How can top-down and bottom-up manipulation be used to mitigate eutrophication? Mesocosm experiment driven modeling zooplankton seasonal dynamic approach in the trophic cascade

Eutrophication leads to algae blooms and reduces the transparency of water bodies, which seriously affects water quality and ecosystem equilibrium, especially in shallow water body ecosystems (SWE). Controlling foodwebs by manipulating fish and macrophytes provides a feasible method to mitigate the effects of eutrophication. The response of zooplankton as the primary consumer to biomanipulation is mostly conceptualized and lacks detailed observation. Mesocosm experiments that altered the biomass of planktivorous fish and macrophytes were set up and their boundary conditions were extended into a series of scenarios for modeling biomanipulation. Thus, this study utilizes a one-dimensional lake ecosystem model Water Ecosystems Tool (WET) which considered each zooplankton group: rotifers, cladocerans, and copepods, to predict the seasonal dynamic effects of biomanipulation on zooplankton in SWE, and the model results are analyzed in comparison with the mesocosm results. Observed data from mesocosm experiments set up in a temperate pond, including water temperature, dissolved oxygen (DO), total nitrogen (TN), total phosphorus (TP), chlorophyll a (Chl a), macrophytes, zooplankton, and fish, were used to calibrate and validate the models. The modeled results showed that in spring and summer zooplanktivorous fish removal would increase all three categories of zooplankton and consequently cause a decrease of phytoplankton, whilst an increase in fish biomass would increase phytoplankton, and concomitantly water turbidity. However, in autumn, rotifers and phytoplankton increased in response to fish removal, but cladocerans and copepods decreased, 27% and 41%, respectively. Across all three vegetated seasons, increasing the biomass of macrophytes revealed a similar pattern: all three categories of zooplankton increased and phytoplankton subsequently decreased. Our study proposes a "fish-zooplankton-macrophyte-phytoplankton" trophic cascade and quantitatively predicts the dynamics of each zooplankton group under biomanipulation through this pathway, and provides support for establishing macrophyte beds and removing zooplanktivorous fish (in spring and summer) as an effective approach to mitigate eutrophication.

Coordination and divergence in community assembly processes across co-occurring microbial groups separated by cell size

Setting the pace of life and constraining the role of members in food webs, body size can affect the structure and dynamics of communities across multiple scales of biological organization (e.g., from the individual to the ecosystem). However, its effects on shaping microbial communities, as well as underlying assembly processes, remain poorly known. Here, we analyzed microbial diversity in the largest urban lake in China and disentangled the ecological processes governing microbial eukaryotes and prokaryotes using 16S and 18S amplicon sequencing. We found that pico/nano-eukaryotes (0.22-20 μm) and micro-eukaryotes (20-200 μm) showed significant differences in terms of both community composition and assembly processes even though they were characterized by similar phylotype diversity. We also found scale dependencies whereby micro-eukaryotes were strongly governed by environmental selection at the local scale and dispersal limitation at the regional scale. Interestingly, it was the micro-eukaryotes, rather than the pico/nano-eukaryotes, that shared similar distribution and community assembly patterns with the prokaryotes. This indicated that assembly processes of eukaryotes may be coupled or decoupled from prokaryotes' assembly processes based on eukaryote cell size. While the results support the important influence of cell size, there may be other factors leading to different levels of assembly process coupling across size classes. Additional studies are needed to quantitatively parse the influence of cell size versus other factors as drivers of coordinated and divergent community assembly processes across microbial groups. Regardless of the governing mechanisms, our results show that there are clear patterns in how assembly processes are coupled across sub-communities defined by cell size. These size-structured patterns could be used to help predict shifts in microbial food webs in response to future disturbance.