Article

Size-based interactions across trophic levels in food webs of shallow Mediterranean lakes

Authors:
  • Çankırı Karatekin Üniversitesi,Çankırı
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Abstract

Body size is a key trait of an organism which determines the dynamics of predator–prey interactions. Most empirical studies on the individual size distribution of the aquatic community have focused on the variations in body size of a single trophic level as a response to certain environmental variables or biotic factors. Few studies, however, have evaluated how individual size structure is altered simultaneously across interacting trophic levels and locations. Such comparative examinations of the size distribution in predator and prey communities may bring insight into the strength of the interactions between adjacent trophic levels. We assessed the potential predation effect of size-structured predators (i.e. predation by individuals of different sizes) on prey size structure using data from 30 shallow Turkish lakes spanning over five latitudinal degrees. We correlated size diversity and size evenness of predator and prey assemblages across the planktonic food web after accounting for the confounding effects of temperature and resource availability which may also affect size structure. We expected to find a negative relationship between size diversity of predators and prey due to the enhanced strength of top-down control with increasing predator size diversity. We also hypothesised that competitive interactions for resources in less productive systems would promote a higher size diversity. We further expected a shift towards reduced size diversity and evenness at high temperatures. In contrast to our hypothesis, we found a positive correlation between size structures of two interacting trophic levels of the planktonic food web; thus, highly size-diverse fish assemblages were associated with highly size-diverse zooplankton assemblages. The size evenness of fish and phytoplankton assemblages was negatively and positively related to temperature, respectively. Phytoplankton size diversity was only weakly predicted by the resource availability. Our results suggest that size structure within a trophic group may be controlled by the size structure at adjacent trophic levels, as well as by temperature and resource availability. The positive relationship between the size diversity of fish and zooplankton suggests that higher diversity of the resources drives a higher size diversity of consumers or vice versa, and these effects are beyond those mediated by taxonomic diversity. In contrast, the size diversity and size evenness of phytoplankton are mainly influenced by physical factors in this region and perhaps in warm shallow lakes in general.

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... Traits are useful tools to describe the function of populations and species dynamics (Pianka, 1970). Size is one of the most frequently used traits in analysing fish populations (e.g., Emmrich et al., 2011Emmrich et al., , 2014Brucet et al., 2017Brucet et al., , 2018. In particular, size diversity is a straight-forward way to estimate variation in the individual body size structure of a community (e.g., Brucet et al., 2006Brucet et al., , 2010 and can be used to reveal interas well as intraspecific variability in communities (Violle et al., 2012). ...
... Size diversity is a simple and intuitive way to assess variation in the individual body size structure of a community (Brucet et al., 2006(Brucet et al., , 2010(Brucet et al., , 2018Quintana et al., 2008;Ye et al., 2013;García-Comas et al., 2016). It is based on the Shannon diversity index (Pielou, 1969), but integrates the amplitude of the size range and evenness instead of the number of species and their relative abundance in a community (Brucet et al., 2017). Since size diversity uses individual body size, it captures inter-as well as intraspecific variability in communities, which are important for many ecological and evolutionary processes such as predator-prey relationships and competition (Violle et al., 2012). ...
... It does not require statistical fitting and is represented by a single value, where small values indicate a narrow size range while higher values indicate a wider range of sizes (Brucet et al., 2006). In general, high size diversity values are the results of equally distributed fish lengths combined with many different fish lengths (Quintana et al., 2008;Brucet et al., 2017). ...
... Most ecological studies of Mediterranean ponds have been focused on their taxonomic diversity [2], the dynamics of a certain species and communities [6][7][8][9] and its relationship with nutrients dynamics and hydrological patterns [10][11][12][13], as well as with anthropogenic pressures [14]. However, there are not many studies considering size-based interactions among adjacent trophic levels (predators and preys) of the food web [15][16][17] despite such interactions play a key role in the trophic structure and functioning of aquatic ecosystems [15][16][17][18]. It is worth to mention that size-based interactions are particularly relevant in the species-poor communities like those of Mediterranean brackish ponds where trophic interactions are mainly body-size dependent [19,20]. ...
... Most ecological studies of Mediterranean ponds have been focused on their taxonomic diversity [2], the dynamics of a certain species and communities [6][7][8][9] and its relationship with nutrients dynamics and hydrological patterns [10][11][12][13], as well as with anthropogenic pressures [14]. However, there are not many studies considering size-based interactions among adjacent trophic levels (predators and preys) of the food web [15][16][17] despite such interactions play a key role in the trophic structure and functioning of aquatic ecosystems [15][16][17][18]. It is worth to mention that size-based interactions are particularly relevant in the species-poor communities like those of Mediterranean brackish ponds where trophic interactions are mainly body-size dependent [19,20]. ...
... Size structure within a trophic group of the planktonic food web can be determined by the size structure of the adjacent trophic levels since both predation and food selection are size-dependent [16]. However, each trophic group may be affected in different ways by top-down (e.g., size-based predation) or bottom-up (e.g., size diversity of resources) controls. ...
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In this study, performed in Mediterranean brackish ponds during spring season, we assessed the effects of biotic interactions and abiotic factors on the size and taxonomic structure of the phytoplankton and zooplankton. We used a taxonomic and a size diversity index as a descriptor of the community structure. We predicted that the size diversity of each trophic level would be mainly related to biotic interactions, such as size-based fish predation (in the case of zooplankton) and food resource availability (in the case of phytoplankton), whereas taxonomic diversity would be more affected by abiotic variables (e.g., conductivity, pond morphology). Our results showed a negative relationship between phytoplankton size diversity and food resource availability leading to low size diversities under food scarcity due to dominance of small species. Conductivity also negatively affected the phytoplankton size diversity, although slightly. Regarding zooplankton size diversity, none of predictors tested seemed to influence this index. Similar fish size diversities among ponds may prevent a significant effect of fish predation on size diversity of zooplankton. As expected, taxonomic diversity of phytoplankton and zooplankton was related to abiotic variables (specifically pond morphometry) rather than biotic interactions, which are usually body size dependent, especially in these species-poor brackish environments.
... Most studies have focused on the abundance, biomass, or average size of predators and prey ( Knight, McCoy, Chase, McCoy, & Holt, 2005;Pace, Cole, Carpenter, & Kitchell, 1999). Size-based interactions between predators and prey at adjacent trophic levels in the food web have often not been considered, despite that they play a key role in the trophic structure and functioning of aquatic ecosystems ( Brose et al., 2006;Brown, Gillooly, Allen, Savage, & West, 2004;Brucet et al., 2017;Emmerson & Raffaelli, 2004). Body size is a fundamental functional trait of organisms because it is linked with life-history patterns such as reproduction, growth, and respiration ( Brown et al., 2004;Calder, 1984;Peters, 1983). ...
... temperature, productivity) can affect the individual body size structure (i.e. the distribution of the number of organisms over a size range) of aquatic communities ( Ye, Chang, Garc ıa-Comas, Gong, & Hsieh, 2013;YvonDurocher, Montoya, Trimmer, & Woodward, 2011;Zhang et al., 2013). Many of these existing investigations have focused on variations in body size structure at a single trophic level, but the different trophic groups in a food web may respond differently to biotic and environmental factors ( Brose et al., 2006;Brucet et al., 2017;Quintana et al., 2015). Changes in resource availability are known to shape phytoplankton size structure in both freshwater ( Brucet et al., 2017;Quintana et al., 2015) and marine ecosystems ( Garzke, Ismar, & Sommer, 2015;Mara~ n on, Cerme~ no, Latasa, & Tadonl ek e, 2012;Sommer, Peter, Genitsaris, & Moustaka-Gouni, 2016). ...
... Many of these existing investigations have focused on variations in body size structure at a single trophic level, but the different trophic groups in a food web may respond differently to biotic and environmental factors ( Brose et al., 2006;Brucet et al., 2017;Quintana et al., 2015). Changes in resource availability are known to shape phytoplankton size structure in both freshwater ( Brucet et al., 2017;Quintana et al., 2015) and marine ecosystems ( Garzke, Ismar, & Sommer, 2015;Mara~ n on, Cerme~ no, Latasa, & Tadonl ek e, 2012;Sommer, Peter, Genitsaris, & Moustaka-Gouni, 2016). For example, high resource availability may promote growth of large-sized bloomforming phytoplankton ( Downing, Watson, & McCauley, 2001). ...
