Community Size Structure Research Articles

Linking community size structure and ecosystem functioning using metabolic theory

Understanding how biogeochemical cycles relate to the structure of ecological communities is a central research question in ecology. Here we approach this problem by focusing on body size, which is an easily measured species trait that has a pervasive influence on multiple aspects of community structure and ecosystem functioning. We test the predictions of a model derived from metabolic theory using data on ecosystem metabolism and community size structure. These data were collected as part of an aquatic mesocosm experiment that was designed to simulate future environmental warming. Our analyses demonstrate significant linkages between community size structure and ecosystem functioning, and the effects of warming on these links. Specifically, we show that carbon fluxes were significantly influenced by seasonal variation in temperature, and yielded activation energies remarkably similar to those predicted based on the temperature dependencies of individual-level photosynthesis and respiration. We also show that community size structure significantly influenced fluxes of ecosystem respiration and gross primary production, particularly at the annual time-scale. Assessing size structure and the factors that control it, both empirically and theoretically, therefore promises to aid in understanding links between individual organisms and biogeochemical cycles, and in predicting the responses of key ecosystem functions to future environmental change.

Novel communities from climate change

Climate change is generating novel communities composed of new combinations of species. These result from different degrees of species adaptations to changing biotic and abiotic conditions, and from differential range shifts of species. To determine whether the responses of organisms are determined by particular species traits and how species interactions and community dynamics are likely to be disrupted is a challenge. Here, we focus on two key traits: body size and ecological specialization. We present theoretical expectations and empirical evidence on how climate change affects these traits within communities. We then explore how these traits predispose species to shift or expand their distribution ranges, and associated changes on community size structure, food web organization and dynamics. We identify three major broad changes: (i) Shift in the distribution of body sizes towards smaller sizes, (ii) dominance of generalized interactions and the loss of specialized interactions, and (iii) changes in the balance of strong and weak interaction strengths in the short term. We finally identify two major uncertainties: (i) whether large-bodied species tend to preferentially shift their ranges more than small-bodied ones, and (ii) how interaction strengths will change in the long term and in the case of newly interacting species.

Climate change impacts on body size and food web structure on mountain ecosystems

The current distribution of climatic conditions will be rearranged on the globe. To survive, species will have to keep pace with climates as they move. Mountains are among the most affected regions owing to both climate and land-use change. Here, we explore the effects of climate change in the vertebrate food web of the Pyrenees. We investigate elevation range expansions between two time-periods illustrative of warming conditions, to assess: (i) the taxonomic composition of range expanders; (ii) changes in food web properties such as the distribution of links per species and community size-structure; and (iii) what are the specific traits of range expanders that set them apart from the other species in the community-in particular, body mass, diet generalism, vulnerability and trophic position within the food web. We found an upward expansion of species at all elevations, which was not even for all taxonomic groups and trophic positions. At low and intermediate elevations, predator : prey mass ratios were significantly reduced. Expanders were larger, had fewer predators and were, in general, more specialists. Our study shows that elevation range expansions as climate warms have important and predictable impacts on the structure and size distribution of food webs across space.

Phytoplankton response to deep seawater nutrient addition in the North Pacific Subtropical Gyre

We examined the phytoplankton response to the addition of nutrient-enriched deep seawater (DSW) and of nitrate only during shipboard experiments performed between July 2004 and May 2007 in the North Pacific Subtropical Gyre (NPSG). Chlorophyll a (chl a), community size structure and composition, carbon fixation (CF) rates, and nutrient concentrations were measured daily for 5 to 7 d under simulated in situ conditions. Despite the fact that the NPSG is a permanently stratified, oligotrophic biome, there was a seasonal response to the addition of DSW and nitrate. In summertime experiments, chl a and CF rates increased up to 18and 22-fold, respectively, relative to unamended controls after an incubation period of 5 to 6 d. Nutrients were assimilated to below control concentrations. A shift from a Prochlorococcus-dominated to a Synechococcusand diatom-dominated community, and an 8to 12-fold increase in the chl a content of the >2 μm size fraction were observed. Addition of nitrate only increased chl a and CF rates 10and 7-fold, respectively. In wintertime experiments, chl a and CF rates increased up to 4and 9-fold, respectively, after an incubation period of up to 7 d. Only 50% of nutrients were assimilated and nitrate alone stimulated a 2-fold increase in chl a and CF rates. We explore the role of nutrient limitation, community composition, grazing, and light in explaining our observations. Findings from this seasonal and multi-year field experiment demonstrate how little we know about bloom development in the NPSG and highlight the potential risk in extrapolating the response of phytoplankton to natural or artificial fertilization from short-term studies.

