Human activities during the Anthropocene are associated with biodiversity loss and taxonomic homogenization of ecological communities, but the mechanisms underlying these trends are often obscure. Here we estimated effects of eutrophication and heavy metal contamination, separately and in combination, in explaining zooplankton species composition during the past 125-145 years using analysis of daphniid diapausing egg banks from four lakes in the Northeastern USA. We tracked historic changes in pollution in our study lakes by measuring concentrations of heavy metal contamination and indicators of nutrient contamination and productivity preserved in lake sediment archives. To determine the relative influence of metal contamination and eutrophication on changes in daphniid composition we conducted redundancy analysis followed by variation partitioning for each lake. We then examined how these community shifts influenced patterns of diversity and homogenization.
Results/Conclusions
Estimates of past lake production (via fossil pigments) and metal contamination (via sedimentary metals) demonstrated that eutrophication alone (19-39%) and in combination with metal pollution (17-54%) explained 36-79% of historical variation in daphniid species relative abundances in heavily fertilized lakes. In contrast, metal pollution alone explained the majority (72%) of historical variation in the daphniid assemblage at the oligotrophic site. We observed repeated episodes of species colonization of eutrophying lakes, resulting in increased species richness and gamma diversity through time. At the same time, daphniid assemblages in three eutrophied lakes became more similar to each other (homogenized) but this pattern was only seen when accounting for species presence/absence. We did not observe consistent patterns of divergence between the assemblages in the eutrophying lakes and the low-nutrient reference site. Given the pervasive nature of fertilization, we suggest that many inhabited lake districts may exhibit similar shifts in daphniid assemblage composition driven by colonization and increasing densities of large-bodied Daphnia species.