Article
1. Trophic cascade studies have so far mostly focused on changes in the abundance, biomass, or average size of prey and predators. In contrast, individual size-based interactions, playing a key role in the trophic structure and functioning of aquatic ecosystems, have been less explored. 2. We conducted a 3-month in situ experiment in Lake Myvatn, Iceland, with two fish treatments (with and without fish, Gasterosteus aculeatus). After the first month of the experiment, Anabaena blooms appeared in the lake. We studied the effects of fish predation and occurrence of cyanobacteria blooms on the individual size structure (i.e. the distribution of the number of organisms over a size range) of zooplankton and phytoplankton. We also assessed the potential consequences for trophic transfer efficiency (TTE) (measured as the predator to prey biomass ratio) in the planktonic food web. 3. Our results showed that fish predation and cyanobacteria bloom had a negative relationship with size diversity of zooplankton, which became dominated by small-sized individuals in both cases. The phytoplankton size diversity changed over time particularly due to the blooming of large-sized Anabaena, and its increase was apparently mainly driven by changes in resources. 4. Low zooplankton size diversity related to fish predation reduced TTE, particularly in the enclosures with fish. This may be because low zooplankton size diversity represents a lower partition of resources among consumers, thereby decreasing the trophic energy transfer. With the occurrence of Anabaena bloom, high phyto-plankton size diversity coincided with a lower energy transfer in all enclosures likely due to reduced zooplankton grazing when large-sized colony-forming Ana-baena dominated. 5. In conclusion, our results indicate that both top-down and bottom-up forces significantly influence the size structure of planktonic communities. The changes in size structure were related to shifts in the energy transfer efficiency of the Lake Myvatn food web. Thus, our study underpins the importance of taking into account size-based interactions in the study of trophic cascades, particularly in a
... Identifying direct and indirect ecological interactions between invasive species and resident communities would therefore be a prerequisite for quantifying invader impacts, yet their empirical assessment through traditional methods (e.g., stomach contents) remains challenging when considering the whole food web scale (i.e., primary producers to apex predator). To answer these methodological limitations, species body size is a widely used trait to infer trophic interactions, particularly in aquatic ecosystems (Petchey et al., 2008;Gravel et al., 2013;Pomeranz et al., 2019), as it allometrically relates to most species biological rates (e.g., respiration, reproduction; Brown et al., 2004) and population characteristics (e.g., trophic levels, abundances; Brucet et al., 2017). Trophic interactions could then be scaled up to population dynamic models to investigate species persistence toward destabilizing factors at the whole food web scale (Brose et al., 2006). ...
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The impacts of invasive species on resident communities are driven by a tangle of ecological interactions difficult to quantify empirically. Combining a niche model with a population dynamic model, both allometrically parametrized, may represent a consistent framework to investigate invasive species impacts on resident communities in a food web context when empirical data are scarce. We used this framework to assess the ecological consequences of an invasive apex predator (Silurus glanis) in peri-Alpine lake food webs. Both increases and decreases of resident species abundances were highlighted and differed when accounting for different S. glanis body sizes. Complementarily, the prominence of indirect effects, such as trophic cascades, suggested that common approaches may only capture a restricted fraction of invasion consequences through direct predation or competition. By leveraging widely available biodiversity data, our approach may provide relevant insights for a comprehensive assessment and management of invasive species impacts on aquatic ecosystems.
... In practice, restoring macrophytes in large floodplain lakes may be hindered by large seasonal fluctuations of water levels [25]. The long-term fishing ban could alter the body sizes of zooplankton and thus modify the sizes and species of algae without changing the number of population [56] and prevent macrophytes from recovery due to more herbivorous fish grazing on plants. ...
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Exogenous drivers may cause a gradual and reversible change in a lake equilibrium, or they may force it over a threshold to a persistent alternative stable state, described as a regime shift in the ecosystem. In the mid-and-lower Yangtze River Basin (MLYB), major environmental problems in shallow lakes have been eutrophication and abrupt algal blooms under anthropogenic disturbances for the recent century. Much value is therefore placed on understanding the changes in shallow-lake ecosystems that characteristically precede changes in the state of the lake. Here, we describe a case study of the paleolimnological signature in diatom assemblages of various types of regime shifts caused by historically documented anthropogenic drivers in a temperate shallow lake: Taibai Lake. We evaluate the effectiveness of paleolimnological data as a surrogate for long-term monitoring. Algorithms using sequential t and F statistics detected breakpoints in the time series of diatom assemblages, in 1994–1996, 1974–1977, 1952–1956, and 1931–1934, respectively. The regression statistics suggest that the hydrodynamic–ecosystem and aquacultural–ecosystem relationships fit better in the breakpoint regression model, and the relationship between nutrient loading and ecosystem state suits the linear model. Feedback loops help reconstruct dynamic changes in Taibai influenced by major stressors. Our study exemplifies the value of system approaches to identifying regime shifts and their possible causes in shallow lakes from paleolimnological records. The case study of Taibai set an example of reconstructing the ecological regime shifts in shallow lakes in the MLYB and understanding the state changes in lake ecosystems, which will benefit effective lake management.
... For S. glanis, the Shannon continuous index (prey body size diversity) and the Evenness (prey body size evenness) were calculated according to Quintana et al. (2016) and Brucet et al. (2017) to inform on the prey body size structure along the S. glanis body size gradient as follows: ...
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Identifying the trophic attributes of co-occurring invaders is a prerequisite to anticipate and mitigate their simultaneous impacts. We used an allometric niche model (aNM) to infer the trophic attributes of a mysid (Hemimysis anomala) and a catfish (Silurus glanis) that recently colonized the largest natural lake in France (Lake Bourget) and we raised the potential consequences of this multispecies invasion on this ecosystem following changes in trophic links in the invaded food web. H. anomala was predicted to mainly rely on primary producers, while S. glanis was expected to experience an ontogenetic change in its trophic attributes (i.e., a diet shift from invertebrate to fish at ~ 90 cm and no more predators at ~ 60 cm). At the whole-lake scale, both invaders would have additive impacts, by competing for resources (diet overlap) with a set of resident species and increasing the trophic vulnerability of fish, invertebrates and primary producers. The trophic link changes caused by the current S. glanis population (i.e., mainly composed of 40–60 cm individuals) and a “future” population (i.e., higher proportion of > 100 cm individuals) suggested an increasing predation pressure on resident fish. In both scenarios, both invaders were also highlighted to be putative prey for resident species, especially H. anomala. The aNM represents a flexible tool for biological invasion management, taking advantage of widely available monitoring data, to identify vulnerable resident species and implement early management actions (e.g. size-based functional controls) when empirical data on invaders are sparse, particularly in the early phases of their colonization.
... Such type of sensors generate a detailed description of temporal variation in temperature and oxygen dynamics and arise as an integrative indicator of ecosystem functioning. Ecosystem responses to temperature have increased its relevance under a climate change scenario (Brucet et al. 2010(Brucet et al. , 2017Jeppesen et al. 2010;Meerhoff et al. 2012). Moreover, fluctuations in oxygen values have been related to trophic state (Vollenweider and Kerekes 1982;Nielsen et al. 2013). ...
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Zooplankton assemblages in the confined coastal lagoons of La Pletera salt marshes (Baix Ter wetlands, Girona, Spain) are dominated by two species: one calanoid copepod ( Eurytemora velox ) and the other rotifer ( Brachionus gr. plicatilis ). They alternate as the dominant species (more than 80% of total zooplankton biomass), with the former being dominant in winter and the latter in summer. Shifts between these taxa are sudden, and intermediate situations usually do not last more than 1 month. Although seasonal shifts between zooplankton dominant species appear to be related with temperature, other factors such as trophic state or oxygen concentration may also play an important role. Shifts between species dominances may be driven by thresholds in these environmental variables. However, according to the alternative stable states theory, under conditions of stable dominance a certain resistance to change may exist, causing that gradual changes might have little effect until a tipping point is reached, at which the reverse change becomes much more difficult. We investigated which are the possible factors causing seasonal zooplankton shifts. We used high-frequency temperature and oxygen data provided by sensors installed in situ to analyse if shifts in zooplankton composition are determined by a threshold in these variables or, on the other hand, some gradual change between stable states occur. Moreover, following the postulates of the alternative stable states theory, we looked at possible hysteresis to analyse if these seasonal zooplankton shifts behave as critical transitions between two different equilibriums. We also examined if top-down or bottom-up trophic interactions affect these zooplankton shifts. Our results show that shifts between dominant zooplankton species in La Pletera salt marshes are asymmetric. The shift to a Eurytemora situation is mainly driven by a decrease in temperature, with a threshold close to 19 °C of daily average temperature, while the shift to Brachionus does not. Usually, the decrease in water temperature is accompanied by a decrease in oxygen oscillation with values always close to 100% oxygen saturation. Moreover, oxygen and temperature values before the shift to calanoids are different from those before the reverse shift to Brachionus , suggesting hysteresis and some resistance to change when a critical transition is approaching. Top-down and bottom-up forces appear to have no significant effect on shifts, since zooplankton biomass was not negatively correlated with fish biomass and was not positively related with chlorophyll, in overall data or within shifts.
