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Scientific Basis for Assessment of Nutrient Impacts on San Francisco Bay
Citation:
Sutula, M., R. Kudela, Jim Hagy, M. Berg, S. Bricker, J. Cloern, R. Dugdale, L. Harding, AND D. Senn. Scientific Basis for Assessment of Nutrient Impacts on San Francisco Bay. CERF 2015, Portland, OR, November 08 - 12, 2015.
Impact/Purpose:
Abstract for CERF
Description:
San Francisco Bay (SFB) is a large, nutrient-enriched estuary that appears resistant to symptoms of nutrient over-enrichment, such as high phytoplankton biomass and hypoxia. This resistance traces to high turbidity, strong tidal mixing, and grazing that limit production and accumulation of phytoplankton biomass. Recent trends of increasing chlorophyll-a suggest that this resistance is weakening, motivating efforts to develop water-quality goals for SFB that are protective from impairments. Here we present analyses to derive thresholds of chlorophyll-a in which SFB is likely to be “protected” and “at risk” of exceeding existing dissolved oxygen water quality objectives and harmful algal bloom alert levels. Analyses were conducted on 20 years (1993-2014) of phytoplankton biomass (chlorophyll-a) and species composition, and dissolved oxygen (DO), and three years of algal toxin data (2012-2014). We found increased presence of harmful algal species and decreasing DO were significantly correlated with chlorophyll-a. Quantile regression and conditional probability analysis (CPA) were used to assess the risk of HABs and low DO with increasing chlorophyll-a as an indicator of nutrient enrichment and were used to identify thresholds. These analyses will support recommendations for a framework to characterize the ecological condition of SFB, based on scientific data and expert judgment. This framework may serve as the basis to further explore, through iterations of monitoring, research and numerical modeling, nutrient loads that can sustain ecosystem health in SFB.