Eutrophication and climate warming alter spatial (depth) co-occurrence patterns of lake phytoplankton assemblages
The composition and dynamics of plankton communities are critically affected by human-induced environmental changes. We analysed 33 years of phytoplankton monthly data collected in Lake Zurich (Switzerland), assigning organisms (genus level) to taxonomic groups (class, family), Reynolds associations and size categories. The aim was to understand how eutrophication and climate change have influenced taxa co-occurrence patterns within and between groups over the lake water column (14 depths, 0–135 m), using null-models to test for non-random spatial (depth) assembly. We found that the whole community showed high taxa co-occurrence levels, significantly deviating over time from random assembly concurrently with lake warming and reduced nutrient loading. This pattern was driven mostly by the depth structure of metalimnetic assemblages during summer and autumn. The prevalence of non-random spatial patterns changed for different taxonomic and functional groups, with only few significant deviations from null-model expectations. Within taxonomic and functional groups (particularly Classes and size categories), the frequency of spatial overdispersion of taxa decreased over time while the frequency of clustering increased. Our data suggest that the relative importance of mechanisms determining phytoplankton metacommunity dynamics have changed along with environmental gradients shaping water column structure.
KeywordsPhytoplankton Co-occurrence Variance ratio Eutrophication Climate change Community assembly
We thank O. Koster (WVZ) for providing access and valuable insights to the Lake Zurich data, R. Ptacnik for critically reviewing the manuscript, and the anonymous reviewers for their constructive suggestions. FP research was in part supported by Eawag action field project Aquaprobe to BWI and OS.
OS and BWI originally formulated the idea; FP prepared the data; FP and BM performed statistical analyses; FP drafted the manuscript; all authors commented on the manuscript.
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