Raphael Kudela
James Hagy
Lawrence W. Harding Jr.
David Senn
James E. Cloern
Suzanne B. Bricker
Gry Mine Berg
Marcus W. Beck
Martha Sutula
2017
<p><span>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 <a title="Learn more about Phytoplankton" href="http://www.sciencedirect.com/topics/agricultural-and-biological-sciences/phytoplankton" data-mce-href="http://www.sciencedirect.com/topics/agricultural-and-biological-sciences/phytoplankton">phytoplankton</a>. Observations in recent years suggest that this resistance may be weakening, shown by: significant increases of chlorophyll-</span><i>a</i><span><span> </span>(</span><i>chl-a</i><span>) 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<span> </span></span><i>chl-a</i><span><span> </span>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,<span> </span></span><i>chl-a,</i><span><span> </span>and algal toxins to derive quantitative relationships between three indicators (HAB abundance, toxin concentrations, DO) and<span> </span></span><i>chl-a</i><span>. Quantile regressions relating HAB abundance and DO to<span> </span></span><i>chl-a</i><span><span> </span>were significant, indicating SFB is at increased risk of adverse HAB and low DO levels if<span> </span></span><i>chl-a</i><span><span> </span>continues to increase. Conditional probability analysis (CPA) showed<span> </span></span><i>chl-a</i><span><span> </span>of 13 mg m</span><sup>−3</sup><span><span> </span>as a “protective” threshold below which probabilities for exceeding alert levels for HAB abundance and toxins were reduced. This threshold was similar to<span> </span></span><i>chl-a</i><span><span> </span>of 13–16 mg m</span><sup>−3</sup><span><span> </span>that would meet a SFB-wide 80% saturation Water Quality Criterion (WQC) for DO. Higher “at risk”<span> </span></span><i>chl-a</i><span><span> </span>thresholds from 25 to 40 mg m</span><sup>−3</sup><span><span> </span>corresponded to 0.5 probability of exceeding alert levels for HAB abundance, and for DO below a WQC of 5.0 mg L</span><sup>−1</sup><span><span> </span>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<span> nutrient management</span><span> </span>actions. This approach is transferrable to other estuaries to derive<span> </span></span><i>chl-a</i><span><span> </span>thresholds protective against eutrophication.</span></p>
application/pdf
10.1016/j.ecss.2017.07.009
en
Elsevier
Novel analyses of long-term data provide a scientific basis for chlorophyll-a thresholds in San Francisco Bay
article