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Scientific Bases for Numerical Chlorophyll Criteria in Chesapeake Bay |
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| Authors: | L W Harding Jr R A Batiuk T R Fisher C L Gallegos T C Malone W D Miller M R Mulholland H W Paerl E S Perry P Tango |
| Institution: | 1. Horn Point Laboratory, University of Maryland Center for Environmental Science, 2020 Horns Point Road, Box 775, Cambridge, MD, 21613, USA 2. Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, Los Angeles, CA, 90095, USA 3. Chesapeake Bay Program Office, U.S. Environmental Protection Agency, 410 Severn Avenue, Annapolis, MD, 21403, USA 4. Smithsonian Environmental Research Center, PO Box 28, Edgewater, MD, 21037, USA 5. U.S. Naval Research Laboratory, 4555 Overlook Ave., SW, Washington, DC, 20375, USA 6. Department of Ocean, Earth and Atmospheric Science, Old Dominion University, 4600 Elkhorn Ave., Norfolk, VA, 23529, USA 7. Institute of Marine Sciences, University of North Carolina at Chapel Hill, 3431 Arendell Street, Morehead City, NC, 28557, USA 8. 2000 Kings Landing Road, Huntingtown, MD, 20639, USA 9. Maryland Department of Natural Resources, 580 Taylor Avenue, Annapolis, MD, 21401, USA |
| Abstract: | In coastal ecosystems with long flushing times (weeks to months) relative to phytoplankton growth rates (hours to days), chlorophyll a (chl-a) integrates nutrient loading, making it a pivotal indicator with broad implications for ecosystem function and water-quality management. However, numerical chl-a criteria that capture the linkage between chl-a and ecosystem impairments associated with eutrophication (e.g., hypoxia, water clarity and loss of submerged aquatic vegetation, toxic algal blooms) have seldom been developed despite the vulnerability of these ecosystems to anthropogenic nutrient loading. Increases in fertilizer use, animal wastes, and population growth in the Chesapeake Bay watershed since World War II have led to increases in nutrient loading and chl-a. We describe the development of numerical chl-a criteria based on long-term research and monitoring of the bay. Baseline chl-a concentrations were derived using statistical models for historical data from the 1960s and 1970s, including terms to account for the effects of climate variability. This approach produced numerical chl-a criteria presented as geometric means and 90th percentile thresholds to be used as goals and compliance limits, respectively. We present scientific bases for these criteria that consider specific ecosystem impairments linked to increased chl-a, including low dissolved oxygen (DO), reduced water clarity, and toxic algal blooms. These multiple lines of evidence support numerical chl-a criteria consisting of seasonal mean chl-a across salinity zones ranging from 1.4 to 15 mg m?3 as restoration goals and corresponding thresholds ranging from 4.3 to 45 mg m?3 as compliance limits. Attainment of these goals and limits for chl-a is a precondition for attaining desired levels of DO, water clarity, and toxic phytoplankton prior to rapid human expansion in the watershed and associated increases of nutrient loading. |
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