Nutrient Dynamics in European Water Systems
Synthesis Results
3. Fate of nutrients in coastal areas (1 of 4)
Nutrients brought to the coastal areas through rivers, groundwater and atmosphere are made available to a large scale of transforming processes that will eventually determine the fate of these nutrients; export to the open sea, cycling in pelagic and/or benthic production and mineralisation processes, burial in sediment.
Besides investigations at the scale of processes, a great amount of effort has been invested into modelling these transformation cycles. Significant progresses have been made in our understanding of these processes and in our capacity to predict their effect on nutrient fate in coastal ecosystems.
3.1 Sediment Biogeochemistry
Within the EROS-21 project, sediment biogeochemistry was studied as an important part of the whole ecosystem response of the north-western Black Sea shelf to variations in nutrient and organic input from the Danube and other major river.
Friedl et al. (1998) present results of benthic lander incubations at Black Sea stations influenced by the Danube or Dniester inputs. Benthic regeneration of phosphate and silicate was very important. The estimate shows that the benthic recycling of phosphorus and silica may be of the same order of magnitude as the input by the Danube River.
Friedrich et al. (2002) present similar results but add seasonal dynamics. Mass balance calculations of the nitrogen fluxes indicate that denitrification is an important remineralisation pathway in the summer (Figure 3.1(a)). During spring, oxic remineralisation and denitrification are important for the decomposition of organic matter. This strong benthic nutrient recycling is an important factor sustaining high productivity in the coastal zone of the Black Sea.
Denitrification
A process by which oxidized forms of nitrogen such as nitrate are reduced to form nitrites, nitrogen oxides, ammonia, or free nitrogen: commonly brought about by the action of denitrifying bacteria and usually resulting in the escape of nitrogen to the air. See Glossary for a complete list of all terms.
Figure 3.1(a). Rough estimate of diagenetic processes due to oxygen consumption and flux of reduced components at the Danube delta front in summer. Fluxes expressed in mmol m-2day-1 (Friedrich et al., 2002).
Diagenetic
The chemical and physical changes occurring in sediments before consolidation or while in the environment of deposition. See Glossary for a complete list of all terms.
Wijsman et al., (2002) provided a coupled diagenetic model for the shelf sediments of the Black Sea exposed to Danube run-off. They predict from the model runs that there are critical organic loading levels of sediments, where the sediment chemistry suddenly switches from oxic mineralisation to iron-managanese dominated mineralisation, and from these to sulphate reduction dominated mineralisation (Figure 3.1(b)).These sudden changes also have consequences for the buffer capacity of these sediments towards sulphide and phosphorus release.
Buffer capacity
A measure of the resistance of water to withstand pH fluctuations.
See Glossary for a complete list of all terms.
In an overview of the ROBUST project, de Wit et al. (2001) discuss the link between iron, calcium, sulphide and phosphorus dynamics in sediments. They identify different benthic 'buffers' that can gradually be filled; iron and calcium sequester phosphorus in the sediment, preventing its release and re-use for further primary production.
In a hypertrophic experiment of a coastal lagoon (Valle Smarlacca, Italy) described by Heijs et al. (2000) the 2cm water layer overlying the organic-rich sediment was permanently anoxic and sulphide was always present. A slurry experiment showed that Ca-bound phosphorus was released after the addition of sulphide whereas it is generally acknowledged that sulphide release phosphate from the Fe-bound pool only. In eutrophic coastal lagoons this process acts as a positive feedback on eutrophication. This results in increased sulphide production in the sediment.
Figure 3.1(b). Effect of the organic matter flux to the sediment on the relative importance (%) of the various mineralization pathways. The Mn-Fe and Fe-S swaps are indicated (Wijsman et al., 2002).