Size Structure of Marine Soft-Bottom Macrobenthic Communities across Natural Habitat Gradients: Implications for Productivity and Ecosystem Function

Size distributions of biotic assemblages are important modifiers of productivity and function in marine sediments. We investigated the distribution of proportional organic biomass among logarithmic size classes (2−6J to 216J) in the soft-bottom macrofaunal communities of the Strait of Georgia, Salish Sea on the west coast of Canada. The study examines how size structure is influenced by 3 fundamental habitat descriptors: depth, sediment percent fines, and organic flux (modified by quality). These habitat variables are uncorrelated in this hydrographically diverse area, thus we examine their effects in combination and separately. Cluster analyses and cumulative biomass size spectra reveal clear and significant responses to each separate habitat variable. When combined, habitat factors result in three distinct assemblages: (1) communities with a high proportion of biomass in small organisms, typical of shallow areas (<10 m) with coarse sediments (<10% fines) and low accumulation of organic material (<3.0 gC/m2/yr/δ15N); (2) communities with high proportion of biomass in the largest organisms found in the Strait, typical of deep, fine sediments with high modified organic flux (>3 g C/m2/yr/δ15N) from the Fraser River; and (3) communities with biomass dominated by moderately large organisms, but lacking the smallest and largest size classes, typical of deep, fine sediments experiencing low modified organic flux (<3.0 gC/m2/yr/δ15N). The remaining assemblages had intermediate habitat types and size structures. Sediment percent fines and flux appear to elicit threshold responses in size structure, whereas depth has the most linear influence on community size structure. The ecological implications of size structure in the Strait of Georgia relative to environmental conditions, secondary production and sediment bioturbation are discussed.

Warming alters community size structure and ecosystem functioning

Global warming can affect all levels of biological complexity, though we currently understand least about its potential impact on communities and ecosystems. At the ecosystem level, warming has the capacity to alter the structure of communities and the rates of key ecosystem processes they mediate. Here we assessed the effects of a 4°C rise in temperature on the size structure and taxonomic composition of benthic communities in aquatic mesocosms, and the rates of detrital decomposition they mediated. Warming had no effect on biodiversity, but altered community size structure in two ways. In spring, warmer systems exhibited steeper size spectra driven by declines in total community biomass and the proportion of large organisms. By contrast, in autumn, warmer systems had shallower size spectra driven by elevated total community biomass and a greater proportion of large organisms. Community-level shifts were mirrored by changes in decomposition rates. Temperature-corrected microbial and macrofaunal decomposition rates reflected the shifts in community structure and were strongly correlated with biomass across mesocosms. Our study demonstrates that the 4°C rise in temperature expected by the end of the century has the potential to alter the structure and functioning of aquatic ecosystems profoundly, as well as the intimate linkages between these levels of ecological organization.

Robustness of size–structure across ecological networks in pelagic systems

The study of biomass size distributions has become an important tool for addressing aquatic ecosystem complexity and the consequences of anthropogenic disturbances. However, it remains unclear how changes in pelagic food web topology affect the biomass size–structure. Employing a dynamic multispecies bioenergetic consumer-resource model, we simulated biomass trajectories over time in 10,000 virtual networks of varying topology to address which food web properties are important in determining size–structure in pelagic systems. The slopes of the normalized biomass size spectra (NBSS) and Pareto’s shape parameter (γ) of our modeled communities are consistent with theoretically expected values for steady-state systems and empirical values reported for several aquatic ecosystems. We found that the main drivers of the NBSS slope and Pareto’s γ were the slope of the relationship between body mass and trophic level, the maximum trophic level of the food web, and the stability of total community biomass. Our analyses showed a clear conservative trend in pelagic community size–structure as demonstrated by the robustness of the NBSS slope and Pareto’s γ against most of the topological changes in virtual networks. Nevertheless, these analyses also caution that major disturbances in large-bodied or top-trophic level individuals may disrupt this stable pattern.

