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.
Observations from single lake and experimental studies predict that vertical habitat heterogeneity in lakes can influence phytoplankton community structure. We examined the nature of water column physical habitat structure (light penetration, thermocline depth and shape and relative thermal resistance to mixing), and in turn, how these structures influenced the distribution of bulk chlorophyll a and the biomass of several major phytoplankton groups across 45 lakes in eastern Canada, within two lake districts which varied in watershed geology and water chemistry. Across all lakes, more pronounced temperature gradients favoured the distribution of bulk phytoplankton into more defined layers. The depth at which peak chlorophyll a was observed was affected by temperature heterogeneity and environmental factors related to light penetration. Peak depths and vertical heterogeneity of the major phytoplankton groups were differentially related to epilimnetic water colour and total phosphorus concentration across all lakes. Further insight was gained by comparing the physical structure and phytoplankton responses in the two regions. Lakes from the Laurentians Region had less wind exposure, shallower thermoclines, but greater vertical temperature variability than in the Eastern Townships Region. As a result, total and major phytoplankton group biomass showed more heterogeneous distributions in the Laurentians. The depth of peaks in total biomass and for the major phytoplankton groups was similar in both regions; the exception being a deeper chlorophyte maximum in the ETR, suggesting that there may be important differences between regions in the taxonomic composition of this group.
1. Patterns in phytoplankton diversity in lakes and their relationships with environmental gradients have been traditionally based on taxonomic analyses and indices, even though measures of functional diversity (FD) might be expected to be more responsive to such gradients. 2. We assessed the influence of water column physical structure, and other components of the overall environment, on lake phytoplankton diversity using two taxonomically based indices [species richness (S) and the Shannon index (H')] and a FD index, to determine whether these different measures respond in similar ways to habitat structure. The study encompassed 45 lakes in Eastern Canada, within two lake districts [the Eastern Townships Region (ETR) and Laurentians Region (LR)] that vary in geology and landscape and in lake morphometry and chemistry. 3. Across all lakes, S and H' were higher in lakes having greater vertical temperature heterogeneity and higher susceptibility to wind mixing. In addition, H' declined with total phosphorus concentration. FD was only related to maximum lake depth, a variable that integrates many other habitat features. 4. Further insight into the factors affecting phytoplankton diversity was obtained by contrasting the two regions. The taxonomically based diversity measures differed little between the regions, while FD was higher in the ETR where more trait variants were present and more evenly distributed amongst species. Whereas factors driving S did not differ between the regions, we found region-dependent patterns in the relationships of H' and FD with maximum lake depth: both indices decreased with maximum depth in the region with lakes more exposed to wind (ETR) but increased in the more hilly landscape where lakes are more sheltered from wind mixing (LR). 5. Our study demonstrates that, for phytoplankton communities, a FD index can show simpler and stronger responses to environmental drivers than a taxonomically based index, while shedding further light onto the functional traits that are important in particular lake categories.
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