A macro-tidal freshwater ecosystem recovering from hypereutrophication: the Schelde case study

Abstract

Abstract. We report a 40 year record of eutrophication and hypoxia on an estuarine ecosystem and its recovery from hypereutrophication. After decades of high inorganic nutrient concentrations and recurring anoxia and hypoxia, we observe a paradoxical increase in chlorophyll-a concentrations with decreasing nutrient inputs. We hypothesise that algal growth was inhibited due to hypereutrophication, either by elevated ammonium concentrations, severe hypoxia or the production of harmful substances in such a reduced environment. We study the dynamics of a simple but realistic mathematical model, incorporating the assumption of algal growth inhibition. It shows a high algal biomass, net oxygen production equilibrium with low ammonia inputs, and a low algal biomass, net oxygen consumption equilibrium with high ammonia inputs. At intermediate ammonia inputs it displays two alternative stable states. Although not intentional, the numerical output of this model corresponds to observations, giving extra support for assumption of algal growth inhibition. Due to potential algal growth inhibition, the recovery of hypereutrophied systems towards a classical eutrophied state, will need reduction of waste loads below certain thresholds and will be accompanied by large fluctuations in oxygen concentrations. We conclude that also flow-through systems, heavily influenced by external forcings which partly mask internal system dynamics, can display multiple stable states.

Fluxes of water, sediments, and biogeochemical compounds in salt marshes

“…Concentrations of DSi in marsh water are two-to fivefold higher in comparison to maximum concentrations observed in the upper reaches of estuaries: the average riverine DSi concentration is 100 μM (Conley 1997), but higher concentrations (up to 250 μM) have been observed in the freshwater tidal reaches of the Scheldt estuary (Belgium) (Carbonnel et al 2009). These are, however, maximum concentrations: in summer, diatom production can reduce DSi concentrations, even in the upstream tidal reaches, to concentrations near 5 μM (e.g., Cox et al 2009;Carbonnel et al 2009). In general, estuarine DSi concentrations also decrease with increasing salinity, due to conservative mixing with seawater and diatom uptake, reaching concentrations below 20 μM at higher salinity (e.g., Van Damme et al 2005).…”

Nutrient dynamics, transfer and retention along the aquatic continuum from land to ocean: towards integration of ecological and biogeochemical models

“…Regression approaches have in common that they rarely account for nonlinear interactions between sources and loss processes, which makes them less useful for assessing the complex interactions induced by disturbance. Moreover, systems may evolve from a source to a sink depending on the forcing and history (Turner et al, 2008;Cox et al, 2009). …”

Modeling photosynthesis‐irradiance curves: Effects of temperature, dissolved silica depletion, and changing community assemblage on community photosynthesis

“…Particularly for our study site, considerable changes in phytoplankton biomass and species composition have been reported over the last decade (Cox et al 2009). Possibly these changes also affected the community photosynthesis parameters, their seasonal variation, and their responses to nutrient limitation.…”

“…Since 1995, the main freshwater (FW) branch has been intensively monitored as part of the Onderzoek naar de Milieueffecten van het SIGMA plan (OMES; Research on the environmental effects of the SIGMA plan) project: monthly samples are taken at~6-km intervals (Table 1 and Fig. 1) Cox et al 2009). …”