Light intensity is known to affect freshwater community dynamics dramatically, although community-level experiments with varying light intensity have rarely been conducted. To explore how complex interspecific interactions alter the effects of light intensity on a focal functional group, we examined the effects of shading on pond ecosystems. At the Cornell University Research Ponds Facility (each pond 0.1 ha surface area and 1.5 m deep), we used opaque floating swimming-pool covers to create duplicate ponds with high (75%), medium (57%), and control (0%) shading. We measured phytoplankton biomass (chlorophyll-a), submersed macrophyte biomass, water-column phosphorus (SRP), and other community indices biweekly from July to September 2015.
Results/Conclusions
Unexpectedly, we found that phytoplankton biomass increased in high-shading relative to medium-shading ponds, and that whereas one of the control ponds had the lowest phytoplankton biomass, the other had phytoplankton biomass as great as in the high-shading ponds. A clue to the cause of this counterintuitive result lies in our finding of a negative correlation between phytoplankton chlorophyll-a and the areal biomass of submersed rooted macrophytes. To understand these dynamics, we modeled competition between phytoplankton and macrophytes for light and nutrients. Our model produces alternative stable states under strong light (i.e., control ponds) and a gradual increase of phytoplankton with decreasing light (as in the medium-shading and high-shading ponds). Although it is difficult to assign with confidence underlying mechanisms in our large-scale community experiment, our observations and modeling suggest that competition with submersed plants can paradoxically increase phytoplankton in low light conditions.