Initial size structure of natural phytoplankton communities determines the response to Daphnia diel vertical migration

Diel vertical migration (DVM) is a common behavior of many pelagic herbivorous zooplankton species in response to predation pressure. It is characterized by a twice daily habitat shift of the zooplankton species: staying in the epilimnion only during night time and migrating down in the crack of dawn in deeper water layers, staying there during the day time. This causes a discontinuous grazing regime and previous studies have shown that the direction and strength of phytoplankton community responses to zooplankton DVM most probably depends on the size of phytoplankton species. To examine the influence of zooplankton DVM on different sized phytoplankton communities, we designed an experiment where we manipulated the size distribution of a natural phytoplankton community a priori in field mesocosms. We investigated the influence of DVM of the cladoceran Daphnia hyalina on two different phytoplankton communities, by the use of deep (10 m) field enclosures. Epilimnetic lake water, containing a summer phytoplankton community, was filtered with two different mesh sizes (11 mm and 64 mm). The 11 mm phytoplankton community (“small”) contained mainly small algal species, while the 64 mm community (“large”) had a wider range of phytoplankton sizes. To simulate zooplankton DVM, D. hyalina were placed in mesh cages that were lowered or raised (“migration”) as dictated by the study design; a “no migration” (representing absence of DVM) treatment was also tested. Phytoplankton abundance was measured using chlorophyll-a and biovolume; size distribution of the algae and nutrient availability was also determined in each treatment. The results indicated that DVM had contrasting effects on the two evaluated phytoplankton communities. Comparison of “migration” and “no migration” zooplankton treatments showed that nutrient availability and total phytoplankton biovolume was higher in (1) “no migration” treatments with phytoplankton communities comprising mainly small algae and (2) “migration” treatments with phytoplankton communities of a broader size spectrum of algae. Hence our study showed two different mechanisms of how zooplankton DVM may influence the phytoplankton community dynamics. Nutrient cycling was an important factor in phytoplankton communities of mainly small algae, whereas the refuge effect was the main driver of phytoplankton dynamics in phytoplankton communities of a large size spectrum of algae.

Size-selective fishing drives species composition in the Celtic Sea

Abstract Shephard, S., Fung, T., Houle, J. E., Farnsworth, K. D., Reid, D. G., and Rossberg, A. G. 2012. Size-selective fishing drives species composition in the Celtic Sea. – ICES Journal of Marine Science, 69: 223–234. Fishing alters community size structure by selectively removing larger individual fish and by changing the relative abundance of different-sized species. To assess the relative importance of individual- and species-level effects, two indices of fish community structure were compared, the relative abundance of large fish individuals (large fish indicator, LFI) and the relative abundance of large fish species (large species indicator, LSI). The two indices were strongly correlated for empirical data from the Celtic Sea and for data from simulated model communities, suggesting that much of the variability in the LFI is caused by shifts in the relative abundance of species (LSI). This correlation is explained by the observation that most of the biomass of a given species is spread over few length classes, a range spanning the factor 2 of individual length, such that most species contributed predominantly to either the small or the large component of the LFI. The results suggest that the effects of size-selective fishing in the Celtic Sea are mediated mainly through changes in community composition.

Redundancy in metrics describing the composition, structure, and functioning of the North Sea demersal fish community

AbstractGreenstreet, S. P. R., Fraser, H. M., Rogers, S. I., Trenkel, V. M., Simpson, S. D., and Pinnegar, J. K. 2012. Redundancy in metrics describing the composition, structure, and functioning of the North Sea demersal fish community. – ICES Journal of Marine Science, 69: 8–22. Broader ecosystem management objectives for North Sea demersal fish currently focus on restoring community size structure. However, most policy drivers explicitly concentrate on restoring and conserving biodiversity, and it has not yet been established that simply restoring demersal fish size composition will be sufficient to reverse declines in biodiversity and ensure a generally healthy community. If different aspects of community composition, structure, and function vary independently, then to monitor all aspects of community general health will require application of a suite of metrics. This assumes low redundancy among the metrics used in any such suite and implies that addressing biodiversity issues specifically will require explicit management objectives for particular biodiversity metrics. This issue of metric redundancy is addressed, and 15 metrics covering five main attributes of community composition, structure, and function are applied to groundfish survey data. Factor analysis suggested a new interpretation of the metric information and indicated that a minimum suite of seven metrics was necessary to ensure that all changes in the general health of the North Sea demersal fish community were monitored properly. Covariance among size-based and species-diversity metrics was low, implying that restoration of community size structure would not necessarily reverse declines in species diversity.

