Research Areas > Nutrients > Factors Controlling Biological Response to Nutrient Loads in Five San Diego Lagoons

Project: Factors Controlling Biological Response to Nutrient Loads in Five San Diego Lagoons

Background and Objective

Southern California coastal estuaries are heavily influenced by surrounding urban areas. Pollutant loading in watershed runoff coupled with restricted tidal exchange has resulted in higher nutrient concentrations and nutrient-related impairments (e.g., low dissolved oxygen, excessive algal growth, and eutrophication) in many systems.  Addressing these impairments requires selection of appropriate management targets for management. However, numeric nutrient thresholds are not yet available to environmental managers, partially due to an incomplete understanding of nutrient cycling within southern California lagoons. Recent SCCWRP research has indicated that within-lagoon sediment processes have an important influence, in that nutrients added to the sediments during the winter are released during the summer, providing an ongoing source of nutrition for macroalgal blooms.

The objective of this project was to build further understanding of the mechanisms and processes that control nutrient cycling in five southern California lagoons (Santa Margarita Estuary, Loma Alta Slough, Buena Vista Lagoon, San Elijo Lagoon, and Famosa Slough). Specifically, this project addressed the following questions:

1) What is the seasonal variability in porewater-derived and in situ benthic nutrient fluxes among the five lagoons?
2) What are the rates of sediment nutrient deposition associated with these trends?
3) How do these rates correlate to primary producer biomass and hypoxia in the lagoons?

Status

This project was initiated in 2007 and completed in 2011.

Methods

Field data were gathered and a dynamic computer model was created for use in establishing meaningful management and restoration targets. Field studies included:

• Measurement of seasonally sediment and particulate nutrient deposition
• Inventory of sediment bulk and porewater nutrient concentrations
• Estimation of in situ benthic (sediment-water interface) nutrient flux
• Measurement of algae biomass and variables affecting its growth

This SCCWRP project was part of a larger monitoring effort by stakeholders involved in TMDL data collection for the five San Diego Lagoons. Data collected by the stakeholders included watershed nutrient wet and dry weather loads, lagoon hydrodynamic and water quality data (water level, nutrients, dissolved oxygen, and other physiochemical data). The data were used to create dynamic computer simulation models that link various sources of nutrients (including sediments), algal growth and biomass, and dissolved oxygen within the lagoons. These models can be used to develop numeric nutrient targets and establish meaningful Total Maximum Daily Loads (TMDLs) or restoration goals.

Findings

The study found that all five estuaries in the study (Santa Margarita River Estuary, Loma Alta Slough, San Elijo Lagoon, Buena Vista Lagoon, and Famosa Slough) were experiencing symptoms of eutrophication. Symptoms included elevated algal biomass and episodes of low dissolved oxygen. For the lagoonal estuaries (Buena Vista, San Elijo Lagoon, and Famosa Slough), internal recycling of nutrients through benthic flux represented 80-90% of the total dry season nutrient loads to surface waters and was responsible for the high macroalgal biomass observed during this time period. For the river mouth estuaries (Santa Margarita River estuary and Loma Alta Slough), internal recycling was less important, representing 20-50% of total loads. River mouth estuaries are less susceptible to eutrophication because they have the ability to scour out final grained sediments that are the source of regenerated nutrients in the summer. Management options to reduce eutrophication were proposed for all five lagoons.

Partners

This project was conducted in collaboration with the University of California Los Angeles and Louisiana State University. Other partners included TMDL stakeholders, the San Diego Regional Water Quality Control Board, Weston Solutions Inc., MACTECH Inc., and CDM Inc.