Estimating estuarine gross production, community respiration and net ecosystem production: a nonlinear inverse technique

We describe a nonlinear inverse technique to estimate gross primary production (GPP), community respiration (CR), and net ecosystem production in the vertically well mixed Parker River, Plum Island Sound estuarine system located in northeastern Massachusetts, USA. The approach uses a calibrated 1D advection–dispersion model to predict oxygen and salt concentrations along the estuarine length, which are compared to oxygen concentrations measured during high speed transects near dawn and dusk over a 2-day period in June 1995. The spatiotemporal shapes of the GPP and CR surfaces are represented with cubic B-splines that are deformed as needed by an algorithm that seeks to minimize error between oxygen observations and model predictions. The spatially resolving solution shows maximum instantaneous GPP of 1030 mmol O2 m−3 d−1 and CR of 340 mmol O2 m−3 d−1 in the upper portions of the estuary; however, the overall estuary is net heterotrophic (−4.8 Mg C d−1). Analysis of the advection–dispersion model reveals that gas invasion and longitudinal dispersion account for up to 21 and 14% of local O2 accumulation relative to CR, respectively. However, an oxygen balance averaged over the 2-day sampling period shows that the negative net ecosystem productivity is balanced by a loss in estuarine oxygen storage. Carbon budget analysis indicates that terrestrial, marine, and marsh allochthonous inputs account for 2, 27 and 71% of the observed net heterotrophy, respectively.