environments, and, most importantly, they can be applied to systems that are difficult to manipulate experimentally or budget accurately. Dissolved nutrient concentrations are most useful in deter- mining nutrient loading rates of aquatic ecosystems. The relative proportions of nutrients supplied to phytoplankton can be a strong selective force shaping phytoplankton communities and affecting the biomass yield per unit of limiting nutrient.
For the application of the empirical model of Vollenweider and Dillon (1974). and Vollenweider (1976). to reservoirs, the critical phosphorus loading levels have been related to dissolved orthophosphate. 19 reservoirs and primary reservoirs are in optimum accordance with the extended model. To permit the use of the model also in the range of very short residence times, an extremely simple analytical model of phytoplankton growth has been introduced. The reliability of the resulting outwash limits has been checked by means of random sampling at two primary reservoirs. Among 17 samples only 1 does not fit into the model.
The examinations showed that the phosphate supply for the planktonic primary production comes almost exclusively from the influxes. The nutrient pool in the hypolimnion and in the sediment does not play a leading part. Evidently, that is often so in reservoirs.
Both the phytoplankton (the most frequent species are Asterionella formosa and Fragilaria crotonensis) and the concentration of phosphate fractions are subject to considerable seasonal variations. The Asterionella spring maximum, typical of this reservoir, is released directly or indirectly by meteorological factors. An adequate orthophosphate concentration is a basic condition which is accomplished, however, nearly every year. The autumnal Fragilaria maximum begins when a definite mixing depth hae been achieved. The breakdown of a diatom maximum is caused almost at all times by exhaustion of the phosphate reserves.
The annual average degree of phosphorus retention (both TP and OP) in reservoirs was found to depend mainly on the theoretical retention time (RT). The phosphorus retention (RP) is shown for some reservoirs in Czech Republic to be well approximated (standard deviation = 10.9) with RP(TP) = 74.7* (1-exp(-0.0255* RT)) (N = 38, 7 ≤ RT ≤ 604 days).
The inclusion of 13 additional reservoirs for which data were assembled from literature resulted in an almost identical relationship: RP(TP) = 76.1* (1-exp(-0.0282* RT)) (N = 51, range of RT nearly unchanged).
The retention of soluble reactive phosphorus RP(OP) studied on data from literature seems to be higher than that of TP.
Statistical comparison with data for RP(TP) of lakes has shown a significantly lower retention in lakes than in reservoirs for comparable RP, resulting in RP(TP) = 66.0* (1-exp(-0.00419* RT)) (N = 59, 5.8 ≤ RT ≤ 260000 days).
The differences between RP(OP) and RP(TP) and differentiation of lakes and reservoirs are discussed and a need for more thorough theoretical analysis stressed. Consequences for watershed management are summarized.
The graph of maximum filtering and grazing rates versus particle size shows that Daphnia hyalina is a microfiltrator and Eudiaptomus gracilis is a macrofiltrator. In contrast to Eudiaptomus gracilis, Daphnia hyalina is also able to utilize larger bacteria (c. 1 μm3) as a food source.
A comparative kinetic study of phosphate-limited growth and phosphate uptake was carried out in chemostat cultures of Anabaena flos-aquae Lyng. Bréb., Ankistrodesmus falcatus (Corda) Ralfs, Asterionella formosa Hass., Fragilaria crotonensis Kitt., and Microcystis sp. Lemm. For each test organism the growth rate (μ) can be described as a function of total cell phosphorus (P), i.e. cell quota (q), or hot-water-extractable P (qsp).
Phosphate uptake rate in each species examined was a function of external P concentration (S) and intracellular P levels. The substrate parameter for uptake (Km) remained constant at all growth rates, but the apparent maximum uptake rate (Vm') increased with decreasing μ, or q, in all test organisms. At low growth rates Vm' was greater than the calculated steady-state uptake rate (μq) by two to three orders of magnitude because Vm' is the uptake rate at S ≫ Km and μq is the rate at S ≪ Km. The difference between Vm' and μm decreased and virtually disappeared as μ approached the physiological maximum growth rate (μm), where q = qm and S ≫ Km.
An inverse relationship existed between Vm' and qsp which could be described by a function that mathematically resembles noncompetitive inhibition in enzyme kinetics. The inverse relation between Vm' and (q – q0) was described by a three-parameter function. This empirical function provided a good description of this inverse relation over the range from Vm' at q approaching q0 up to and including calculated net, maximum uptake rates at q = qm.