Shallow coastal seas are generally characterized by high turn-over rates of resources. Due to wave action and tidal energy, mixing depths are relatively deep, and biogenic particulates are regularly resuspended and deposited, resulting in a strong benthic-pelagic coupling and an efficient coupling of production and remineralization.Temporal and spatial patterns in biogeochemistry and hydrography for such basins can only be reproduced by combining a biogeochemical model with a 3D hydrodynamic model which then provides the basics for an ecosystem model. 

The GETM-ERSEM is an example of such a hybrid ecosystem model, which marries a new and extended version of the Biochemical Flux Model (BFM) to the 3D General Estuarine Transport model (GETM), which in turn uses the General Ocean Turbulence Model (GOTM) for the vertical turbulence structure. The resulting model describes the biogeochemical processes in both the pelagic and the benthic domain and puts benthic-pelagic exchanges at the center of the implementation effort. 

This new GETM-ERSEM model enhances our understanding of the North Sea ecosystem, both in terms of its structure as well as its function. The biological submodel is a Fortran-95 version and builds on the BFM and ERSEM III (European Regional Seas Ecosystem Model, see Baretta et al., 1995 and Vichi et al., 2004). It dynamically describes the biogeochemistry of the water column and sediment for temperate seas. BFM simulates the cycles of Carbon, Oxygen and the macro-nutrients N, P and Si, allowing for variable internal nutrient ratios of organisms, based on external availability and physiological status of the organisms.

The ECMWF ERA 40 reanalysis data were used as the meteorological forcing. Atmospheric deposition of nutrients was not included. The model was spun up for a period of 8 years until a repeating seasonal cycle in the pelagic variables was achieved.

GETM-ERSEM can be run on relatively inexpensive computer clusters without additional effort, which allows both CEFAS and NIOZ to run such a complex model system sufficiently long to project climate change effects for the North Sea on decadal time scales.

GETM-ERSEM is being developed by CEFAS and the Royal NIOZ, with external assistance from Karsten Bolding (Bolding & Burchard ApS, Denmark), Marcello Vichi (CMCC: Euro-Mediterranean Center for Climate Change, Italy) and Job Baretta (Noctiluca NL).

Spatial modelling with the GETM-ERSEM-BFM model: the basic flow of carbon and nutrients within ERSEM-BFM (left) and the application of the water column model for spatial studies (right). Note that the hydrodynamical model GETM provides the horizontal transports for physical, chemical and biological properties. The water column model (right) relies on the hydrodynamical model GOTM for the vertical transports.