Effects of added nutrients and salinity on freshwater ecosystems

Freshwater ecosystems are commonly exposed to excess nutrients and salinity from anthropogenic activities. Excess nutrients can increase the growth of producers while increased salinity can kill sensitive taxa, resulting in wide-ranging direct and indirect effects on food webs, depending on the composition of the community. Using an outdoor mesocosm experiment, we studied the effects of added nutrients and increased salinity on freshwater communities containing phytoplankton, periphyton, filamentous algae, zooplankton, and snails (Lymnaea columella and Physa acuta). Our regression-style design consisted of 14 salt treatments crossed with two nutrient concentrations. We also included the presence or absence of submerged macrophytes (Elodea canadensis), which was treated as a covariate due to varying survival. We found that added nutrients increased the biomass of phytoplankton and filamentous algae, and through bottom-up effects increased the biomass of zooplankton and snail consumers. Increased salinity reduced the abundance of filamentous algae and zooplankton, through direct toxic effects, while increasing the biomass of phytoplankton, likely as a result of reduced grazing pressure. Most of the effects of salinity and nutrients were additive. However, a few interactions were observed. For example, high salinity eliminated the bottom up effect of nutrients on Lymnaea abundance and the effects of nutrient addition on periphyton increased with salinity. Overall, this work shows the impact multiple anthropogenic stressors can have on simple aquatic communities, furthering our knowledge on the vulnerability of freshwater ecosystems in the real world.