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Novel analyses of long-term data provide a scientific basis for chlorophyll-a thresholds in San Francisco Bay

Estuarine, Coastal and Shelf Science

By:
, , , , , ORCID iD , , , and
https://doi.org/10.1016/j.ecss.2017.07.009

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Abstract

San Francisco Bay (SFB), USA, is highly enriched in nitrogen and phosphorus, but has been resistant to the classic symptoms of eutrophication associated with over-production of phytoplankton. Observations in recent years suggest that this resistance may be weakening, shown by: significant increases of chlorophyll-a (chl-a) and decreases of dissolved oxygen (DO), common occurrences of phytoplankton taxa that can form Harmful Algal Blooms (HAB), and algal toxins in water and mussels reaching levels of concern. As a result, managers now ask: what levels of chl-a in SFB constitute tipping points of phytoplankton biomass beyond which water quality will become degraded, requiring significant nutrient reductions to avoid impairments? We analyzed data for DO, phytoplankton species composition, chl-a, and algal toxins to derive quantitative relationships between three indicators (HAB abundance, toxin concentrations, DO) and chl-a. Quantile regressions relating HAB abundance and DO to chl-a were significant, indicating SFB is at increased risk of adverse HAB and low DO levels if chl-a continues to increase. Conditional probability analysis (CPA) showed chl-a of 13 mg m−3 as a “protective” threshold below which probabilities for exceeding alert levels for HAB abundance and toxins were reduced. This threshold was similar to chl-a of 13–16 mg m−3 that would meet a SFB-wide 80% saturation Water Quality Criterion (WQC) for DO. Higher “at risk” chl-a thresholds from 25 to 40 mg m−3 corresponded to 0.5 probability of exceeding alert levels for HAB abundance, and for DO below a WQC of 5.0 mg L−1 designated for lower South Bay (LSB) and South Bay (SB). We submit these thresholds as a basis to assess eutrophication status of SFB and to inform nutrient management actions. This approach is transferrable to other estuaries to derive chl-a thresholds protective against eutrophication.

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Additional publication details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Novel analyses of long-term data provide a scientific basis for chlorophyll-a thresholds in San Francisco Bay
Series title:
Estuarine, Coastal and Shelf Science
DOI:
10.1016/j.ecss.2017.07.009
Volume:
197
Year Published:
2017
Language:
English
Publisher:
Elsevier
Contributing office(s):
National Research Program - Western Branch
Description:
12 p.
First page:
107
Last page:
118
Country:
United States
State:
California
Other Geospatial:
San Francisco Bay