Nutrient Sources and Cycling

Assessing the Bay’s Resilience to High Nutrient Loads

For decades, San Francisco Bay has experienced significant nutrient enrichment, largely from the region's 42 publicly-owned treatment works (POTWs), which process wastewater for the region’s 8 million people. Despite effective removal of other pollutants, POTWs release 50,000 kg of nitrogen daily into the Bay. While the Bay has historically resisted major impacts of nutrient enrichment, increasing phytoplankton levels and harmful algae occurrences raise concerns about its resilience.

The San Francisco Bay Nutrient Management Strategy (NMS) aims to understand nutrient impacts on the Bay, relying on an ever-evolving monitoring program, data synthesis, and numerical models to study nutrient sources, nutrient transformations, and water quality effects, guided by a 5-year plan to inform management decisions.

Our Work Products

Understanding Nutrient Transport

Once introduced into the Bay, nutrients from wastewater and other sources disperse throughout the system via tides, wind, and currents, undergoing complex chemical reactions. Nutrient losses from the Bay occur through biological processes like denitrification and export to the Pacific Ocean. Effective nutrient management requires estimating nutrient transport and transformations, including understanding how individual sources impact nutrient concentrations spatially and temporally.

Nutrient Losses, Oxygen Levels, and Ecosystem Health

Despite substantial nutrient inputs, losses occur through phytoplankton-mediated transformations and transport pathways. The NMS investigates these cycles through intensive monitoring and synthesis, using advanced hydrodynamic and water quality models to test management scenarios and the consequences of future conditions.

Phytoplankton dynamics are influenced by light availability and nutrients, with deep Bay channels maintaining low biomass except during stratification events, where lighter freshwater flows on top of denser saline seawater. Shallower parts of the Bay foster higher concentrations of phytoplankton biomass and can initiate larger algal blooms. Prolonged dissolved oxygen drops in some Lower South Bay sloughs underscore the ongoing need for research and management to address nutrient-related challenges in the Bay ecosystem.

Nutrient Export and Effects on the Outer Coast

Nutrient inputs from the Bay represent a source of nutrients to the Pacific Ocean, with implications for harmful algal blooms, ocean acidification, and lower dissolved oxygen. Initial models suggest significant nitrogen contributions through the Golden Gate, but the coastal ecological impacts remain underexplored.

A collaborative modeling initiative, involving the Southern California Coastal Water Research Project (SCCWRP), UCLA, UC Santa Cruz, and the National Marine Sanctuaries, seeks to quantify the effects of Bay-derived nutrients on the California coast's ecosystem health, focusing on water quality responses and the Bay's incremental impact on coastal regions.

Understanding Nutrients in Shallow Parts of the Bay

Since 2021, the NMS has conducted high-frequency lateral mapping cruises with USGS to study biogeochemical variability on the eastern shoals of the South Bay, areas less monitored than deeper channels. Utilizing the USGS Aiken research boat equipped with advanced sondes, these cruises measure parameters like chlorophyll-a, nutrients, and dissolved oxygen with high resolution. The data support various NMS projects, including moored sensor network design, model validation, and development of satellite algorithms.

During the 2022 Heterosigma akashiwo bloom, rapid response cruises provided key insights into the bloom's dynamics. Initial findings reveal higher primary production and chlorophyll-a on the shoals compared to the channel, with significant spatial variability. Preliminary analyses suggest a strong relationship between gross primary production (GPP), nitrate availability, and pH, indicating photosynthetic activity's impact on carbon dioxide and nitrate levels. This work is critical for understanding the bay's biogeochemistry and informing ecosystem management.

ASSOCIATED STAFF