... We examined the zooplankton size distribution from 148 samples collected exclusively during summer months among 13 years in the upper and lower pelagic layers of a deep mesotrophic lake. To add robustness to our analyses, we expressed the size distributions as SS based on log-binning, as continuous SS and by the size diversity, a measure that has been developed to mimic taxonomic diversity indices (Brucet et al. 2006(Brucet et al. , 2010(Brucet et al. , 2017Quintana et al. 2008Quintana et al. , 2016. We expected that both annually varying predation by the dominant pelagic planktivorous fish and resource availability via phytoplankton biomass would affect the year-to-year variation of the zooplankton size distributions. ...
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Trophic interactions in the pelagic area of lakes and the opposing effects of fish feeding (top-down) and phytoplankton biomass (bottom-up) on zooplankton communities are central topics in limnology. We hypothesized that zooplankton size distributions should be a more sensitive approach to disentangle top-down and bottom-up effects than the commonly measured zooplankton biomass. We examined zooplankton size distributions from 148 samples collected during summer months in the upper and lower pelagic layers of a deep mesotrophic lake among 13 years of sampling. Top-down effects, namely fish size and biomass, and bottom-up effects, including water temperature and total phosphorus and chlorophyll a concentrations, were considered. To add robustness to our analyses, we expressed the zooplankton size distributions as size spectra based on log-binning, as continuous size spectra and by the size diversity, a measure that has been developed to mimic taxonomic diversity indices. Among numerous regressions tested, significant top-down or bottom-up effects could rarely been detected. Our results indicate that the overall zooplankton size distribution was not significantly affected by fish predation and lake productivity measured as total phosphorus or chlorophyll a concentration. However, we found negative correlations between fish biomass and the preferred zooplankton prey, including Bosmina longirostris, Daphnia cucullata and nauplii in the upper depth layer. However, due to their small body size, low biomass and therefore relative small contribution to the zooplankton size distribution, predation on preferred zooplankton species did not translate into a statistically significant modification of the entire size spectrum. Consequently, the size spectrum seems to be relatively robust against predation effects, but might reflect the lake-wide energy availability and transfer efficiency in the food web.
... In our case, it integrates the amplitude of the length range and relative abundance of the different lengths. Thus, the high values of the size diversity would indicate a high diversity of sizes with an equitable numerical frequency of sizes along the distributions [57,58]. In contrast, the low values of size diversity (rarely taking negative values) would indicate a low diversity of fish sizes with an inequitable numerical frequency of sizes along the distribution [43]. ...
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Aphanius iberus is an endemic cyprinodontoid fish species of Mediterranean ponds in danger of extinction. In this study, we studied some abiotic and biotic factors that can influence A. iberus’s size structure and density in Mediterranean brackish ponds. We sampled fish using fyke nets in 10 ponds of Empordà (Spain) during the spring season. Our results showed that a better ecological status (according to the Water Quality of Lentic and Shallow Ecosystems (QAELS) index), pond’s depth and pond’s isolation (reflected by an increase in total nitrogen) were related to larger individual sizes and more size-diverse populations. Increasing the salinity is known to help the euryhaline A. iberus acting as a refuge from competitors. Nevertheless, our results showed that higher conductivities had a negative effect on A. iberus’s size structure, leading to a decrease in the mean and maximum size of the fish. Fish abundance (expressed as captures per unit of effort (CPUE)) seemed to increase with increasing the pond’s depth and total nitrogen (the latter reflecting pond isolation). In conclusion, our results suggest that achieving a better pond ecological status may be important for the conservation of endangered A. iberus, because better size-structured populations (i.e., larger mean and average lengths) were found at higher water quality conditions.
... Phytoplankton size diversity did not show significant responses to the treatments throughout the study period, and negative values were rare (only 3 of the 216 samples analyzed). Size diversity revealed overall high values (the different sizes classes contributed relatively equally to the size distribution, [87]). However, size evenness decreased in both HN-A2 and HN-A2+ during and after the HW and showed interaction effects between nutrient and both A2 and A2+ warming ( Table 2). ...
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Phytoplankton usually responds directly and fast to environmental fluctuations, making them useful indicators of lake ecosystem changes caused by various stressors. Here, we examined the phytoplankton community composition before, during, and after a simulated 1-month heat wave in a mesocosm facility in Silkeborg, Denmark. The experiment was conducted over three contrasting temperature scenarios (ambient (A0), Intergovernmental Panel on Climate Change A2 scenario (circa +3 • C, A2) and A2+ %50 (circa +4.5 • C, A2+)) crossed with two nutrient levels (low (LN) and high (HN)) with four replicates. The facility includes 24 mesocosms mimicking shallow lakes, which at the time of our experiment had run without interruption for 11 years. The 1-month heat wave effect was simulated by increasing the temperature by 5 • C (1 July to 1 August) in A2 and A2+, while A0 was not additionally heated. Throughout the study, HN treatments were mostly dominated by Cyanobacteria, whereas LN treatments were richer in genera and mostly dominated by Chlorophyta. Linear mixed model analyses revealed that high nutrient conditions were the most important structuring factor, which, regardless of temperature treatments and heat waves, increased total phytoplankton, Chlorophyta, Bacillariophyta, and Cyanobacteria biomasses and decreased genus richness and the grazing pressure of zooplankton. The effect of temperature was, however, modest. The effect of warming on the phytoplankton community was not significant before the heat wave, yet during the heat wave it became significant, especially in LN-A2+, and negative interaction effects between nutrient and A2+ warming were recorded. These warming effects continued after the heat wave, as also evidenced by Co-inertia analyses. In contrast to the prevailing theory stating that more diverse ecosystems would be more stable, HN were less affected by the heat wave disturbance, most likely because the dominant phytoplankton group cyanobacteria is adapted to high nutrient conditions and also benefits from increased temperature. We did not find any significant change in phytoplankton size diversity, but size evenness decreased in HN as a result of an increase in the smallest and largest size classes simultaneously. We conclude that the phytoplankton community was most strongly affected by the nutrient level, but less sensitive to changes in both temperature treatments and the heat wave simulation in these systems, which have been adapted for a long time to Water 2020, 12, 3394 2 of 21 different temperatures. Moreover, the temperature and heat wave effects were observed mostly in LN systems, indicating that the sensitivity of phytoplankton community structure to high temperatures is dependent on nutrient availability.
... Similar conditions are found in north temperate, relatively nutrient-poor lowland lakes (e.g., Jeppesen et al. 2000). A study by Brucet et al. (2017) on the predation effect of size-structured predators (i.e., predation by individuals of different sizes) on prey size structure, using data from some of the current study lakes, revealed that highly size-diverse fish assemblages including large piscivorous fish occurring in the northern highland lakes were associated with highly size-diverse zooplankton assemblages, comprising also large-sized cladocerans, as we found in the highland lakes. Meanwhile many of the characteristics of the lowland lakes were shared with southern highland lakes, being overall larger, more saline, and more eutrophic, with higher TN and Chl-a concentrations, cyanobacteria biomass, and a larger proportion of small fish ( Fig. 2 and 3). ...
Article
Climate warming threatens the structure and function of shallow lakes, not least those in the Mediterranean climate. We used a space-for-time substitution approach to assess the response of trophic and community structures as well as the richness and evenness of multiple trophic levels to temperature, hydrological, and nutrient constraints. We selected 41 lakes covering wide climatic, hydrological, and nutrient gradients within a short distance for reducing the effect of biogeographical factors in the western Anatolian plateau of Turkey. Generalized linear model analyses revealed that temperature was overall the most important driving variable, followed by total nitrogen (TN) and salinity. The chlorophyll a:total phosphorus ratio, the cyanobacteria:total phytoplankton biovolume ratio, the fish:zooplankton biomass ratio, the proportion of small fish, and fish richness increased with increasing temperature, whereas macrophyte plant volume inhabited (PVI, %), richness, and evenness decreased. Grazing pressure, macrophyte coverage, piscivore biomass, phytoplankton richness, and evenness decreased significantly with both increasing TN and temperature. Temperature and nutrients also separated the northern highland lakes from other lakes in a non-metric multidimensional scaling analysis. Additionally, salinity reduced richness and evenness of phytoplankton and zooplankton. Our results indicate major changes in lake structure and functioning with warming and eutrophication, and highlight the need for strict control of nutrients and water use.