Limited effects of stocked trout on littoral invertebrates in boreal foothills lakes

Stocking lakes with trout for the purposes of recreational angling is a management strategy that introduces a new predator into these systems and thus deserves careful scrutiny. To assess the impact of non-native trout on littoral invertebrates in naturally fish-bearing lakes in the boreal foothills of Alberta, Canada, we compared their community composition, abundance, and size structure in stocked (n = 5) and unstocked (n = 6) lakes over a 2-year period. We detected no clear negative effects of introduced trout on invertebrate community composition and only few taxa-specific examples of decreased or increased invertebrate abundance. Furthermore, predation by trout had inconsistent direct effects on the size structure of invertebrate populations. Indirect effects were suggested by increased abundances and sizes of some invertebrate taxa in stocked lakes and might also contribute to the limited overall differences that we observed. We propose that net effects of stocked trout on littoral invertebrates are influenced by key characteristics of receiving ecosystems. In our boreal foothills lakes, dense macrophyte cover in warm littoral zones, high productivity, abundant forage fish, and limited densities of trout all likely combine to allow littoral invertebrate communities to withstand the impact of introduced trout with minimal effects.

ICHTHYOFAUNA OF A NEAR-NATURAL PYRENEAN RIVER: SPATIO-TEMPORAL VARIABILITY AND REACH-SCALE HABITAT

The characterization of fish communities and their relationships with abiotic factors is necessary for a number of environmental management issues. This study aims to analyze the spatio-temporal variability in the structure of fish communities in an undisturbed and unpolluted Pyrenean river in relation to aquatic habitat features. Fish and habitat were sampled seasonally in 2002 at fourteen sampling sites along the Erro River, a tributary of the Ebro basin (Navarra, northern Spain). Fish communities comprised eight native species, and their capture efficiency estimates were high and consistent along the river. Differences between species in estimated capture probability values were related to attributes such as body size and/or abundance, underscoring the importance of using species-specific capture probabilities. The observed low variability of species capture probabilities allows their use as reference values for future population size estimations based on single-run electrofishing surveys in similarly sized rivers. The longitudinal fish species distribution was analyzed by means of separate hierarchical cluster analyses performed on total species’ densities and species’ densities partitioned by size-classes, both of which displayed similar patterns of fish community composition along the Erro River. The changes on species presence and relative contributions to local fish communities along the river were also evident from diversity indices, which showed strong and significant positive relationships with distance from the source. Seasonal values in diversity indices were also significantly different within sampling sites, indicating that low-order streams support lower diversity and less-structured fish communities because seasonal flow variations lead to high variability in hydrochemical parameters, habitat size and diversity. Two separate canonical correspondence analyses were performed to jointly summarize the variation in aquatic habitat parameters in relation to fish community size structure in terms of densities or biomasses. In both analyses, the first axis was related to the along-river gradient and the second axis accounted for seasonal variation. The relative positions of fish species along the spatio-temporal gradients are in accordance with their habitat preferences described in the Pyrenean area. These results provide a framework for understanding how the spatiotemporal variations of reach-scale aquatic habitat features structure fish communities in a Pyrenean river with low anthropogenic disturbance.

Seasonal shifts in predator body size diversity and trophic interactions in size‐structured predator–prey systems

1. Theory suggests that the relationship between predator diversity and prey suppression should depend on variation in predator traits such as body size, which strongly influences the type and strength of species interactions. Prey species often face a range of different sized predators, and the composition of body sizes of predators can vary between communities and within communities across seasons. 2. Here, I test how variation in size structure of predator communities influences prey survival using seasonal changes in the size structure of a cannibalistic population as a model system. Laboratory and field experiments showed that although the per-capita consumption rates increased at higher predator-prey size ratios, mortality rates did not consistently increase with average size of cannibalistic predators. Instead, prey mortality peaked at the highest level of predator body size diversity. 3. Furthermore, observed prey mortality was significantly higher than predictions from the null model that assumed no indirect interactions between predator size classes, indicating that different sized predators were not substitutable but had more than additive effects. Higher predator body size diversity therefore increased prey mortality, despite the increased potential for behavioural interference and predation among predators demonstrated in additional laboratory experiments. 4. Thus, seasonal changes in the distribution of predator body sizes altered the strength of prey suppression not only through changes in mean predator size but also through changes in the size distribution of predators. In general, this indicates that variation (i.e. diversity) within a single trait, body size, can influence the strength of trophic interactions and emphasizes the importance of seasonal shifts in size structure of natural food webs for community dynamics.

