|A regime shift in a freshwater estuarine ecosystem recovering from hypereutrophication?|
Cox, T.; Maris, T.; Soetaert, K.; Conley, D.; Van Damme, S.; Meire, P.; Struyf, E. (2009). A regime shift in a freshwater estuarine ecosystem recovering from hypereutrophication?, in: ASLO Aquatic Sciences Meeting 2009: A Cruise Through Nice Waters, 25-30 January 2009, Nice, France. pp. 56
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After decades of hypereutrophication, with strong hypoxia and elevated ammonia concentrations, water quality in the freshwater part of the Schelde estuary changed rapidly. Average May-Sep oxygen concentrations more than quintupled, from 8% of saturation (1996) to 51% (2007). Ammonium concentrations decreased from a 296 µM May to Sep average (1996) to 16 µM (2007). Average May-Sep chlorophyll a concentrations increased from 15 µg/l (1996) to 178 µg/l (2007). From 2006 oversaturated oxygen concentrations are occasionally observed, caused by algal primary production. We hypothesise that inhibition of primary production and algal growth due to anoxia and/or ammonia toxicity prevented the build-up of algal biomass until a decade ago. We study the steady states and dynamics of a simple mathematical model, including this algal growth inhibition. It displays a regime shift from a heterotroph state to an autotroph state, when ammonia load is reduced below a threshold. At intermediate loads it predicts fluctuations in oxygen concentration in response to discharge peaks. Data and model results are consistent, and thus indicate a possible regime shift from heterotrophy to autotrophy in this ecosystem.