Comparing Measures of Estuarine Ecosystem Production in a Temperate New England Estuary
Anthropogenic nutrient enrichments and concerted efforts at nutrient reductions, compounded with the influences of climate change, are likely changing the net ecosystem production (NEP) of our coastal systems. To quantify these changes, scientists monitor a range of physical, chemical, and biological parameters sampled at various frequencies. Water column chlorophyll concentrations are arguably the most commonly used indicator of net phytoplankton production, as well as a coarse indicator of NEP. We compared parameters that estimate production, including chlorophyll, across an experimental nutrient gradient and in situ in both well-mixed and stratified estuarine environments. Data from an experiment conducted in the early 1980s in mesocosms designed to replicate a well-mixed mid-Narragansett Bay (Rhode Island) water column were used to correlate changes in chlorophyll concentrations, pH, dissolved oxygen (O2), dissolved inorganic nitrogen, phosphate, and silicate concentrations, cell counts, and 14C carbon uptake measurements across a range of nutrient enrichments. The pH, O2, nutrient, and cell count measurements reflected seasonal cycles of spring blooms followed by late summer/early fall respiration periods across nutrient enrichments. Chlorophyll concentrations were more variable and rates of 14C productivity were inconsistent with observed trends in nutrient concentrations, pH, and O2 concentrations. Similar comparisons were made using data from a well-mixed lower Narragansett Bay station and a more stratified upper Narragansett Bay station in 2007 and 2008. Trends among pH, O2, and nutrient concentration parameters were similar to those observed in the mesocosm dataset, suggesting that continuous free water measurements of pH and O2 seem to reliably reflect ecosystem metabolism and, while not perfect measures, may be underused indicators of NEP.
KeywordspH Dissolved oxygen Production Estuary Eutrophication Nutrients Ecosystem
We would like to thank all of the scientists and support staff who contributed to the MERL mesocosm experiments and the development of such thorough datasets that are still used, even after 30 years. The manuscript was improved by feedback from Dr. Jason Grear, Dr. Richard Pruell, Roxanne Johnson, and three anonymous reviewers. This is ORD Tracking Number ORD-013005 of the Atlantic Ecology Division, National Health and Environmental Effects Research Laboratory, Office of Research and Development, US Environmental Protection Agency. Although the research described in this article has been funded in part by the US Environmental Protection Agency, it has not been subjected to Agency review. Therefore, it does not necessarily reflect the views of the Agency. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
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