Remote sensing of size structure of phytoplankton communities using optical properties of the Chukchi and Bering Sea shelf region

Abstract. Recent ocean warming and subsequent sea ice decline resulting from climate change could affect the northward shift of the ecosystem structure in the Chukchi Sea and Bering Sea shelf region (Grebmeier et al., 2006b). The size structure of phytoplankton communities provides an index of trophic levels that is crucial to understanding the mechanisms underlying such ecosystem changes and their implications for the future. This study proposes a new ocean color algorithm for deriving this characteristic by using the region's optical properties. The size derivation model (SDM) estimates the phytoplankton size index FL on the basis of size-fractionated chlorophyll-a (chl-a) using the light absorption coefficient of phytoplankton, aph(λ), and the backscattering coefficient of suspended particles including algae, bbp(λ). FL was defined as the ratio of algal biomass attributed to cells larger than 5 μm to the total. It was expressed by a multiple regression model using the aph(λ) ratio, aph(488)/aph(555), which varies with phytoplankton pigment composition, and the spectral slope of bbp(λ), γ, which is an index of the mean suspended particle size. A validation study demonstrated that 69% of unknown data are correctly derived within FL range of ±20%. The spatial distributions of FL for the cold August of 2006 and the warm August of 2007 were compared to examine application of the SDM to satellite remote sensing. The results suggested that phytoplankton size was responsive to changes in sea surface temperature. Further analysis of satellite-derived FL values and other environmental factors can advance our understanding of ecosystem structure changes in the shelf region of the Chukchi and Bering Seas.

Effects of Bythotrephes longimanus (Crustacea, Cladocera) on the abundance, morphology, and prey community of Leptodora kindtii (Crustacea, Cladocera)

We hypothesized that native Leptodora kindtii would be shorter and have smaller feeding baskets in central Ontario lakes with greater abundances of small-bodied zooplankton prey, and that differences in zooplankton size among lakes could be attributed to the invasive cladoceran Bythotrephes longimanus. We evaluated these conjectures by comparing size metrics of Leptodora and the size of their preferred cladoceran prey in lakes invaded or not by Bythotrephes. Leptodora was less abundant in invaded lakes, but were smaller bodied with smaller feeding baskets only in lakes with long invasion histories. Small cladoceran abundance was greater in non-invaded lakes and was directly related to Leptodora abundance although not to Leptodora size. Mean Leptodora body size declined with increasing abundance of Bythotrephes. We evaluated three possible explanations for these patterns in Leptodora—(a) competition with Bythotrephes for zooplankton prey, (b) direct predation by Bythotrephes, and (c) size-selective predation by fish. While we were unable to unequivocally distinguish among these hypotheses, our observations are most consistent with predation by Bythotrephes changing zooplankton community composition and size structure in a manner that is detrimental to Leptodora. Our results indicate that Bythotrephes invasion may trigger more complex and subtle changes in food webs than previously thought.

Size structure of a heavily fished benthic/demersal community by shrimp trawling in the Colombian Caribbean Sea

The benthic and demersal communities in the Colombian Caribbean Sea (CCS) are heavily fished by the shrimp trawling fishery, which presents very high discard levels. Here, we conducted an analysis of the size structure of these benthic and demersal communities in the northern and southern zones of the CCS. Sampling was conducted onboard shrimp trawlers throughout an entire year. No significant differences were found in the size distributions of the two zones, among sites within southern ecoregions, or among the analyzed cruises. This homogeneity in size structure is remarkable since the zones analyzed possess very different species compositions and environmental conditions. The observed size structures were adequately described by non-linear distributions rather than the traditionally employed linear normalized biomass size spectra. It is hypothesized that the non-linearity is due to the effect of fishing and particularly, of discarding. This study emphasizes the need for a greater understanding of the impacts that trawl fishing has on community size structure and the applicability of this knowledge towards fishery resource management in ecosystems with high diversity.

