Modelling oxygen dynamics in an intermittently stratified estuary: Estimation of process rates using field data

The relationship between bottom water dissolved oxygen concentration, verti cal stratification, and temperature was investigated for the Neuse River es tuary, North Carolina, a shallow, intermittently-mixed estuary using approx imately 10 years of weekly/biweekly, mid-channel data. A generalized additi ve model (GAM) was used to initially explore the major relationships among observed variables. The results of this statistical model guided the specif ication of a process-based model of oxygen dynamics that is consistent with theory pet simple enough to be parameterized using available field data. T he nonlinear optimization procedure employed allows for the direct estimati on of microbial oxygen consumption and physical reoxygenation rates, includ ing the effects of temperature and vertical stratification. These estimated rates may better represent aggregate system behaviour than closed chamber measurements made in the laboratory and in situ. The resulting model descri bes 79% of the variation in dissolved oxygen concentration and is robust wh en compared across separate locations and time periods. Model predictions s uggest that the spatial extent and duration of hypoxia in the bottom waters of the Neuse are controlled by the balance between the net oxygen depletio n rate and the frequency of vertical mixing events. During cool months, oxy gen consumption rates remain low enough to keep oxygen concentration well a bove levels of concern even under extended periods of stratification. A con centration below 4 mg 1(-1) is only expected under extended periods without vertical mixing when bottom water temperature exceeds 15 degreesC, while a concentration below 2 mg 1(-1) is only expected when water temperature exc eeds 20 degreesC. To incorporate thr effects of parameter uncertainty, mode l error, and natural variability on model prediction, we used Monte Carlo s imulation to generate distributions for the predicted number of days of hyp oxia during the summer season. The expected number of days with a dissolved oxygen concentration less than 4 mg 1(-1) is 46.8 with a standard deviatio n of 4.7, while 23.8 days are expected to have an oxygen concentration belo w 2 mg 1(-1) with a standard deviation of 4.2 days. When joined with models relating nutrient loading and productivity to benthic and pelagic respirat ion rates, this model will be useful for probabilistically predicting the i mpact of nutrient management on the frequency of low oxygen events. (C) 200 1 Academic Press.