... These diverse examples demonstrate that size spectra models can be applied in many different contexts. Furthermore, when the necessary adjustments are made to the sampling methods, size spectra analyses are applicable to other types of ecosystems, including large rivers 48,49,50 , lakes 51,52,53 , and marine environments 54,55,56 . ...
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Freshwater ecosystems are becoming saltier due to human activities. The effects of increased salinity can lead to cascading trophic interactions, affecting ecosystem functioning and energy transfer, through changes in community and size structure. These effects can be modulated by other environmental factors, such as nutrients. For example, communities developed under eutrophic conditions could be less sensitive to salinization due to cross-tolerance mechanisms. In this study, we used a mesocosm approach to assess the effects of a salinization gradient on the zooplankton community composition and size structure under eutrophic conditions and the cascading effects on algal communities. Our results showed that zooplankton biomass, size diversity and mean body size decreased with increased chloride concentration induced by salt addition. This change in the zooplankton community did not have cascading effects on phytoplankton. The phytoplankton biomass decreased after the chloride concentration threshold of 500 mg L⁻¹ was reached, most likely due to direct toxic effects on the osmotic regulation and nutrient uptake processes of certain algae rather than as a response to community turnover or top-down control. Our study can help to put in place mitigation strategies for salinization and eutrophication, which often co-occur in freshwater ecosystems.
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Elevated lake chloride concentration has been observed in many regions, due to human activities such as mining, agriculture, and urbanisation. Meanwhile, lakes are also experiencing increasing frequency and intensity of heatwaves. The combination of elevated salinity and heatwaves has not been thoroughly studied in freshwater communities, limiting our ability to predict outcomes of future disturbances. We conducted a mesocosm experiment to investigate the individual and interactive effects of increased salinity and heatwaves on a freshwater zooplankton community. The combined effects of the two stressors were examined in two scenarios: when they occurred simultaneously and when a heatwave was preceded by an 8‐week increase in salinity. We expected to see a synergistic effect when the two stressors were applied simultaneously, as organisms might experience energy deficiency due to physiological changes caused by salinity stress and be overwhelmed by the heat treatment. When the two stressors were applied sequentially, we expected them to act independently as the two stressors trigger different physiological responses and physiological homeostasis may have already recovered from previous salt exposure and not influence an organism's response to a subsequent stressor. Individually, increased salinity and heatwave conditions both impaired zooplankton communities with largest effects on copepod nauplii and cladocerans. Together, these stressors caused antagonistic effects on total zooplankton abundance and biomass in both the simultaneous and sequential scenarios, with the combined effects being similar to the salt‐only effects. Our experiment illustrates the potential for heatwaves to have hidden effects when they occur in lakes experiencing salinisation. The findings suggested that the two stressors negatively impacted some zooplankton taxonomic groups, and at the community level, they acted antagonistically such that the occurrence of a 3‐day heatwave did not cause any additional loss of abundance or biomass regardless of whether the community was exposed to the sequential or simultaneous scenario. Our findings also illustrated that even when the two stressors were decoupled in time, the community could still be influenced by a previous stressor.
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Planktivorous fish predation directly affects zooplankton biomass, community and size structure, and may indirectly induce a trophic cascade to phytoplankton. However, it is not clear how quickly the zooplankton community structure and the cascading effects on phytoplankton recover to the unaffected state (i.e. resilience) once short-term predation by fish stops. The resilience has implications for the ecological quality and restoration measures in aquatic ecosystems. To assess the short-term zooplankton resilience against fish predation, we conducted a mesocosm experiment consisting of 10 enclosures, 6 with fish and 4 without fish. Plankton communities from a natural lake were used to establish phytoplankton and zooplankton in the mesocosms. High biomasses (about 20 g wet mass m ⁻³ ) of juvenile planktivorous fish (perch, Perca fluviatilis) were allowed to feed on zooplankton in fish enclosures for four days. Thereafter, we removed fish and observed the recovery of the zooplankton community and its cascading effect on trophic interactions in comparison with no fish enclosures for four weeks. Short-term fish predation impaired resilience in zooplankton community by modifying community composition, as large zooplankton, such as calanoids, decreased just after fish predation and did not re-appear afterwards, whereas small cladocerans and rotifers proliferated. Total zooplankton biomass increased quickly within two weeks after fish removal, and at the end even exceeded the biomass measured before fish addition. Despite high biomass, the dominance of small zooplankton released phytoplankton from grazer control in fish enclosures. Accordingly, the zooplankton community did not recover from the effect of fish predation, indicating low short-term resilience. In contrast, in no fish enclosures without predation disturbance, a high zooplankton:phytoplankton biomass ratio accompanied by low phytoplankton yield (Chlorophyll-a:Total phosphorus ratio) reflected phytoplankton control by zooplankton over the experimental period. Comprehensive views on short and long-term resilience of zooplankton communities are essential for restoration and management strategies of aquatic ecosystems to better predict responses to global warming, such as higher densities of planktivorous fish. © 2019 Ersoy et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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The size spectrum of an ecological community characterises how a property, such as abundance or biomass, varies with body size. Size spectra are often used as ecosystem indicators of marine systems. They have been fitted to data from various sources, including groundfish trawl surveys, visual surveys of fish in kelp forests and coral reefs, sediment samples of benthic invertebrates and satellite remote-sensing of chlorophyll. 2.Over the past decades several methods have been used to fit size spectra to data. We document eight such methods, demonstrating their commonalities and differences. Seven methods use linear regression (of which six require binning of data), while the eighth uses maximum likelihood estimation. We test the accuracy of the methods on simulated data. 3.We demonstrate that estimated size-spectrum slopes are not always comparable between the seven regression-based methods because such methods are not estimating the same parameter. We find that four of the eight tested methods can sometimes give reasonably accurate estimates of the exponent of the individual size distribution (which is related to the slope of the size spectrum). However, sensitivity analyses find that maximum likelihood estimation is the only method that is consistently accurate, and the only one that yields reliable confidence intervals for the exponent. 4.We therefore recommend the use of maximum likelihood estimation when fitting size spectra. To facilitate this we provide documented R code for fitting and plotting results. This should provide consistency in future studies and improve the quality of any resulting advice to ecosystem managers. In particular, the calculation of reliable confidence intervals will allow proper consideration of uncertainty when making management decisions. This article is protected by copyright. All rights reserved.
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Body size exerts multiple effects on plankton food-web interactions. However, the influence of size structure on trophic transfer remains poorly quantified in the field. Here, we examine how the size diversity of prey (nano-microplankton) and predators (mesozooplankton) influence trophic transfer efficiency (using biomass ratio as a proxy) in natural marine ecosystems. Our results support previous studies on single trophic levels: transfer efficiency decreases with increasing prey size diversity and is enhanced with greater predator size diversity. We further show that communities with low nano-microplankton size diversity and high mesozooplankton size diversity tend to occur in warmer environments with low nutrient concentrations, thus promoting trophic transfer to higher trophic levels in those conditions. Moreover, we reveal an interactive effect of predator and prey size diversities: the positive effect of predator size diversity becomes influential when prey size diversity is high. Mechanistically, the negative effect of prey size diversity on trophic transfer may be explained by unicellular size-based metabolic constraints as well as trade-offs between growth and predation avoidance with size, whereas increasing predator size diversity may enhance diet niche partitioning and thus promote trophic transfer. These findings provide insights into size-based theories of ecosystem functioning, with implications for ecosystem predictive models.
