Plankton community patterns across a trophic gradient: The role of zooplankton functional groups

Footprint of the plastisphere on freshwater zooplankton

Changes in the functional groups of zooplankton were studied in autumn in a temperate floodplain lake (Lake Sakadaš, Kopački Rit Nature Park, Croatia) and in the Drava River (in the Croatian part of the river). Various abiotic parameters as well as available food sources (phytoplankton and microphytes (algae and cyanobacteria) developing on epixylon, epilithon and artificially introduced microplastics called "plastisphere") were also studied. The lake was hydrologically isolated from the main river during the study, while the water level of the Drava River fluctuated, resulting in larger variations in limnological parameters. Due to stable conditions in the lake, zooplankton abundance, biomass, and species richness were higher than in the Drava River. In both environments, zooplankton species feeding on bacteria, detrital suspensions, and small algae were most abundant, with predators and microfilter-feeders being more abundant in the lake. Microphytes were diverse and mostly small and medium-sized in phytoplankton and all substrate types. Stable lake conditions promoted higher abundance of the zooplankton group, which effectively uses larger algae as a food source. The lower abundance of zooplankton feeding on larger algae and predatory species in the river suggests that the epilithon and plastisphere community was a less mature community compared to the lake, and the heterotrophic component with ciliates and/or other small heterotrophs was not well developed. The importance of plastispheres was particularly evident under the turbid hydrologic conditions that prevailed in the river at the end of the study, when phytoplankton biomass decreased and zooplankton abundance steadily increased, suggesting that microphytes colonised on microplastics were an additional food source for higher trophic levels.

Environmental seasonality on predator-prey systems under nutrient and toxicant constraints

Bioaccumulation of toxicants in aquatic food webs can pose risks to ecosystem function and human health. Toxicant models of aquatic ecosystems can be improved by incorporating realistic environmental impacts such as nutrient availability and seasonality. It is well known that the carrying capacity of predator-prey systems can vary seasonally due to environmental cycles resulting from natural and human activities. As such, incorporating seasonal variation in the carrying capacity of a predator-prey system provides a better understanding of the underlying population dynamics bioaccumulation of toxicants. Here, we develop a seasonally varied predator-prey model subject to concurrent nutrient and toxicant stressors. We investigate the effects of seasonality on population dynamics to increase understanding of the complex governing processes of the trophic transfer of nutrients, energy, and toxicants. We observe that the strength of seasonality can shift solutions from periodic to quasi-periodic and models that neglect environmental seasonality may be under-predicting adverse effects of toxicity.

Intraguild Predation Dynamics in a Lake Ecosystem Based on a Coupled Hydrodynamic-Ecological Model: The Example of Lake Kinneret (Israel)

The food web of Lake Kinneret contains intraguild predation (IGP). Predatory invertebrates and planktivorous fish both feed on herbivorous zooplankton, while the planktivorous fish also feed on the predatory invertebrates. In this study, a complex mechanistic hydrodynamic-ecological model, coupled to a bioenergetics-based fish population model (DYCD-FISH), was employed with the aim of revealing IGP dynamics. The results indicate that the predation pressure of predatory zooplankton on herbivorous zooplankton varies widely, depending on the season. At the time of its annual peak, it is 10–20 times higher than the fish predation pressure. When the number of fish was significantly higher, as occurs in the lake after atypical meteorological years, the effect was a shift from a bottom-up controlled ecosystem, to the top-down control of planktivorous fish and a significant reduction of predatory and herbivorous zooplankton biomass. Yet, seasonally, the decrease in predatory-zooplankton biomass was followed by a decrease in their predation pressure on herbivorous zooplankton, leading to an increase of herbivorous zooplankton biomass to an extent similar to the base level. The analysis demonstrates the emergence of non-equilibrium IGP dynamics due to intra-annual and inter-annual changes in the physico-chemical characteristics of the lake, and suggests that IGP dynamics should be considered in food web models in order to more accurately capture mass transfer and trophic interactions.

A case study of an enhanced eutrophication model with stoichiometric zooplankton growth sub-model calibrated by Bayesian method

Urban lakes in China have suffered from severe eutrophication over the past several years, particularly those with relatively small areas and closed watersheds. Many efforts have been made to improve the understanding of eutrophication physiology with advanced mathematical models. However, several eutrophication models ignore zooplankton behavior and treat zooplankton as particles, which lead to the systematic errors. In this study, an eutrophication model was enhanced with a stoichiometric zooplankton growth sub-model that simulated the zooplankton predation process and the interplay among nitrogen, phosphorus, and oxygen cycles. A case study in which the Bayesian method was used to calibrate the enhanced eutrophication model parameters and to calculate the model simulation results was carried out in an urban lake in Tianjin, China. Finally, a water quality assessment was also conducted for eutrophication management. Our result suggests that (1) integration of the Bayesian method and the enhanced eutrophication model with a zooplankton feeding behavior sub-model can effectively depict the change in water quality and (2) the nutrients resulting from rainwater runoff laid the foundation for phytoplankton bloom.