Ecological Stoichiometry, Biogeochemical Cycling, Invasive Species, and Aquatic Food Webs: San Francisco Estuary and Comparative Systems

Abstract

Eutrophication has altered food webs across aquatic systems, but effects of nutrient stoichiometry (varying nutrient ratios) on ecosystem structure and function have received less attention. A prevailing assumption has been that nutrients are not ecologically relevant unless concentrations are limiting to phytoplankton. However, changes in nutrient stoichiometry fundamentally affect food quality at all levels of the food web. Here, 30-year records of nitrogen and phosphorus concentrations and ratios, phytoplankton, zooplankton, macroinvertebrates, and fish in the San Francisco Estuary (Bay Delta) were examined to collectively interpret ecosystem changes within the framework of ecological stoichiometry. Changes in nutrient concentrations and nutrient ratios over time fundamentally affect biogeochemical nutrient dynamics that can lead to conditions conducive to invasions of rooted macrophytes and bivalve molluscs, and the harmful cyanobacterium Microcystis. Several other aquatic ecosystems considered here have exhibited similar changes in food webs linked to stoichiometric changes. Nutrient stoichiometry is thus suggested to be a significant driver of food webs in the Bay Delta by altering food quality and biogeochemical dynamics. Since nitrogen-to-phosphorus ratios have increased over time, an overall implication is that remediation of fish populations in the San Francisco Estuary will require significant nitrogen reductions to restore the historic ecological stoichiometric balance and the food web.