Present nitrogen and carbon dynamics in the Scheldt estuary using a novel 1-D model

A 1-D, pelagic, reactive-transport model of a completely mixed, turbid, heterotrophic estuary – the Scheldt estuary – is presented. The model resolves major carbon and nitrogen species and oxygen, as well as pH. The model features two organic matter degradation pathways, oxic mineralisation and denitrification, and includes primary production as well as nitrification. Apart from advective-dispersive transport along the length axis, the model also describes O<sub>2</sub>, CO<sub>2</sub>, and N<sub>2</sub> air-water exchange. The aim of this study is to present a model which is as simple as possible but still fits the data well enough to determine the fate and turnover of nutrients entering the estuary and their spatial patterns in the years 2000 to 2004. Nitrification is identified as one of the most important processes in the estuary, consuming a comparable amount of oxygen as oxic mineralisation (1.7 Gmol O<sub>2</sub> y<sup>−1</sup> vs. 2.7 Gmol O<sub>2</sub> y<sup>−1</sup>). About 10% of the 2.5 Gmol of nitrogen entering the estuary per year is lost within the estuary due to denitrification. Nitrogen and carbon budgets are compared to budgets from the seventies and eighties, showing that nitrification activity has peaked in the eighties, while denitrification steadily declined. Our model estimates an average CO<sub>2</sub> emission of 3.3 Gmol y<sup>−1</sup> in the years 2001 to 2004, which is a comparatively low estimate in the context of previous estimates of CO<sub>2</sub> export from the Scheldt estuary

Ecosystem engineering creates a direct nutritional link between 600-m deep cold-water coral mounds and surface productivity

Cold-water corals (CWCs) form large mounds on the seafloor that are hotspots of biodiversity in the deep sea, but it remains enigmatic how CWCs can thrive in this food-limited environment. Here, we infer from model simulations that the interaction between tidal currents and CWC-formed mounds induces downwelling events of surface water that brings organic matter to 600-m deep CWCs. This positive feedback between CWC growth on carbonate mounds and enhanced food supply is essential for their sustenance in the deep sea and represents an example of ecosystem engineering of unparalleled magnitude. This ’topographically-enhanced carbon pump’ leaks organic matter that settles at greater depths. The ubiquitous presence of biogenic and geological topographies along ocean margins suggests that carbon sequestration through this pump is of global importance. These results indicate that enhanced stratification and lower surface productivity, both expected consequences of climate change, may negatively impact the energy balance of CWCs

Application of two way nesting model to upscale sediment processes of the Southern Bight of the North Sea: full model validation

The BRAIN project FaCE-iT (Functional biodiversity in a Changing sedimentary Environment: Implications for biogeochemistry and food webs in a managerial setting) funded by BELSPO aims at evaluating the influence of offshore wind farms settlements and dredging activities on the distribution of sediment grain size over the Southern Bight of the North Sea (SBNS) and the Belgian Coastal Zone (BCZ), as well as associated impacts on biodiversity and biogeochemistry.In this framework an implementation of the tri-dimensional hydrodynamical and sediment transport model ROMS-COAWST was set-up to conduct scenario experiment relating offshore activities to resulting alteration of the seafloor structure. This implementation combines high resolution nested grids covering the Belgian Coastal Zone, embedded into a coarser grid covering the Southern Bight of the North Sea and is forced by ECMWF ERA-Interim data at the air-sea interface, CMEMS data at the open boundaries, TPXO data to introduce/force the tidal impact, and consider the discharge of four main rivers. Currently, the work focuses on assessing the skills of this modelling system to resolve the dynamics of the complex shallow and highly tidal region. The 3-year climatological run for 2006-2009 was performed to test the model ability to simulate the interannual dynamics. The model skills were evaluated by validation against remote-sensing temperature fields, tidal elevations and currents at the Meetnet pylons, and in situ temperature and salinity data provided by the Lifewatch network. We evaluate how grid refinement and different set-up of the nesting strategy enhance essential model skills in relation with sediment transport The further step will be to confront the sediment transport dynamics stemming from the nested system to that resolved from the coarser parent alone. A diagenetic model developed in the frame of FaCE-iT will be joint with the sediment model in order to upscale locally derived alteration of the biogeochemistry and benthic functionality stemming from seafloor texture alteration.Functional biodiversity in a Changing sedimentary Environment: Implications for biogeochemistry and food webs in a managerial setting (FaCE-It

Balance of assimilative and dissimilative nitrogen processes in a diatom-rich tidal flat sediment

