Studied sites and field works

EUROTROPH will study four sites which are quite different coastal ecosystems in terms of metabolism:
  1. The Randers Fjord in Denmark Studied sites
  2. The Scheldt estuary in Belgium/The Netherlands
  3. The plume of the Scheldt in the Southern Bight of the North Sea
  4. The Posidonia seagrass bed in the Bay of Palma (Spain)
Two types of field works are carried out during EUROTROPH:

  • Monitoring: each studied sites will be monitored for basic parameters in direct relation with the trophic status (pH, alkalinity, pCO2, O2). The Belgian oceanographic research vessel, the R. V. Belgica, will be equiped to monitor continuously the pCO2 during each cruise in the southern bight of the North Sea (more than 200 days per year).

  • Joint field works: two two-weeks joint field work will be carried out in each site during which each partner will simultaneously proceed experiments. The first years will be devoted to the Randers Fjord, the second one to the Bay of Palma and the third one to the Scheldt estuary and its plume at sea.


    The Randers Fjord is the longest river mouth in Denmark and the upstream River "Guden Å" is, with a length of 146 km, the largest river in Denmark. The river and fjord drain an area of 3260 km 2 and receive treated sewage water from 600,000 inhabitants. The tide is negligible (10 cm) and the water column of the fjord is highly stratified. The decision to treat the sewage water was taken 25 years ago. At this time, the effect of loading with untreated sewage was very visible and the whole river system was characterized as highly eutrophic (Guden Å undersøøgelsen, 1975). Based on a Guden Å study (1975) it was decided that the loading of nutrients and organic carbon from point sources should be reduced and cities with more than 200 persons were required to build sewage treatment plants with C, N and P removal capabilities, and had to comply with the discharge standards of 8-10 mg BOD, 8 mg N and 0.5 mg P per l. The construction of sewage treatment plants took place during the 1970 and 1980 and, based on measurements from 1978 to present, it can be estimated that the effect was a the reduction of the carbon and nitrogen loading of approximately 50% and that of phosphorus was of 70%. Evolution during the past 25 years has been changed from a system dominated by point source discharge to a system dominated by diffuse loading from the surrounding agricultural.

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    The Western Scheldt estuary is a typical hypernutrified heterotroph ecosystem, where primary production is low due to limited light penetration. The estuary is well-mixed and the tidal range is up to 6 meters. In the upper part, the dissolved inorganic nitrogen concentration is about 500 micromoles and the total annual organic carbon load is about 100,000 tons for a river discharge of only 3 km3 per year. The respiration is intense in the upper estuary and the turbidity maximum is often characterized by very low oxygen concentrations (down to 10-20% saturation). Indeed, the Scheldt catchment presently receives sewage discharges, with only partial treatment, from cities such as Lille, Antwerp, Brussels and Gent. However, it is scheduled that Brussels will shortly begin sewage treatment, and this study site is therefore ideal to monitor the direct effect of technological interventions on a highly disturbed estuarine ecosystem.

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    The plume is the coastal area where estuarine water is further diluted with seawater outside the estuarine embayment. This area is often rich in both nutrients and organic carbon and, in contrast with the inner estuary, light availability is higher. Plumes can thus be sites of intense eutrophication and secondary effects such as oxygen depletion caused by subsequent decay of plant material may be observed. The result is a shift from long food chains (phytoplankton/zooplankton) to short food chains (flagellates/bacteria) of low diversity and ecological value. The Scheldt plume is notably well-known for intense Phaeocystis algae blooms. The simultaneity of light availability and high concentrations of nutrients and organic carbon makes it difficult to assess the global trophic status of the plume and, in turn, its response to future improvement of the quality of estuarine water.

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    The Bay of Palma has a surface of about 330 km2 and a maximum depth of 70 m. The bottom of the bay is mainly composed of biogenic coarse and fine sands with a high (more than 90%) content in calcium carbonate and is covered by macrophyte meadows (Posidonia Oceanica and Caulerpa prolifera) between the 3 and 40 m isobaths. The city of Palma de Mallorca (around 350.000 inhabitants) is located in the bay and constitutes the major source of nutrient. Increased nutrient inputs have been identified as one of the main forces driving the regression of P. Oceanica meadows in the bay. Since 26% of the annual production of carbon by P. Oceanica is buried in the sediment and the degradation of the rhizomes and roots is slow, this seagrass is a net carbon and nutrient sink in Mediterranean coastal ecosystems.

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