Model Details

The setup for this model application is the North Sea and Channel area (domain 5.8^oW to 18.0^oE, 48.4^oN to 60.3^oN), on a spherical grid with approximately 10 kilometer resolution, with 25 sigma layers in the vertical. See below a map of theNorth Sea domain and a detailed map of the model showing the size of the 10x10 km grid cells grid along the Dutch coast.

Map of the North Sea domain for the GETM-ERSEM model

10x10 km grid of the GETM-ERSEM model

The GETM model is a full 3D hydrodynamical model which uses the General Ocean Turbulence Model (GOTM) for the vertical turbulence structure. Spherical coordinates (expressed in latitude and longitude) are used in the horizontal plane, while sigma coordinates are used in the vertical (equidistant in shallow water and contracted near the surface and the sea bed in deeper water). The hydrostatic equations are solved on an Arakawa C grid in the horizontal using a mode splitting technique. Sea surface elevation at the boundary is derived from Topex Poseidon satellite altimetry data, and the model allows for the drying and flooding of tidal flats. For further detail cf. Burchard and Bolding (2002) and Stips et al. (2004).

The temporally and spatially varying suspended matter (SPM) concentrations are calculated as a function of the local sea-bed composition and wave-shear stress, with wave height and -period calculated from the wind forcing data using an equilibrium JONSWAP formulation subject to a shoaling function, and wave-orbital velocities calculated using linear wave theory. An improved SPM model is under construction (van der Molen et al., 2009).

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ERSEM uses a functional group approach and contains 6 phytoplankton functional groups, 4 zooplankton functional groups and 5 benthic functional groups. (Meiobenthos, deposit-feeding macrobenthos, filter-feeding macrobenthos, benthic predators and epibenthic fauna such as shrimp). Bacterioplankton and benthic bacteria, both aerobic and anaerobic, are also included. The sediment is subdivided into three layers: an oxic layer, a denitrification layer and an anoxic layer.

The three layers have a fixed total thickness of 30 cm. The oxic and denitrification layers have a variable thickness, calculated by the model, while the remaining part is therefore the anoxic layer.

The light climate is determined by the vertical distribution and abundance of phytoplankton and (in)organic suspended matter. Phytoplankton use special pigments, chlorophylls, to capture and transform solar energy inorganic carbon into organic carbon biomass from carbon dioxide in the water (primary production). Both algal pigments and suspended particulate matter absorb light: Together they determine the underwater light climate and consequently how far light penetrate into the water column. In ERSEM chlorophyll is a state variable and as such dynamically modelled. By doing this we can account for the different light sensitivities of different algal groups and their different dynamics in time and space.

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 Pelagic productivity in coastal waters is besides the light constrained by nutrient availability. Nutrient availability limits the production of phytoplankton, which forms the basis of the food web, providing food for primary consumers, such as protozoa and meso-zooplankton, which the higher-level consumers (fish, squid) prey upon. In nutrient-replete conditions unicellular phytoplankton in the 5 to 100 µm size range is responsible for the bulk of the primary production in the sea and grazers such as meso-zooplankton control the phytoplankton biomass. However, as soon as nutrients are depleted, most primary production is generated by still smaller (0.5 - 10 µm) algae (cyano-bacteria and pico-phytoplankton). The main grazers under those circumstances are the heterotrophic nanoflagellates and the ciliates which are considered the dominant grazers of both bacteria and phytoplankton in the sea. The pelagic food web representation in this model now allows for complex interactions between the classical (phytoplankton, zooplankton, fish) food chain and the microbial food web. (phytoplankton, DOC/nutrients, bacterioplankton and  microzooplankton). Larger consumers, such as meso-zooplankton in this way are (indirectly) supported by the bacterioplankton, which is too small for them to graze directly.

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The food web of organisms living in or in the bottom of coastal waters is generally dependent on the pelagic food web through the sinking of living and dead (detritus) organic matter. This sinking is affected by various processes. The first process is the passive sedimentation of partulates (zooplankton faecal pellets, diatoms skeletons), the second one is the active filtration of organic matter from the water by suspension feeders such as bivalves (mussels, oysters). Especially food of high quality such as the sinking diatoms from the spring bloom is eagerly used by these organisms. The small food web in the sea-bottom is represented by the aerobic and anaerobic bacteria and the meio-benthos (1-5 um). The model contains 4 macro-benthos groups which are of similar size but have distinct functions in the system. Besides the suspension feeders we defined deposit-feeders, living in the sediment and grazing on bacteria and meiobenthos,  infaunal predators, also living in the sediment but preying on deposit and suspension feeders and epifaunal predators, living on the sediment and mainly preying on filter feeders. The main input for the food web in the benthos is detritus, used by the bacteria as their main food source. The bacteria form the basis of the food web in the benthic system. The higher trophic levels in the benthos are indirectly fed by these bacteria. In the model we limit ourselves to a coarse division in functional groups, ignoring the large variance in size even within a species.

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Baretta, J.W., Ebenhöh, W., Ruardij, P., 1995. "The European Regional Seas Ecosystem Model, a complex marine ecosystem model", Journal of Sea Research, 33, 233-246.

Burchard, H., Bolding, K., 2002. "GETM, a general estuarine transport model: scientific documentation" Technical Report no. EUR 20253 EN, European Commission, 157 pp.

Stips, A., Bolding, K., Pohlman, T., Burchard, H., 2004, Simulating the temporal and spatial dynamics of the North Sea using the new model getm, General Estuarine Transport Model), Ocean Dynamics, vol. 54, pp. 266-283, doi: 10.1007/s10236-003-0077-0

Van der Molen, J., Bolding, K., Greenwood, N., Mills, D.K., 2009. A multiple grain-size model of suspended particulate matter in combined currents and waves in shelf seas. Journal of Geophysical Research, 114, doi:10.1029/2008JF001150.

Vichi, M., Ruardij, P., Baretta, J.W., 2004. "Link or sink: a modelling interpretation of the open Baltic biogeochemistry", Biogeosciences, Vol. 1, 1, pp 79-100, SRef-ID: 1726-4189/bg/2004-1-79.