Seasonal and Interannual Variability in Net Ecosystem Production of Two Subtropical Estuaries

Maria Herrmann, Pennsylvania State University Main Campus, Department of Meteorology and Atmospheric Science, University Park, United States, Raymond Najjar, The Pennsylvania State University, Meteorology and Atmospheric Science, University Park, United States, Henry Briceno, Florida International University, Southeastern Environmental Research Center, Miami, FL, United States, Chuanmin Hu, University of South Florida St. Petersburg, Optical Oceanography, St Petersburg, FL, United States and Nicole Millette, Virginia Institute of Marine Science, College of William & Mary, Gloucester Point, United States
Abstract:
A central organizing concept in estuarine biogeochemistry is net ecosystem production (NEP). However, estimates of seasonal and interannual variability of whole-system estuarine NEP, which provide insight into how estuaries respond to climatic and anthropogenic forcing, are rare. The main objectives of this study are to (1) determine the seasonal and interannual variability in whole-system NEP of Biscayne and Tampa Bay, two subtropical shallow estuaries located in southeastern Florida, USA, and (2) determine the potential driving mechanisms of NEP in these estuaries. We compute monthly NEP over more than a decade in each system by constructing the dissolved oxygen budget for the estuary from monthly snapshot oxygen survey data (i.e., collected once per month). High-frequency observations of oxygen in similar subtropical estuaries are used to quantify the error associated with the snapshot monthly sampling. Previously published analysis of Biscayne Bay from June 1996 to September 2008 will be extended through 2014 and compared to the results generated from the historical oxygen observations in Tampa Bay. In Biscayne Bay, oxygen air–water exchange and NEP closely balanced each other, with the long-term mean NEP (± 2 standard errors) equal to −5.3 ± 0.3 mol O2m−2year−1, indicating net heterotrophy. Significant seasonality was found, with lowest NEP in September. At monthly time scales, NEP was significantly positively correlated with chlorophyll and total phosphorus and significantly negatively correlated with canal flow. Interannual variability in NEP was substantial, and the bay temporarily shifted from net heterotrophy to net autotrophy after the 2005 Atlantic hurricane season, probably in response to increases in nutrients from runoff and resuspension. These findings show that monthly oxygen surveys can be used to quantify whole-system estuarine NEP and that Biscayne Bay NEP is sensitive to climatic and anthropogenic forcing on seasonal and interannual timescales.
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