Seasonal Productivity Estimates from the Diurnal Oxygen Budget in Two Subtropical Estuaries

Subtropical estuaries worldwide are facing increasing pressure from human population growth, development, and climate change. Carbon is a useful currency for understanding how estuaries respond to these pressures and yet relatively little is known about carbon cycling in subtropical estuaries. Here we compute gross primary production (GPP), community respiration (R), and net ecosystem production (NEP) from the diurnal cycle in dissolved oxygen measured during week-long deployments in two estuaries in the southeastern United States, Biscayne Bay and Tampa Bay. The dominant term in the oxygen budget is the rate of oxygen change between day and night; gas transfer and the longer-term oxygen change are relatively small. GPP and R nearly balance; NEP is about an order of magnitude smaller. Mean GPP was the same in Tampa Bay and Biscayne Bay, but seasonality was much larger in Biscayne Bay. Our GPP estimates in Biscayne Bay are more than an order of magnitude greater than the only other productivity estimates available for this system, which are planktonic net primary productivity (NPP) measurements from the late 1970s. The difference likely reflects the importance of seagrass to primary production as well as long-term increases in nutrients in Biscayne Bay. Tampa Bay is much better monitored for productivity and mean planktonic NPP rates are about a factor of two smaller than our GPP estimates, a difference that likely reflects the limited temporal extent of our measurements as well as the difference between NPP and GPP.