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We conducted a fish survey in 40 lakes in western and central Turkey. Fifty species (one to eleven per lake) were recorded, including eighteen endemic and seven alien species. We investigated which local geo-climatic and other environmental variables shaped the fish assemblages. Altitude and temperature turned out to be the most important factors for total species richness as well as richness of omnivorous and zooplanktivorous species and the Shannon–Wiener diversity index, with more species and higher diversity occurring in the warmer lowland lakes. Altitude may affect the fish assemblage directly through dispersal limitation or indirectly by creating a gradient in temperature with which it was strongly correlated. Cyprinidae was the most species-rich and widespread family. Atherinidae, Gobiidae, and Mugilidae (families of marine origin) were mainly found in the lowland regions, while Salmonidae exclusively appeared in the high-altitude lakes. The presence of widely distributed translocated native and alien species revealed a large human impact on the fish assemblages, potentially threatening the rich endemic fish fauna in lakes in this region.
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Planktivorous and benthivorous fish have been documented to influence the density and size structure of their prey communities in lakes. We hypothesized that piscivorous fish modify their prey fish communities in the same way and sought to find evidence for such predation effects from a comparison across 356 lakes located in nine European ecoregions.Wecategorized individual fish as being piscivore, nonpiscivore, or prey of piscivores, depending on species and individual size. We calculated piscivore, nonpiscivore, and piscivore prey densities, respectively, and fit linear abundance size spectra (SS) on lake-specific piscivore, nonpiscivore, and piscivore-prey size distributions. Multiple linear regressions were calculated to quantify the effect of piscivore density and SS slopes on nonpiscivore and piscivore-prey densities and SS slopes by accounting for potentially confounding factors arising from lake morphometry, productivity, and local air temperature. Piscivore density correlated positively with piscivore-prey density but was uncorrelated with density of nonpiscivores. Across a subset of 76 lakes for which SS slopes of piscivores were statistically significant, SS slopes of piscivores were uncorrelated with SS slopes of either nonpiscivores or piscivore prey. However, densities of piscivores, nonpiscivores, or piscivore prey were a significant negative predictor of SS slopes of the respective groups. Our analyses suggest that direct predation effects by piscivorous fish on density and size structure of prey fish communities are weak in European lakes, likely caused by low predator–prey size ratios and the resulting size refuges for prey fish. In contrast, competition may substantially contribute to between-lake variability in fish density and size.
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The most suitable method for estimation of size diversity is investigated. Size diversity is computed on the basis of the Shannon diversity expression adapted for continuous variables, such as size. It takes the form of an integral involving the probability density function (pdf) of the size of the individuals. Different approaches for the estimation of pdf are compared: parametric methods, assuming that data come from a determinate family of pdfs, and nonparametric methods, where pdf is estimated using some kind of local evaluation. Exponential, generalized Pareto, normal, and log-normal distributions have been used to generate simulated samples using estimated parameters from real samples. Nonparametric methods include discrete computation of data histograms based on size intervals and continuous kernel estimation of pdf. Kernel approach gives accurate estimation of size diversity, whilst parametric methods are only useful when the reference distribution have similar shape to the real one. Special attention is given for data standardization. The division of data by the sample geometric mean is proposed as the most suitable standardization method, which shows additional advantages: the same size diversity value is obtained when using original size or log-transformed data, and size measurements with different dimensionality (longitudes, areas, volumes or biomasses) may be immediately compared with the simple addition of ln k where k is the dimensionality (1, 2, or 3, respectively). Thus, the kernel estimation, after data standardization by division of sample geometric mean, arises as the most reliable and generalizable method of size diversity evaluation.
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Omnivory is important in determining species interactions and weakening possible trophic cascade effects. The present study is focused on determining if an indirect effect of fish presence (trophic cascade effect) can be observed regardless of a high abundance of omnivores. With this aim a field experiment was carried out in a Mediterranean salt marsh. The natural top predator of these systems is a small and endangered fish, the Iberian toothcarp (Aphanius iberus). We wanted to assess if the presence of this fish could trigger a trophic cascade in the aquatic community. Six mesocosms were installed in a salt marsh lagoon without A. iberus presence, for two months. Twenty adult females of A. iberus were added to three mesocosms (fish treatment), while the other three were used as control (without any A. iberus addition). Physical and chemical water characteristics, as well as aquatic invertebrate samples (from bacteria to amphipoda, the largest organisms detected beside fish) were collected at the begining and the end of the experiment. To assess the trophic position of organisms, stable isotope analyses were used (13C and 15N). Our results show that the presence of A. iberus produces a trophic cascade even though only two trophic levels were differentiated by the stable isotope analysis. Body size appears to be determinant, since trophic interactions observed within the food web were mainly body-size dependent. Moreover, an unexpected positive but weak effect of A. iberus on macrophyte biomass was observed, caused probably by the decrease of small phytoplankton abundance. This effect may suppose an advantage for A. iberus, since macrophytes provide refuge and a food supply for this small fish.
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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.
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We assessed the importance of temperature, salinity, and predation for the size structure of zooplankton and provided insight into the future ecological structure and function of shallow lakes in a warmer climate. Artificial plants were introduced in eight comparable coastal shallow brackish lakes located at two contrasting temperatures: cold-temperate and Mediterranean climate region. Zooplankton, fish, and macroinvertebrates were sampled within the plants and at open-water habitats. The fish communities of these brackish lakes were characterized by small-sized individuals, highly associated with submerged plants. Overall, higher densities of small planktivorous fish were recorded in the Mediterranean compared to the cold-temperate region, likely reflecting temperature-related differences as have been observed in freshwater lakes. Our results suggest that fish predation is the major control of zooplankton size structure in brackish lakes, since fish density was related to a decrease in mean body size and density of zooplankton and this was reflected in a unimodal shaped biomass-size spectrum with dominance of small sizes and low size diversity. Salinity might play a more indirect role by shaping zooplankton communities toward more salt-tolerant species. In a global-warming perspective, these results suggest that changes in the trophic structure of shallow lakes in temperate regions might be expected as a result of the warmer temperatures and the potentially associated increases in salinity. The decrease in the density of large-bodied zooplankton might reduce the grazing on phytoplankton and thus the chances of maintaining the clear water state in these ecosystems.
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A recently documented correlate of anthropogenic cli-mate change involves reductions in body size, the nature and scale of the pattern leading to suggestions of a third universal response to climate warming. Because body size affects thermoregulation and energetics, changing body size has implications for resilience in the face of climate change. A review of recent studies shows het-erogeneity in the magnitude and direction of size responses, exposing a need for large-scale phylogeneti-cally controlled comparative analyses of temporal size change. Integrative analyses of museum data combined with new theoretical models of size-dependent thermo-regulatory and metabolic responses will increase both understanding of the underlying mechanisms and phys-iological consequences of size shifts and, therefore, the ability to predict the sensitivities of species to climate change. Body size reductions: is the signal clear and what does it matter? A recently documented correlate of rising temperatures associated with anthropogenic climate change involves reductions in the body size of many organisms. This phe-nomenon is being reported from a growing number of species on multiple continents and appears to apply to both endotherms and ectotherms, in both terrestrial and aquatic environments (Table 1). The geographic pattern and phylogenetic scale of findings to date suggest a broad-scale physiological response to some major environmental change over the past 50–100 years. This has led to the suggestion that body-size reduction is a third universal response to global warming [1], alongside changes in the phenology [2] and distributions [3] of species. Changes in body size have important implications for the thermal biology and energetics of endotherms and ectotherms. This is because body size directly affects ener-gy and water requirements for thermoregulation [4–6], energy and mass acquisition and utilization rates [7] and life-history characteristics [8,9]. Changing body size will, therefore, have implications for resilience in the face of climate change. Here, we examine the relationship
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Biomass distribution and energy flow in ecosystems are traditionally described with trophic pyramids, and increasingly with size spectra, particularly in aquatic ecosystems. Here, we show that these methods are equivalent and interchangeable representations of the same information. Although pyramids are visually intuitive, explicitly linking them to size spectra connects pyramids to metabolic and size-based theory, and illuminates size-based constraints on pyramid shape. We show that bottom-heavy pyramids should predominate in the real world, whereas top-heavy pyramids indicate overestimation of predator abundance or energy subsidies. Making the link to ecological pyramids establishes size spectra as a central concept in ecosystem ecology, and provides a powerful framework both for understanding baseline expectations of community structure and for evaluating future scenarios under climate change and exploitation.
