Long-Term Changes in Nutrient Loading and Stoichiometry and Their Relationships with Changes in the Food Web and Dominant Pelagic Fish Species in the San Francisco Estuary, California

Abstract

Nutrient enrichment is an important stressor in coastal ecosystems. This analysis tests the hypothesis that changes in nutrient loads, imbalances in nitrogen:phosphorus, and changes in nitrogen form, especially shifts to increasing loads of chemically reduced, rather than oxidized nitrogen, can have major impacts on food webs, from primary producers through secondary producers to fish. The application of cumulative sums of variability, the running total of deviations from normalized values over time, is a sensitive method for comparing rates of change between and among all parameters, including organisms of all trophic levels. This approach was applied to the San Francisco Estuary, California, demonstrating that abrupt changes in nutrient loads and nutrient form over the past several decades were correlated with food web changes, including pelagic fish collapse. Remediation of pelagic fish populations should be centered on reduction of nitrogen loads and reestablishment of balanced nutrient ratios delivered from point source discharges.