Cold-water coral reefs and adjacent sponge grounds are distributed widely in the deep ocean, where only a small fraction of the surface productivity reaches the seafloor as detritus. It remains elusive how these hotspots of biodiversity can thrive in such a food-limited environment, as data on energy flow and organic carbon utilization are critically lacking. Here we report in situ community respiration rates for cold-water coral and sponge ecosystems obtained by the non-invasive aquatic Eddy Correlation technique. Oxygen uptake rates over coral reefs and adjacent sponge grounds in the Traena Coral Field (Norway) were 9-20 times higher than those of the surrounding soft sediments. These high respiration rates indicate strong organic matter consumption, and hence suggest a local focusing onto these ecosystems of the downward flux of organic matter that is exported from the surface ocean. Overall, our results show that coral reefs and adjacent sponge grounds are hotspots of carbon processing in the food-limited deep ocean, and that these deep-sea ecosystems play a more prominent role in marine biogeochemical cycles than previously recognized.

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The Schelde estuary, characterised as a turbid, polluted and eutrophic system, has nowadays reached a turning point in the restoration of its water quality. During the past century, human activities have reduced the intertidal areas, essential in the estuarine ecosystem for nutrient cycling and the self-cleaning capacity. Today, in combination with a master plan to protect the population from storm surges, an opportunity rises to restore areas with a tidal influence. One specific option of combining safety and ecology is the creation of flood control areas (FCA) under the influence of a controlled reduced tide (CRT). These specific areas will differ in many ways from fully tidal areas. However, these areas can fulfill important ecological functions with effects on aeration, nitrification, denitrification, sedimentation and primary production in the estuary. Opportunities for ecological development within a CRT have been investigated for a specific case. The ecology within a CRT showed to be very case specific, depending e.g. on the morphology of the area, the sluice design and the local water quality. Depending on the sluice design, water quality can be improved and sedimentation can be influenced. Possible measures to design a CRT with a rich habitat variation are discussed.

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Based on an analysis in the frequency domain of the governing equation of oxygen dynamics in aquatic systems, we derive a new method for estimating gross primary production (GPP) from oxygen time series. The central result of this article is a relation between time averaged GPP and the amplitude of the diel harmonic in an oxygen time series. We call this relation the Fourier method for estimating GPP. To assess the performance and accuracy of the method, we generate synthetic oxygen time series with a series of gradually more complex models, and compare the result with simulated GPP. We demonstrate that the method is applicable in systems with a range of rates of mixing, air–water exchange and primary production. We also apply the new method to oxygen time series from the Scheldt estuary (Belgium) and compare it with 14C‐based GPP measurements. We demonstrate the Fourier method is particularly suited for estimating GPP in estuarine and coastal systems where tidal advection has a large imprint in observed oxygen concentrations. As such it enlarges the number of systems where GPP can be estimated from in situ oxygen concentrations. By shifting the focus to the frequency domain, we also gain some useful insights on the effect of observational error and of stochastic drivers of oxygen dynamics on metabolic estimates derived from oxygen time series.

Abstract Sediment transport in estuaries and the formation of estuarine turbidity maxima (ETM) highly depend on the ability of suspended particulate matter (SPM) to flocculate into larger aggregates. While most literature focuses on the small-scale impact of biological flocculants on the formation of larger aggregates, the influence of the flocculation process on large-scale estuarine SPM profiles is still largely unknown. In this paper, we study the impact of flocculation of SPM on the formation of ETM. For this, a semianalytical width-integrated model called iFlow is utilized and extended by a flocculation model. Starting from a complex one-class flocculation model, we show that flocculation may be described as a linear relation between settling velocity and suspended sediment concentration to capture its leading-order effect on the ETM formation. The model is applied to a winter case in the Scheldt estuary (Belgium, Netherlands) and calibrated to a unique, long-term, two-dimensional set of turbidity (cf. SPM) observations. First, model results with and without the effect of flocculation are compared, showing that the spatial and temporal variations of the settling velocity due to flocculation are essential to reproduce the observed magnitude of the suspended sediment concentrations and its dependence on river discharge. Second, flocculation results in tidally averaged land-inward sediment transport. Third, we conduct a sensitivity analysis of the freshwater discharge and floc breakup parameter, which shows that flocculation can cause additional estuarine turbidity maxima and can prevent flushing of the ETM for high freshwater inflow.

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.