We assessed the relative roles of local environmental conditions and dispersal on community structure in a landscape of lakes for the major trophic groups. We use taxonomic presence-absence and abundance data for bacteria, phytoplankton, zooplankton, and fish from 18 lakes in southern Quebec, Canada. The question of interest was whether communities composed of organisms with more limited dispersal abilities, because of size and life history (zooplankton and fish) would show a different effect of lake distribution than communities composed of good dispersers (bacteria and phytoplankton). We examine the variation in structure attributable to local environmental (i.e., lake chemical and physical variables) vs. dispersal predictors (i.e., overland and watercourse distances between lakes) using variation partitioning techniques. Overall, we show that less motile species (crustacean zooplankton and fish) are better predicted by spatial factors than by local environmental ones. Furthermore, we show that for zooplankton abundances, both overland and watercourse dispersal pathways are equally strong, though they may select for different components of the community, while for fish, only watercourses are relevant dispersal pathways. These results suggest that crustacean zooplankton and fish are more constrained by dispersal and therefore more likely to operate as a metacommunity than are bacteria and phytoplankton within this studied landscape.

The idea that alternative stable states may exist in communities has been a recurring theme in ecology since the late 1960s, and is now experiencing a resurgence of interest. Since the first papers on the subject appeared, two perspectives have developed to describe how communities shift from one stable state to another. One assumes a constant environment with shifts in variables such as population density, and the other anticipates changes to underlying parameters or environmental “drivers”. We review the theory behind alternative stable states and examine to what extent these perspectives are the same, and in what ways they differ. We discuss the concepts of resilience and hysteresis, and the role of stochasticity within the two formulations. In spite of differences in the two perspectives, the same type of experimental evidence is required to demonstrate the existence of alternative stable states.

1. While studies of phytoplankton and terrestrial plant communities have increasingly emphasised the use of functional traits in ecological research, few have yet to apply this approach to zooplankton communities. 2. This study reviews laboratory and observational studies on zooplankton feeding and life history and provides a series of functional trait tables for the North American freshwater zooplankton. Qualitative and quantitative trait tables highlight areas where data were more scarce and point to which types of studies could fill in gaps in our knowledge of zooplankton niches. 3. Data were most complete for the Cladocera across most traits, while feeding information for cyclopoids was most sparse. Qualitative data that distinguished congeneric species were lacking for most groups. 4. A regional community dendrogram for common north-eastern North American zooplankton species was generated and shows that taxonomic differences between species do not capture fully functional differences based on the traits of body length, habitat, trophic group and feeding type. 5. The data collected here, combined with readily measurable species attributes, can be used to generate a multivariate measure of the functional niche of each species found in a community. Armed with this information, functional relationships that are useful for ecological studies of lake ecosystems can be more easily conducted.

The importance of terrestrial-derived organic matter for lake zooplankton communities remains debated, partly because little is known about the basic pathways by which allochthonous carbon is transferred to zooplankton, and whether these vary among the major taxonomic and functional groups. We quantified allochthony of three zooplankton groups (Cladocera, Calanoida, and Cyclopoida) across 18 lakes in Quebec, spanning broad gradients of dissolved organic matter (DOM) and lake trophy, using a multi-isotope (delta2H + delta13C), multi-source (terrestrial, phytoplanktonic, benthic) approach. All three zooplankton groups had significant levels of allochthony, but differed greatly in their respective patterns across lakes. Allochthony in Calanoida and Cyclopoida was linked to detrital food chains based on particulate organic matter (POM) and on DOM, respectively, whereas in Cladocera it appeared related to both pathways; not surprisingly this latter group had the highest mean allochthony (0.31; compared to 0.18 in Cyclopoida and 0.16 in Calanoida). This study highlights the complexity of the pathways of delivery and transfer of terrestrial organic matter in freshwaters, and underscores the role that microbial food webs play in this transfer.

The vertical distribution of chlorophyll in stratified lakes and reservoirs frequently exhibits a maximum peak deep in the water column, referred to as the deep chlorophyll maximum (DCM). DCMs are ecologically important hot spots of primary production and nutrient cycling, and their location can determine vertical habitat gradients for primary consumers. Consequently, the drivers of DCM structure regulate many characteristics of aquatic food webs and biogeochemistry. Previous studies have identified light and thermal stratification as important drivers of summer DCM depth, but their relative importance across a broad range of lakes is not well resolved. We analyzed profiles of chlorophyll fluorescence, temperature, and light during summer stratification from 100 lakes in the Global Lake Ecological Observatory Network (GLEON) and quantified two characteristics of DCM structure: depth and thickness. While DCMs do form in oligotrophic lakes, we found that they can also form in eutrophic to dystrophic lakes. Using a random forest algorithm, we assessed the relative importance of variables associated with light attenuation vs. thermal stratification for predicting DCM structure in lakes that spanned broad gradients of morphometry and transparency. Our analyses revealed that light attenuation was a more important predictor of DCM depth than thermal stratification and that DCMs deepen with increasing lake clarity. DCM thickness was best predicted by lake size with larger lakes having thicker DCMs. Additionally, our analysis demonstrates that the relative importance of light and thermal stratification on DCM structure is not uniform across a diversity of lake types.

A standardized neighbor removal experiment was conducted in 12 plant communities located on three continents to test the null hypothesis that competition intensity (CI) was independent of the amount of plant biomass present. Six plots were chosen in each community to cover the range oflocal variation in plant biomass. In each plot the relative growth rate (RGR) of transplanted Poapratensis (Poaceae) seedlings was compared in the presence and absence of neighbors. Neighbors were removed experimentally using R. J. READER ET AL. Ecology, Vol. 75, No. 6 herbicide. Removing neighbors increased RGR of transplants significantly in most plots. CI increased with an increase in the amount of neighbor biomass present in one community where the range of neighbor biomass was greater than in any other community. In contrast, CI did not change significantly with an increase in neighbor biomass in other communities where the range of neighbor biomass was smaller. For the communities combined, CI was not related to neighbor biomass in a consistent fashion. These results indicate that competition may reduce growth over a wide range of habitat productivity, but the relationship between CI and neighbor biomass may differ among communities.

Theory predicts, and some evidence demonstrates that in lakes, the depth of the thermocline can have a large structural influence on the spatial distribution, and strongly influences the composition of plankton communities. However, experimental assessments of responses of the planktonic food web to thermocline depth have not yet been done at the whole-basin scale. We conducted an experiment wherein we artificially lowered the thermocline in an isolated basin of a three-basin lake, maintaining another isolated basin as a control. The vertical distribution and taxonomic composition of both phytoplankton and zooplankton were monitored throughout the summer months. Greater phytoplankton production, especially in the epilimnion, attributable mainly to increases in the chlorophytes was observed with thermocline deepening, but at the deepest thermoclines, production was limited. Total zooplankton biomass was unaffected by thermocline depth, suggesting top-down control by predators. Zooplankton biomass peaks were less pronounced in the manipulated basin, but tended to follow the thermocline whether at its normal position or as it was deepened. Zooplankton composition was significantly altered by large increases in densities of predatory cyclopoid copepods and rotifers; taxa commonly found in turbulent environments. Overall, both phytoplankton and zooplankton communities demonstrated important shifts in structure and composition in response to thermocline deepening.

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