Seasonal and Interannual Variability in Net Ecosystem Production of a Subtropical Coastal Lagoon Inferred from Monthly Oxygen Surveys

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 Bay, a subtropical, shallow estuary located in southeastern Florida, USA, and (2) determine the potential driving mechanisms of NEP in this estuary. We compute monthly NEP over more than 12 years 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 were used to quantify the error associated with the snapshot monthly sampling. 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 O ₂  m ⁻²  year ⁻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.