Norwegian Institute for Air Research
Netherlands Institute for Ecology
Tyndall Centre for Climate Change Research
Institute for Environmental Studies, Free University Amsterdam
University of Plymouth
Centre for Social and Economic Research on the Global Environment
Land-Ocean Interactions in the Coastal Zone
 


Nutrient Dynamics in European Water Systems
Synthesis Results

4. Effects of altered nutrient discharges on the functioning of coastal and estuarine food webs (1 of 3)

 
Man-made alteration in the emission of nutrients to a water body mainly consists in an increase of the loading for one or more chemical species. Ecosystems respond to these changes by developing characteristic structural and functional patterns. Both the change in nutrient supply and and the related symptoms are usually described under the term of eutrophication.
 
4.1 Nutrient loading and the response in coastal pelagic communities
 

The main objective of the COMWEB project was to develop efficient analytical, numerical and experimental methods for assessing and predicting the effects of nutrient (nitrogen, phosphorus, silica) supply on the stability and persistence of pelagic food web structure and function in coastal water (Olsen et al., 2001).

The field experiments were conducted at different regional locations in order to cover the diversity of European coastal waters (Baltic, Mediterranean, North Sea and Norwegian coastal waters). The COMWEB project was characterised by a common platform of conceptual and experimental approaches that were applied to all sites.

The generalised trophic structure used in the flow network construction included 3 or 4 functional autotrophic (A1-4) and 4 heterotrophic compartments (H1-4) as shown in Figure 4.1(a). The indices 1-4 correspond to the size class to which these organisms belong and/or to their relative position along trohpic chains.

The efficiency of the top-down control determines when nutrients accumulate in vegetal biomass or reach higher trophic levels (Olsen et al., 2001).

 
Figure 4.1(a). Generic food web structure used during flow network construction of carbon, nitrogen and phosphorus for the COMWEB project (Olsen et al., 2001).
Figure 4.1(a). Generic food web structure used during flow network construction of carbon, nitrogen and phosphorus for the COMWEB project (Olsen et al., 2001).
 

One of the common patterns highlighted by Olsen et al. (2001) was that primary production, mesozooplankton grazing and growth, the fraction of primary production consumed by grazers and bacterial production relative to primary production were all well related to the nutrient loading rate. However, closer examination of the North Sea coastal system (Gasparini et al., 2000 & Rousseau et al., 2000) shows that the major response to nutrient additions in disequilibrium (much higher nitrogen additions than phosphorus and silica) is translated onto a bloom of Phaeocystis globosa, a species that is not grazed by copepods and actually inhibits copepods grazing on diatoms.

Phaeocystis production is mainly processed by the microbial food web, and transfer of this production to (via microzooplankton) mesozooplankton is particularly poor: only 1.65 compared to 34% transfer efficiency from diatom production to mesozooplankton grazing. A scheme for the food web structure and flows is given in Figure 4.1(b).
 
Figure 4.1(b). Carbon budget established on basis of integrated flows for the spring period (26 February-June 6). Flows are expressed in mgCm-3period-1. Underlined figures are not directly measured but estimated. O.M. represents the pool of organic matter. (Rousseau et al., 2000).
Figure 4.1(b). Carbon budget established on basis of integrated flows for the spring period (26 February-June 6). Flows are expressed in mgCm-3period-1. Underlined figures are not directly measured but estimated. O.M. represents the pool of organic matter. (Rousseau et al., 2000).
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