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We know little about how ecosystems of different complexity will respond to global warming1±5. Microcosms permit experimental control over species composition and rates of environmental change. Here we show using microcosm experiments that extinc- tion risk in warming environments depends on trophic position but remains unaffected by biodiversity. Warmed communities disproportionately lose top predators and herbivores, and become increasingly dominated by autotrophs and bacterivores. Changes in the relative distribution of organisms among trophically de®ned functional groups lead to differences in ecosystem func- tion beyond those expected from temperature-dependent physio- logical rates. Diverse communities retain more species than depauperate ones, as predicted by the insurance hypothesis, which suggests that high biodiversity buffers against the effects of environmental variation because tolerant species are more likely to be found6,7. Studies of single trophic levels clearly show that warming can affect the distribution and abundance of species2,4,5, but complex responses generated in entire food webs greatly complicate inferences based on single functional groups.
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Trait-based approaches are increasingly used in ecology. Phytoplankton communities, with a rich history as model systems in community ecology, are ideally suited for applying and further developing these concepts. Here we summarize the essential components of trait-based approaches and review their historical and potential application to illuminating phytoplankton community ecology. Major ecological axes relevant to phytoplankton include light and nutrient acquisition and use, natural enemy interactions, morphological variation, temperature sensitivity, and modes of reproduction. Trade-offs between these traits play key roles in determining community structure. Freshwater and marine environments may select for a different suite of traits owing to their different physical and chemical properties. We describe mathematical techniques for integrating traits into measures of growth and fitness and predicting how community structure varies along environmental gradients. Finally, we outline challenges and future...
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One important aspect of climate change is the increase in average temperature, which will not only have direct physiological effects on all species but also indirectly modifies abundances, interaction strengths, food-web topologies, community stability and functioning. In this theme issue, we highlight a novel pathway through which warming indirectly affects ecological communities: by changing their size structure (i.e. the body-size distributions). Warming can shift these distributions towards dominance of small- over large-bodied species. The conceptual, theoretical and empirical research described in this issue, in sum, suggests that effects of temperature may be dominated by changes in size structure, with relatively weak direct effects. For example, temperature effects via size structure have implications for top-down and bottom-up control in ecosystems and may ultimately yield novel communities. Moreover, scaling up effects of temperature and body size from physiology to the levels of populations, communities and ecosystems may provide a crucially important mechanistic approach for forecasting future consequences of global warming.
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We present a new map depicting the first global biogeographic regionalization of Earth's freshwater systems. This map of freshwater ecoregions is based on the distributions and compositions of freshwater fish species and incorporates major ecological and evolutionary patterns. Covering virtually all freshwater habitats on Earth, this ecoregion map, together with associated species data, is a useful tool for underpinning global and regional conservation planning efforts (particularly to identify outstanding and imperiled freshwater systems); for serving as a logical framework for large-scale conservation strategies; and for providing a global-scale knowledge base for increasing freshwater biogeographic literacy. Preliminary data for fish species compiled by ecoregion reveal some previously unrecognized areas of high biodiversity, highlighting the benefit of looking at the world's freshwaters through a new framework.
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Climate change has complex structural impacts on coastal ecosystems. Global warming is linked to a widespread decline in body size, whereas increased flood frequency can amplify nutrient enrichment through enhanced run-off. Altered population body-size structure represents a disruption in top-down control, whereas eutrophication embodies a change in bottom-up forcing. These processes are typically studied in isolation and little is known about their potential interactive effects.Here, we present the results of an in situ experiment examining the combined effects of top-down and bottom-up forces on the structure of a coastal marine community. Reduced average body mass of the top predator (the shore crab, Carcinus maenas) and nutrient enrichment combined additively to alter mean community body mass. Nutrient enrichment increased species richness and overall density of organisms. Reduced top-predator body mass increased community biomass. Additionally, we found evidence for an allometrically induced trophic cascade. Here, the reduction in top-predator body mass enabled greater biomass of intermediate fish predators within the mesocosms. This, in turn, suppressed key micrograzers, which led to an overall increase in microalgal biomass. This response highlights the possibility for climate-induced trophic cascades, driven by altered size structure of populations, rather than species extinction.
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We recorded vertical profiles of size distributions of large particles (> 100 mu m) to a 1000-m depth in the Atlantic, Indian, and Pacific Oceans and in the Mediterranean Sea with the Underwater Video Profiler. Of the 410 profiles used in our analysis, 193 also included temperature, salinity, and high-performance liquid chromatography (HPLC)-resolved pigments, which were used to characterize the size structure of the phytoplankton community. Classification analysis identified six clusters of vertical profiles of size distributions of particles. Each cluster was characterized by the size distribution of its particles in the mesopelagic layer and the change of the particle-size distribution with depth. Clusters with large particles in the mesopelagic layer corresponded to surface waters dominated by microphytoplankton, and those with small particles corresponded to surface waters dominated by picophytoplankton. We estimated the mass flux at 400 m using a relationship between particle size and mass flux. Principal-component regression analysis showed that 68% of the variance of the mass flux at 400 m was explained by the size structure of the phytoplankton community and integrated chlorophyll a in the euphotic zone. We found that coefficient k in the Martin power relationship, which describes the decrease in the vertical mass flux with depth, varies between 0.2 and 1.0 in the world ocean, and we provided an empirical relationship to derive k from the size structure of phytoplankton biomass in the euphotic zone. Biogeochemists and modelers could use that relationship to obtain a realistic description of the downward particle flux instead of using a constant k value as often done.
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A method for the measurement of the size diversity based on the classical Shannon–Wiener expression was proposed as a proxy of the shape of the size distribution. The summatory of probabilities of a discrete variable (such as species relative abundances) in the original Shannon–Wiener expression was substituted by an integral of the probability density function of a continuous variable (such as body size). Here, we propose an update of this method by including the measurement of the size e-evenness, just dividing the exponential of the size diversity by its possible maximum for a given size range. Assuming a domain of the size range of (0,1), for a given logarithmic mean (m ln) and a logarithmic standard deviation r ln ð Þ, the distribution with the highest diversity is the Log-Normal. The size e-evenness ranges between 0 and 1 because of the division by the maximum exponential diversity. Size e-evenness is useful to discriminate whether variations in size diversity are due to changes in the shape of the size distribution or caused by differences in size dispersion.
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Understanding the consequences of species loss in complex ecological communities is one of the great challenges in current biodiversity research. For a long time, this topic has been addressed by traditional biodiversity experiments. Most of these approaches treat species as trait-free, taxonomic units characterizing communities only by species number without accounting for species traits. However, extinctions do not occur at random as there is a clear correlation between extinction risk and species traits. In this review, we assume that large species will be most threatened by extinction and use novel allometric and size-spectrum concepts that include body mass as a primary species trait at the levels of populations and individuals, respectively, to re-assess three classic debates on the relationships between biodiversity and (i) food-web structural complexity, (ii) community dynamic stability, and (iii) ecosystem functioning. Contrasting current expectations, size-structured approaches suggest that the loss of large species, that typically exploit most resource species, may lead to future food webs that are less interwoven and more structured by chains of interactions and compartments. The disruption of natural body-mass distributions maintaining food-web stability may trigger avalanches of secondary extinctions and strong trophic cascades with expected knock-on effects on the functionality of the ecosystems. Therefore, we argue that it is crucial to take into account body size as a species trait when analysing the consequences of biodiversity loss for natural ecosystems. Applying size-structured approaches provides an integrative ecological concept that enables a better understanding of each species' unique role across communities and the causes and consequences of biodiversity loss.
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Understanding and predicting species diversity in ecological communities is one of the great challenges in community ecology. Popular recent theory contends that the traits of species are "neutral" or unimportant to coexistence, yet abundant experimental evidence suggests that multiple species are able to coexist on the same limiting resource precisely because they differ in key traits, such as body size, diet, and resource demand. This book presents a new theory of coexistence that incorporates two important aspects of biodiversity in nature--scale and spatial variation in the supply of limiting resources.Introducing an innovative model that uses fractal geometry to describe the complex physical structure of nature, Mark Ritchie shows how species traits, particularly body size, lead to spatial patterns of resource use that allow species to coexist. He explains how this criterion for coexistence can be converted into a "rule" for how many species can be "packed" into an environment given the supply of resources and their spatial variability. He then demonstrates how this rule can be used to predict a range of patterns in ecological communities, such as body-size distributions, species-abundance distributions, and species-area relations. Ritchie illustrates how the predictions closely match data from many real communities, including those of mammalian herbivores, grasshoppers, dung beetles, and birds.This book offers a compelling alternative to "neutral" theory in community ecology, one that helps us better understand patterns of biodiversity across the Earth.