Tidal flat sediments are subject to repetitive mixing and resuspension events. In a short-term (24 h) <sup>15</sup>N-labelling experiment, we investigated reactive nitrogen cycling in a tidal flat sediment following an experimentally induced resuspension event. We focused on (a) the relative importance of assimilatory versus dissimilatory processes and (b) the role of benthic microalgae therein. <sup>15</sup>N-labelled substrate was added to homogenized sediment, and <sup>15</sup>N was subsequently traced into sediment and dissolved inorganic nitrogen (DIN) pools. Integration of results in a N-cycle model allowed us to quantify the proportion of major assimilatory and dissimilatory processes in the sediment. <br><br> Upon sediment disturbance, rates of dissimilatory processes like nitrification and denitrification were very high, but declined rapidly towards a steady state. Once this was reached, the balance between assimilation and dissimilation in this tidal mudflat was mainly dependent on the nitrogen source: nitrate was utilized almost exclusively dissimilatory via denitrification, whereas ammonium was rapidly assimilated, with about a quarter of this assimilation due to benthic microalgae (BMA). Benthic microalgae significantly affected the nitrogen recycling balance in sediments, because in the absence of BMA activity the recovering sediment turned from a net ammonium sink to a net source. <br><br> The driving mechanisms for assimilation or dissimilation accordingly appear to be ruled to a large extent by external physical forcing, with the entire system being capable of rapid shifts following environmental changes. Assimilatory pathways gain importance under stable conditions, with a substantial contribution of BMA to total assimilation

The Westerschelde estuary: two food webs and a nutrient rich desert

Hummel et al. (1988) hypothesised the concomitant existence of two separate food chains in the Westerschelde: a photo-autotrophic coastal food chain in the marine part and a heterotrophic chain in the brackish part. The present study intends to re-examine the hypothesis on the basis of recently published data. Biomass gradients of the important functional units along an estuarine transect were observed to differ from those reported by Hummel et al. (1988) in some important aspects. The biomodal primary production gradient reported by Spaendonck et al. (in press) does not resemble the phytoplankton biomass curve, gradually increasing from the sea to Antwerp proposed by Hummel et al. (1988). Estimates of mesozooplankton biomass were found to be about an order of magnitude lower than reported by Hummel et al. (1988) and to display a completely different and more complex spatial pattern. However, the new gradient found is more in line with the hypothesis of two food chains than the gradient reported by Hummel et al. (1988). In the macrobenthos the biomass peak in the brackish part reported by Hummel et al. (1988) could not be confirmed. This finding does not falsify the original hypothesis as the function of this detritus dependent macrobenthic fauna is largely taken over by the hyperbenthic mysids, a group of previously unknown importance in the system. The existence of two food chains is also supported by the gradients observed in fish and epibenthic invertebrates, functional units not addressed by Hummel et al. (1988). In the zone between the two different food chains the dominant animal groups of the pelagic system have only a low biomass, this is the nutrient rich desert of the title. The zone upstream of the Dutch-Belgian border supports no hyperbenthos, no epibenthos and no mesozooplankton because of the low dissolved oxygen concentrations (less than 40% saturation), but there is a prominent peak in the microzooplankton. Clearly, in the brackish part, the richness of most functional units can only be explained on the basis of an input of organic matter from outside, consumed through a heterotrophic food chain. A second, smaller peak is observed close to the mouth of the estuary and is dependent on the primary production in the marine part of the estuary. Even for individual species this clear bimodal pattern can be observed. This disqualifies simplistic physiological models of estuarine succession as a basis for the findings. In the oxygenated part of the system there is no good general correlation between macrobenthic biomass (mostly suspension-feeders) and primary production. Macrobenthic biomass is highly variable in this zone, probably as a result of local differences in current velocity maxima. The new data confirm the view of Hummel et al. (1988) but it is concluded that these authors must have formulated their hypothesis intuitively and could not have done so from the data available at the time

Laboratory study of the impact of repetitive electrical and mechanical stimulation on brown shrimp Crangon crangon

Pulse trawling is currently the best available alternative to beam trawling in the brown shrimp Crangon crangon and Sole Solea solea (also known as Solea vulgaris) fisheries. To evaluate the effect of repetitive exposure to electrical fields, brown shrimp were exposed to the commercial electrodes and pulse settings used to catch brown shrimp (shrimp startle pulse) or Sole (Sole cramp pulse) 20 times in 4 d and monitored for up to 14 d after the first exposure. Survival, egg loss, molting, and the degree of intranuclear bacilliform virus (IBV) infection were evaluated and compared with those in stressed but not electrically exposed (procedural control) and nonstressed, nonexposed (control) brown shrimp as well as brown shrimp exposed to mechanical stimuli. The lowest survival at 14 d (57.3%) occurred in the Sole cramp pulse treatment, and this was significantly lower than in the group with the highest survival, the procedural control (70.3%). No effect of electrical stimulation on the severity of IBV infection was found. The lowest percentage of molts occurred in the repetitive mechanical stimulation treatment (14.0%), and this was significantly lower than in the group with the highest percentage of molts, the procedural control (21.7%). Additionally, the mechanically stimulated brown shrimp that died during the experiment had a significantly larger size than the surviving individuals. Finally, no effect of the shrimp startle pulse was found. Therefore, it can be concluded that repetitive exposure to a cramp stimulus and mechanical stimulation may have negative effects on the growth and/or survival of brown shrimp. However, there is no evidence that electrical stimulation during electrotrawls would have a larger negative impact on brown shrimp stocks than mechanical stimulation during conventional beam trawling