Grazers, pathogens and shelf-shading enhance phytoplankton species richness more and reduce productivity less when environments are less dynamic: A theoretical study

Abstract

Human activities, such as reservoir construction, can result in hydraulically less dynamic systems and cause downstream systems to shift from sudden resource supply transitions to gradual. In this research, how such human activities might influence phytoplankton is further explored theoretically. Building on previous modeling results, new findings suggest that preferential grazing, pathogens, self-shading, and all these combined interact differentially with the mode of resource loading (sudden and gradual transitions) to shape phytoplankton assemblage characteristics. Most notably, phytoplankton species richness was much greater in scenarios that considered pathogen effects, self-shading effects, and combined preferential grazing, pathogen and shelf-shading effects when resource supply transitions were gradual compared to when they were sudden, i.e., 2.7-fold increase, 2.4-fold increase and 1.7-fold increase, respectively. Furthermore, reduced productivity with the additions of preferential grazing, pathogens, self-shading, and all these combined was lessened when resource supply transitions were gradual. Smaller differences to phytoplankton species evenness, overyielding, species interactions, niche breadth, and resource drawdown were also observed when comparing scenarios with sudden and gradual resource supply transitions. The nuanced details of preferential grazing, pathogens and shelf-shading uncovered here underscore the fundamental principle framed in the philosophy of Okham's Razor. How much complexity must we account for to understand how plankton systems will respond to altered land use in watersheds, and associated changed hydrology and nutrient loading? In this cursory effort, with simplistic additions of preferential grazing, pathogens and self-shading, findings highlight the need to explore empirically the effects of impoundment construction on downstream estuarine ecosystems.