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Due to the covariation between temperature and resource availability in the surface ocean, a correct assessment of resource supply is crucial to determine if temperature has a direct effect on phytoplankton size structure. To remove the effect of resources, López-Urrutia and Morán analyzed data subsets with narrow ranges of variation in Chlorophyll a (Chl a) concentration and found that temperature is correlated with Chl a partitioning among size classes, from which they concluded that temperature is an important variable to explain the variability of phytoplankton size structure. Our analysis, however, shows that resource supply varies widely also within these subsets and, importantly, that it is inversely correlated with temperature. Therefore, the relationship between temperature and size structure reflects instead the effect of resources. When groups of samples with similar resource supply conditions are considered, no correlation between temperature and phytoplankton size structure is observed, which invalidates the conclusion of López-Urrutia and Morán. Even within restricted ranges of variation for phytoplankton biomass and production, changes in resource supply alone are sufficient to explain the variability of phytoplankton size structure in the sea.
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In warm lakes, fish aggregate within macrophytes, thereby weakening the role of these as a daytime refuge for zooplankton and altering the zooplankton size structure, predation pressure and water clarity. To elucidate the role of macrophytes as a refuge for zooplankton and their effect on zooplankton size distribution, we established three sets of strandardised artificial plant beds in eleven lakes in Turkey with contrasting fish predation risk and turbidity. Zooplankton were sampled within and outside of each plant beds during day and night. Fish, collected overnight in multimesh-sized gillnets, were abundant both inside and outside the artificial plant beds, impoverishing the usefulness of plants as a daytime refuge for particularly large-bodied zooplankton. Zooplankton size diversity was negatively related to fish abundance. Diel vertical migration was the frequent anti-predator avoidance behavior, but reverse migration was also observed when Chaoborus was present. In contrast to the small-bodied taxa, large- and medium-sized taxa showed intraspecific size-based migration (i.e. individuals of different sizes had different migration patterns). Predators influenced the size structure and diel movement of zooplankton, but the response changed with the size of zooplankton and water clarity.
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In warm lakes, fish aggregate within macrophytes, thereby weakening the role of these as a daytime refuge for zooplankton and altering the zooplankton size structure, predation pressure, and water clarity. To elucidate the role of macrophytes as a refuge for zooplankton and their effect on zooplankton size distribution, we established three sets of stran-dardized artificial plant beds in 11 lakes in Turkey with contrasting fish predation risk and turbidity. Zoo-plankton were sampled within and outside of each plant beds during day and night. Fish, collected overnight in multimesh-sized gillnets, were abundant both inside and outside the artificial plant beds, impoverishing the usefulness of plants as a daytime refuge for particularly large-bodied zooplankton. Zooplankton size diversity was negatively related to fish abundance. Diel vertical migration was the frequent anti-predator avoidance behavior, but reverse migration was also observed when Chaoborus was present. In contrast to the small-bodied taxa, large-and medium-sized taxa showed intraspecific size-based migration (i.e., individuals of different sizes had different migration patterns). Predators influenced the size structure and diel movement of zooplankton, but the response changed with the size of zooplankton and water clarity.
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Body size has been widely recognized as a key factor determining community structure in ecosystems. We analysed size diversity patterns of phytoplankton, zooplankton and fish assemblages in 13 data sets from freshwater and marine sites and found only weak relationships, indicating that predation and competition are not the only determinants of size distributions. However, when significant, greater biomass of predators was associated with reduced size diversity of prey at all trophic levels. In contrast, competition effects depend on the trophic level considered. At upper trophic levels (zooplankton and fish), size distributions were more diverse when potential resource availability was low, suggesting that competitive interactions for resources promote a diversification of aquatic communities by size. This pattern was not found in phytoplankton size distributions where size diversity mostly increased with low zooplankton grazing and high nutrient availability. Our results provide that predation pressure leads to an accumulation of organisms in the less predated size classes, while resource competition promoted a wider size distribution.
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We conducted a meta-analysis of temperature, phytoplankton size structure, and productivity in cold, temperate, and warm waters of the world's oceans. Our data set covers all combinations of temperature and resource availability, thus allowing us to disentangle their effects. The partitioning of biomass between different size classes is independent of temperature, but depends strongly on the rate of resource use as reflected in the rate of primary production. Temperature and primary production explained 2% and 62%, respectively, of the variability in the contribution of microphytoplankton to total biomass. This contribution increases rapidly with total biomass and productivity, reaching values > 80% when chlorophyll a concentration is > 2 mu g L-1 or primary production is > 100 mg C L-1 d(-1), irrespective of water temperature. Conversely, picophytoplankton contribution is substantial (> 40%), at all temperatures, only when chlorophyll a concentration is, 1 mu g L-1 or primary production is < 50 mu g C L-1 d(-1). The temperature-size rule cannot explain these changes, which instead reflect fundamental reorganizations in the species composition of the assemblage, arising from taxon- and size-dependent differences in resource acquisition and use. Given that resource availability, rather than temperature per se, is the key factor explaining the relative success of different algal size classes, there will be no single, universal effect of global warming on phytoplankton size structure.
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Cladocerans are valuable indicators of environmental change in lakes. Their fossils provide information on past changes in lake environments. However, few studies have quantitatively examined the relationships between contemporary and sub-fossil cladoceran assemblages and no investigations are available from Mediterranean lakes where salinity, eutrophication and top-down control of large-bodied cladocerans are known to be important. Here we compared contemporary Cladocera assemblages, sampled in summer, from both littoral and pelagic zones, with their sub-fossil remains from surface sediment samples from 40 Turkish, mainly shallow, lakes. A total of 20 and 27 taxa were recorded in the contemporary and surface sediment samples, respectively. Procrustes rotation was applied to both the principal components analysis (PCA) and redundancy analysis (RDA) ordinations in order to explore the relationship between the cladoceran community and the environmental variables. Procrustes rotation analysis based on PCA showed a significant accord between both littoral and combined pelagic–littoral contemporary and sedimentary assemblages. RDA ordinations indicated that a similar proportion of variance was explained by environmental variation for the contemporary and fossil Cladocera data. Total phosphorus and salinity were significant explanatory variables for the contemporary assemblage, whereas salinity emerged as the only significant variable for the sedimentary assemblage. The residuals from the Procrustes rotation identified a number of lakes with a high degree of dissimilarity between modern and sub-fossil assemblages. Analysis showed that high salinity, deep water and high macrophyte abundance were linked to a lower accord between contemporary and sedimentary assemblages. This low accord was, generally the result of poor representation of some salinity tolerant, pelagic and macrophyte-associated taxa in the contemporary samples. This study provides further confirmation that there is a robust relationship between samples of modern cladoceran assemblages and their sedimentary remains. Thus, sub-fossil cladoceran assemblages from sediment cores can be used with confidence to track long-term changes in this environmentally sensitive group and in Mediterranean lakes, subjected to large inter-annual variation in water level, salinity and nutrients.
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Body size has been widely recognised as a key factor determining community structure in ecosystems. We analysed size diversity patterns of phytoplankton, zoo-plankton and fish assemblages in 13 data sets from freshwater and marine sites with the aim to assess whether there is a general trend in the effect of predation and resource competition on body size distribution across a wide range of aquatic ecosystems. We used size diversity as a measure of the shape of size distribution. Size diversity was computed based on the Shannon-Wiener diversity expression, adapted to a continuous variable, i.e. as body size. Our results show that greater predation pressure was associated with reduced size diversity of prey at all trophic levels. In contrast, competition effects depended on the trophic level considered. At upper trophic levels (zoo-plankton and fish), size distributions were more diverse when potential resource availability was low, suggesting that competitive interactions for resources promote diver-sification of aquatic communities by size. This pattern was not found for phytoplankton size distributions where size
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Our aim was to document geographical patterns of variation in the body-size structure of European lake fish assemblages along abiotic gradients, and any differences in fish assemblage structure. We hypothesized that patterns in the body-size structure of entire lake fish assemblages are primarily temperature driven and consistent with the dominant pattern of the temperature–size rule, which suggests a decrease in adult body size with increasing developmental temperature for many ectothermic species.
