Autochthony, allochthony and the role of consumers in influencing the sensitivity of aquatic systems to nutrient enrichment

Primary consumers may mitigate or exacerbate the effects of nutrient enrichment by grazing on plant matter and recycling nutrients. Few studies have quantified these effects for a suite of consumers and in the context of other processes regulating plant standing crop and nutrient supply. We quantified the abundance, feeding and diet of zooplankton, benthic filter-feeders, and planktivorous and detritivorous fish in the James River Estuary and found that consumer-mediated fluxes of CHLa and N were small in comparison to other fluxes regulating phytoplankton abundance (production, respiration, advection) and N availability (external inputs, internal recycling). Chlorophyll-a ingestion by consumers was equivalent to 15% of daily phytoplankton production and N recycling by consumers corresponded to 29% of phytoplankton N demand. The bulk of phytoplankton production (74%) was lost to autotrophic and heterotrophic respiration. Recycling of N contained in autochthonous and allochthonous organic matter was sufficient to meet 100% of phytoplankton N demand with external inputs corresponding to 39% of phytoplankton N demand. Experiments using 2000 L outdoor mesocosms with natural plankton communities showed that the presence of consumers (shad) resulted in higher N availability, elevated CHLa and reduced macrozooplankton abundance. Overall, our study showed that the direct effect of consumers on fluxes of CHLa and N was small as the bulk of phytoplankton biomass and N passed to microbial decomposers. However selective grazing by planktivorous fishes reduced zooplankton densities resulting in greater phytoplankton yield at lower nutrient concentrations.