Nutrient Dynamics in European Water Systems
Synthesis Results

3. Fate of nutrients in coastal areas (4 of 4)

Studies on sediment dominated by seagrass confirm the trend that 'vegetated' sites are autotrophic or only slightly heterotrophic and that nitrogen-uptake and storage in biomass is a dominant factor in the sediment nitrogen cycle. Welsh et al. (2000) drew up a nitrogen budget of a Zostera noltii system (Arcachon Bay). DIN (dissolved inorganic nitrogen) fluxes towards the plants and sediments were entirely dominated by the plants. Surprisingly, the plant community showed a high dark assimilation activity for inorganic-nitrogen and differences in light and dark fluxes of DIN, nitrate and ammonium were never significant (Figure 3.2(b)). In general denitrification was very low. In general, denitrification was very low. Similarly, rates of denitrification coupled to nitrification were consistently low, probably due to the competition between nitrifying bacteria and the Zostera roots for ammonium

The function of seagrass beds as sinks for nitrogen is furthermore supported by many other studies within ELOISE. Risgaard-Petersen & Ottosen (2000) in eelgrass beds showed N-sequestration in spring and summer, nitrogen release during autumn and very low denitrification rates throughout the year. De Wit et al. (2001) also concluded that plant biomass and detritus is a major temporary sink for nutrients (nitrogen & phosphorus) at vegetated sites.

Boschker et al. (2000) showed in samples from Zostera beds across north-western Europe that there was no significant biomass transfer from the seagrass to the local bacteria. The Zostera detritus appeared to be either buried or exported from the beds.

Alongside the empirical and budget methods, mechanistic tools have been developed to simulate the main benthic nutrient processes. Current modelling process have however become increasingly inaccessible to the experimental geochemist since they often require and in-depth knowledge of the underlying numerics and programming language (Meysman et al., 2003a).

A very generic, non-linear coupled diagenetic model was developed within the ECOFLAT project (Meysman et al., 2003a & 2003b). The authors propose a new modelling approach implementing the object-orientated technology and the concept of a problem-solving environment to increase the popularity, the productivity and the possibilities of modelling applications. Figure 3.2(c) displays an overview of the different object-type building blocks within the MEDIA object-orientated modelling environment.

They were successfully implemented in the object-orientated problem-solving environment MEDIA (Modelling Early DIAgenesis), which enables the design of customer-tailored diagenetic models and provides an efficient numerical solution for these models (Meysman et al., 2003b).