Article
1. Aquatic macrophytes are commonly used to assess the ecological condition of lakes. Little is known, however, about long-term macrophyte dynamics in shallow lakes. In the absence of historical data, the remains of macrophytes (fruits, seeds and vegetative fragments) found in lake sediments may provide just such information. In order to interpret confidently past change in aquatic plant communities from their sedimentary remains, it is vital to establish the similarity between the con-temporary and fossil assemblages. 2. We investigated the relationship between present lake vegetation and plant macrophyte remains in surface sediments. Thirty-five shallow lakes, spanning around six degrees of latitude and mostly located in the semi-arid Mediterranean climatic zone of Turkey, were sampled for aquatic plants, surface sediment plant remains and a range of other key environmental variables. 3. Around 50% of the taxa recorded in the modern vegetation were represented in the sediment. Sedimentary macrofossils of some taxa were under-or over-represented relative to their frequency in the modern vegetation, for example Potamogeton spp. and Characeae, respectively. Despite this disparity, there was good agreement between the assemblage composition of the modern and sedimentary samples. Furthermore, conductivity and trophic state (as indicated by total nitrogen, total phosphorus and chlorophyll-a) were the environmental variables most clearly correlated with both the contemporary and macrofossil assemblages in these lakes. 4. We conclude that aquatic macrophyte macrofossils can be used as reliable indicators of ecological status and to determine qualitative changes in assemblages of aquatic plants consequent to environ-mental change (e.g. in lake trophic status and/or salinity). This may be especially useful for lakes in arid and semi-arid Mediterranean regions, which are particularly vulnerable to hydrological constraints under climate change.
Article
This chapter demonstrates that methods to describe ecological communities can be better understood, and can reveal new patterns, by labeling each species that appears in a community’s food web with the numerical abundance and average body size of individuals of that species. We illustrate our new approach, and relate it to previous approaches, by analyzing data from the pelagic community of a small lake, Tuesday Lake, in Michigan. Although many of the relationships we describe have been well studied individually, we are not aware of any single community for which all of these relationships have been analyzed simultaneously. An overview of some of the results of the present study, with further theoretical extensions, has been published elsewhere (Cohen et al., 2003). Our new approach yields four major results. Though many patterns in the structure of an ecological community have been traditionally treated as independent, they are in fact connected. In at least one real ecosystem, many of these patterns are relatively robust after a major perturbation. Some of these patterns may be predictably consistent from one community to another. Locally, however, some community characteristics need not necessarily coincide with previously reported patterns for guilds or larger geographical scales. We describe our major findings under these headings: trivariate relationships (that is, relationships combining the food web, body size, and species abundance); bivariate relationships; univariate relationships; and the effects of food web perturbation.
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The biodiversity-ecosystem functioning debate is a central topic in ecology. Recently, there has been a growing interest in size diversity because body size is sensitive to environmental changes and is one of the fundamental characteristics of organisms linking many ecosystem properties. However, how size diversity affects ecosystem functioning is an important yet unclear issue. To fill the gap, with large-scale field data from the East China Sea, we tested the novel hypothesis that increasing zooplankton size diversity enhances top-down control on phytoplankton (H1) and compared it with five conventional hypotheses explaining the top-down control: flatter zooplankton size spectrum enhances the strength of top-down control (H2); nutrient enrichment lessens the strength of top-down control (H3); increasing zooplankton taxonomic diversity enhances the strength of top-down control (H4); increasing fish predation decreases the strength of top-down control of zooplankton on phytoplankton through trophic cascade (H5); increasing temperature intensifies the strength of top-down control (H6). The results of univariate analyses support the hypotheses based on zooplankton size diversity (H1), zooplankton size spectrum (H2), nutrient (H3) and zooplankton taxonomic diversity (H4), but not the hypotheses based on fish predation (H5) and temperature (H6). More in-depth analyses indicate that zooplankton size diversity is the most important factor in determining the strength of top-down control on phytoplankton in the East China Sea. Our results suggest a new potential mechanism that increasing predator size diversity enhances the strength of top-down control on prey through diet niche partitioning. This mechanism can be explained by the optimal predator-prey body-mass ratio concept. Suppose each size group of zooplankton predators has its own optimal phytoplankton prey size, increasing size diversity of zooplankton would promote diet niche partitioning of predators and thus elevates the strength of top-down control.
Article
The established approach to model seston size distributions involves the grouping of particles within logarithmic size classes and the examination of the relationship between density, or normalized biomass, and the characteristic sizes of the classes. Here we examine the distributional basis of the established approach and draw a connection between the biomass size spectrum and the Pareto distribution, a model widely used in other disciplines dealing with size-structured systems. We provide efficient estimators of the parameters and also suggest that datasets exhibiting significant departures from a smooth power function decline can be adequately modeled using a Pareto type II distribution.
Article
Body size is recognized as an important determinant of trophic structure as it affects individual energetic demands, population density, and the interaction between potential prey and predators. However, its relationship with trophic position remains unclear. It has been hypothesized that a positive relationship between body size and trophic position would be associated to some particular trophic structures, which would allow larger organisms to satisfy their energetic demand and sustain viable population sizes at higher trophic positions, where fewer resources are available. To test this hypothesis, we analyzed the diet of 619 killifishes from four species (Austrolebias cheradophilus, A. luteoflammulatus, A. viarius and Cynopoecilus melanotaenia), collected in temporary ponds occurring in the grasslands of Rocha, Uruguay. Trophic position, diet richness, number of energy sources, and evenness were estimated for 20 size classes, formed by consecutive groups of 31 individuals. Gape limitation and preference for the larger available prey were evaluated as explanations for observed patterns with an individual based model (IBM). In agreement with the hypothesis, killifishes presented a strong positive relationship between trophic position and body size (R 2 00.86), associated with a trophic structure that could allow larger organisms to have access to more energy from the environment. This was reflected in a positive relationship between body size and 1) prey richness, 2) number of basal energy sources (i.e. plants, detritus, phytoplankton and terrestrial prey), and 3) evenness in prey use. IBM results showed that changes in trophic structure with body size are well explained by gape limitation, but not by size preferences. Our results suggest that the fulfilment of the greater energetic demands of larger organism will depend on community diversity, which typically increases with ecosystem size, indicating a novel connection between area, diversity, body size, and food chain length. Human activities impact natural ecosystems by changing their diversity, area and productivity. However, the con-nections between these changes and community organiza-tion or species persistence are not always evident (Gotelli and Ellison 2006). Top predators usually have a key role in community structure and dynamics (Gotelli and Ellison 2006), and are widely used by humans as resources (Sibert et al. 2006). Large predators are also particularly sensitive to human activities, showing higher extinction rates (Olden et al. 2007), which highlights the theoretical and applied relevance of understanding the determinants of their persistence (Marquet and Taper 1998, Valkenburgh et al. 2004). Food webs represent the predatorÁprey mediated routes through which energy and matter flow across species in ecosystems; their structure and dynamics determine the potential for large predators to satisfy their energetic demands and persist (Cohen et al. 2003, McCann 2007). Body size is one of the most important traits differ-entiating individuals that compose a community (Wood-ward et al. 2005, Raffaelli 2007). Body size was early recognized as a potential determinant of species interactions
Article
Summary1. The size structure of communities is shaped by biotic and abiotic interactions. Therefore, comparative analyses of size spectra may reveal the major drivers governing patterns and processes in size-structured communities.2. We tested the suitability of non-taxonomic, size-related variables as tools for elucidating systematic shifts in lake fish assemblages along the gradients of environmental factors and lake-use intensity. Catch data of multimesh gillnets from 78 lowland lakes in northern Germany were analysed.3. We first identified the correlations, and hence inherent redundancy, among 18 size-related variables. The correspondence between eight weakly correlated size variables and descriptors of lake morphometry, lake productivity, lake-use intensity and taxonomic and functional fish-assemblage composition was tested using ordination by non-metric multidimensional scaling (NMDS). The three axes of the NMDS analysis were strongly correlated with five size variables, which in turn corresponded to lake area, mean and maximum depth, total phosphorus and chlorophyll a concentration, predator abundance and predator/prey length ratios (PPLR).4. The number of size classes increased with increasing lake area. The slopes of normalised length spectra were flatter (less negative) and size diversity was higher in deep, less nutrient-rich lakes and in lakes with a higher numerical predator abundance, indicating a higher relative abundance of large fish. The exponent of the Pareto type II mass spectra was larger and maximum fish length was smaller in shallow, nutrient-rich lakes and in lakes with lower predator biomass and smaller PPLR, indicating a higher relative proportion of medium-sized fish.5. Analyses of size spectra at regional scales can contribute important information to the evaluation of the ecological quality of lakes. We suggest further studies at a broader range of environmental and geographical scales to understand the subtle response of size-related variables to biotic interactions, abiotic stressors and geographical patterns.