Santa Margarita River study offers insights on reducing nutrient loading to estuaries

SCCWRP and its partners have completed the first phase of a five-year study exploring how water-quality managers can set appropriate, scientifically defensible targets for reducing nutrient loading in the Santa Margarita River Watershed, an analysis that is expected to have widespread implications for management of similar estuaries across coastal Southern California.

Phase 1 of the study, published in June, involved figuring out how to appropriately apply and adapt mathematical models that predict how nutrients move through the Santa Margarita River estuary and alter its water quality. Nutrient loading to the Santa Margarita River watershed – which spans Riverside and San Diego counties – has led to substantial overproduction of macroalgae in the estuary, impairing water quality and beneficial uses.

San Diego Regional Water Quality Control Board staff is interacting with the study’s stakeholder workgroup on how to use its findings and recommendations to design an alternative Total Maximum Daily Load (TMDL) that will manage nutrient loading to the watershed; the Regional Board is expected to consider the proposed TMDL in late 2016.

The Santa Margarita River Watershed study is a pilot project demonstrating a watershed approach to implementing “nutrient numeric endpoints,” which are the State Water Board staff’s approach to nutrient and eutrophication management of watersheds.

The TMDL for the Santa Margarita estuary will establish an important precedent for how goals get set for California coastal estuaries dominated by macroalgal blooms. Macroalgae are the most common type of algae in Southern California estuaries.