ABSTRACT

The aim of the RIACON project was to evaluate the risk of shoreface nourishment and subaqueous sand extraction for the marine benthic community and its effect in the foodweb. A study was carried out on 5 European coastal sites. The results generally indicate a recovery of most benthic species within 1-2 years time, except for long-living species such as bivalves and sea urchins. Foodweb effects may be problematic when local food resources (e.g. Spisula beds) for diving ducks are affected by shoreface nourishment.

INTRODUCTION

In a previous contribution in the WSNL, the RIACON project was already introduced (Essink, 1996). At 5 sites along the European coastline, the ecological effects of sand deposition in the shoreface were investigated as an alternative method for traditional sand nourishment of beaches threatened by erosion. The study was carried out in Denmark (Torsminde), Germany (Norderney), The Netherlands (Terschelling), Belgium (De Haan) and Spain (Costa Daurada). Also studies of the ecological effects of subaqueous sand extractions were included in the RIACON project, that was co-sponsored by the Commission of the European Communities (MAS2- CT-94-0084).

Reports for each site studied are now available (e.g. Grotjahn & Liebezeit, 1997; Birklund et al., 1997; Van Dalfsen & Essink, 1997) as well as an integrative report (Essink, 1997). In this WSNL-contribution, an overview of the main results, as far as relevant to the coastal zone of the Wadden Sea area, will be presented.

ECOLOGICAL RISK

The ecological risk of shoreface nourishment primarily relates to the benthic community covered with a few meters of sand. Secondary effects may pertain to a reduced food supply for benthic fish and diving ducks. Therefore, it is of importance to known how much time it takes for the benthic community to recover, or, in other words, how long the carrying capacity of the shoreface benthic system for fish and birds gets affected. The same holds for sites of subaqueous sand extraction, where the primary effect is the removal of benthic fauna.

EFFECTS OF NOURISHMENT

At the nourishment sites studied in Denmark, Germany and The Netherlands, the general finding was that shoreface nourishment initially caused a reduction in abundance and biomass of zoobenthic species. Also the species diversity decreased and the structure of the zoobenthic community departed from the one present before the nourishment started. At the Norderney site, after completion of the nourishment, a strong development of opportunistic species (e.g. Scolelepis squamata) was observed.

Approximately one year after the nourishment of the North Sea sites recovery of the zoobenthic community was already in progress. After two years, densities, biomass and diversity had largely regained pre-nourishment values. The community structure in the nourished areas was no longer very different from the one in reference areas. In long-living bivalve and sea urchin species, however, no such recovery was observed. Notwithstanding recruitment following nourishment, the population structure of these species (e.g. Spisula subtruncata, Donax vittatus, Echinocardium cordatum) was unbalanced due to the absence of older year classes.

As these species do not reproduce successfully each year, recovery may take 2 - 5 years. Only on the Belgian nourishment site, an increase of densities of the bivalve Macoma balthica was observed.

Along the North Sea shores, the ecological risk for the food web mainly consists of temporary reduction of food resources for juvenile fish (e.g. plaice, sole and dab, eating polychaetes and crustaceans) and diving ducks (eating bivalves). For fish, the risks are relatively small because of the rather fast recovery of the zoobenthic community and of the relativly small surface of nourishment operations in relation to the entire coastal zone. Diving ducks (e.g. Common Scoter and Eider), during their winter stay in the North Sea coastal zone, are very much dependent on local bivalve food resources. Therefore, these populations are likely to be affected when nourishment is realized on one or more beds of their favorite bivalve food species (e.g. Spisula sp.).

EFFECTS OF SAND EXTRACTION

Sand extraction sites studied (off Torsminde and Terschelling) were in 14 - 20 m deep water. Volumes extracted amounted to 0.25 and 2.1 million m3 respectively, causing a sea floor area of 0.5 and 1.4 km2 resp. to be directly affected due to the removal of a 0.25 - 1.5 m thick layer of sand.

As expected, the direct effect of sand extraction was a significant reduction of species abundance and biomass. The sediment disturbance was followed by a quick opportunistic response: development of spionid polychaetes (Spio filicornis and Spiophanes bombyx).

The recovery of the benthic fauna largely proceeded rather fast: within one year. Long living bivalve species and sea urchins, however, had not yet completely recovered by the end of the project (i.e. 2 years after extraction). This is expected to take some more years (see above).

The coastal zone of the North Sea down to ca. 30 m is an important habitat for a diverse demersal fish community, among which sandeel and plaice are the major consumers of benthic invertebrates. Because of the relatively fast recovery of the zoobenthos and the remnant fauna inbetween the sand suction lanes, relatively small scale sand extraction operations are not considered to cause a significant risk for the demersal fish populations. At North Sea sand extraction sites at ca. 20 m depth, the risk for diving ducks (e.g. Common Scoter) is considered small because their preferred food Spisula subtruncata is not abundantly present there.

Rijkswaterstaat North Sea Directorate has taken the initiative to further investigate the recovery of long-living benthos at the Terschelling borrow site. The authors are involved in this study.

REFERENCES

Birklund, J., H. Toxvig & C. Laustrup, 1997. RIACON. Risk analysis of coastal nourishment techniques. Risk of shoreface nourishment and subaqueous sand extraction for the coastal marine benthic community. Evaluation of the nourishment and sand extraction off Torsminde, Denmark. Danish Coastal Authority/Water Quality Institute (VKI).

Essink, K., 1996. Risk Assessment of Coastal Nourishment Techniques - RIACON. Wadden Sea Newsletter 1996-1: 10-12.

Essink, K., 1997. Risk Assessment of Coastal Nourishment Techniques (RIACON). Final Evaluation Report. National Institute for Coastal and Marine Management/RIKZ, Report RIKZ-97.031.

Grotjahn, M. & G. Liebezeit, 1997. Risk analysis of coastal nourishment techniques (RIACON). Risk of beach nourishment for the foreshore and shallow shoreface benthic communities on the island of Norderney, Germany. - Evaluation of the nourishment in 1994. AQUA MARIN, Norden/TERRAMARE, Wilhelmshaven.

Van Dalfsen, J.A. & K. Essink, 1997. Risk analysis of coastal nourishment techniques in The Netherlands. Part A. The ecological effects of shoreface nourishment off the island of Terschelling, Part B. The ecological effects of subaqueous sand extraction North of the island of Terschelling. National Institute for Coastal and Marine Management/RIKZ, Report RIKZ -97.022.

Authors address:

Karel Essink, Jan A. van Dalfsen

Rijkswaterstaat - RIKZ

Postbus 207

NL - 9750 AE Haren

e-mail: K.Essink@rikz.rws.minvenw.nl