Bio-optical properties of east coast Malaysia waters in relation to remote sensing of chlorophyll

Surface water samples collected during the monsoon and inter-monsoon seasons of 2009 off the east coast of Peninsular Malaysia have been analysed for concentrations of total chlorophyll, suspended particulates and coloured dissolved organic matter (CDOM). Spectral absorption coefficients of dissolved and particulate materials have also been measured. Significant seasonal variabilities in concentrations and optical properties were reported with high concentrations of all parameters during the northeast monsoon (NEM) season and low during the southwest monsoon (SWM) and inter-monsoon seasons. Contrary to previous reports on the oligotrophic nature of the waters during the inter-monsoon season, relatively high concentrations of chlorophyll (>3 mg m−3) were observed at offshore stations in the study area in the spring and fall inter-monsoon months. The chlorophyll-specific absorption spectrum changes with the seasons with the greatest absorption per unit chlorophyll during the SWM and the least during the inter-monsoon seasons, probably in response to seasonal changes in phytoplankton community and cell size structure. The water is classified as optical case 2. At the blue end of the spectrum (440 nm), light absorption by non-phytoplankton materials (CDOM and detritus) accounts for nearly 70% of the total non-water absorption regardless of the season. At the wavelength (676 nm) of the secondary chlorophyll absorption peak in the red part of the spectrum, light absorption by chlorophyll contributes 80–90% to total non-water absorption at most stations and this may provide the basis for remote sensing of phytoplankton chlorophyll in these waters.

Extinctions in complex, size-structured communities

Extinctions in complex, size-structured communities

Effects of production, sedimentation and taphonomic processes on the composition and size structure of sedimenting cladoceran remains in a large deep subalpine lake: paleo-ecological implications

Changes in the composition and size structure of cladoceran remains collected in sediment traps (ST) were compared at a monthly time-resolution to the changes in the cladoceran source communities in order to assess the effects of production, sedimentation and taphonomic processes on the composition, completeness of time series and size structure of cladoceran remains in a large deep monomictic subalpine lake. Cladoceran remains collected in the ST globally reflected seasonal changes in the composition of the source community for the dominant taxa (Daphnia sp., Eubosmina spp. and. D. brachyurum) but failed in capturing accurately the seasonal changes in the abundance of the least abundant species, B. longirostris. Using allometric relationships, the average body size of the organisms that produced the remains retrieved in the ST could be reconstructed. Although the cladoceran average body size estimated from trap remains was always smaller than that of the source communities, temporal changes in the average body size of the dominant taxa in the source community were captured by trap samples. Our results showed that, in this deep subalpine lake, cladoceran remains production, sedimentation and taphonomic processes within the water column did not alter the compositional fidelity of remains at seasonal and annual time scales for the dominant taxa. Results did not show any significant over-representation of Bosmina and subsequent under-representation of Daphnia in contrast to previous studies on small and flat lakes. Our results suggest that the frequently observed under-estimation of Daphnia in fossil assemblages could be a consequence of the mesh size used when processing sediment core samples rather than of varying magnitude of degradation processes between small or flat lakes and deep lakes. Finally, our results support the use of the size of cladoceran remains within paleo-ecological studies to assess past changes in the size structure of the source communities.

Phytoplankton community structure in an Amazon floodplain lake (Lago Catalão, Amazonas, Brazil)

This study analyzed the attributes of the phytoplankton community (species richness, diversity, equitability, density, size structure and rate of community compositional change) associated with some physical and chemical characteristics of a mixed water lake (white and black water) for four consecutive weeks in the dry and rain season, in four sampling sites and at three depths (surface, middle and bottom). The presented differences in limnological characteristicas in the rainy and the dry season. The phytoplankton was composed, mainly by nanoplanktonic algae, especially Chlorophyceae and Cyanobacteria. In both periods the functional group X1 was the predominant association in terms of organism density. Considering the sampling seasons, the highest values of richness, equitability and diversity were recorded in the dry season. Chlorophyceae, Euglenophyceae, Bacillariophyceae and Cyanobacteria were the groups with the highest number of species in the dry season, while Chlorophyceae and Cyanobacteria were the groups with the highest species richness in the rainy season. The main physical and chemical variables that determine the distribution of the species were dissolved oxygen, temperature, electrical conductivity and ratio of mixing zone to euphotic depth.Key words: phytoplankton, floodplain, Amazon Lake.