In this report the results are presented of a macrobenthos survey on the Dutch Continental Shelf (DCS), carried out in spring 2001. The survey forms part of the 'Biological monitoring programme of marine waters' (MON*BIOLOGIE, generally referred to as 'BIOMON') which was initiated by the National Institute for Coastal and Marine Management (RIKZ). The purpose of the programme is to obtain insight into the year-to-year variations of the macrobenthic assemblages and to detect trend-like changes, that possibly indicate anthropogenic influences on the marine environment (e.g. eutrophication, pollution, beam-trawl fishery). Within the framework of this project fieldwork is carried out every year in spring. In 2001 the 100 BIOMON stations were sampled in the period between February 27 and April 4. On the basis of the results collected in 2001 and previous years an analysis is made of the trends and fluctuations of some selected species and of basic community attributes over the period 1986-2001. The community attributes studied were the diversity, abundance and biomass of the total macrofauna and of the 4 major taxonomic groups. Temporal variation or trends were investigated separately for each of the four subareas in the DCS i.e. the Coastal, Offshore areas, Dogger Bank and Oyster Ground.The conclusions of this study can be summarized as follows:1. The sediment composition in the four subareas was quite similar to that found in previous years. Also at most of the individual stations the median grain size and silt content of the sediment had hardly changed. There were only three stations with strong year to year differences in sediment composition. At the stations OFF 2 and OFF 6 the median grain size strongly fluctuated and at station OYS 8 the silt content. lt is suggested that these stations are situated at a sharp local gradient.2. At the community level, there were only a few slight changes compared to preceding years. In the offshore area and in the coastal area there seemed to be a tendency for increased diversity. This was not due to increased species richness but to a more even distribution in the abundance of the various species. With respect to the share of the different taxonomic groups to the total biomass, a decrease has been observed in previous years in molluscs, particularly at the Dogger Bank and in the Oyster Ground. This decrease has come to an end in 2001 and turned into an increase. Since molluscs also increased numerically, the increase in biomass is caused by higher densities rather than by a larger size of the animals.3. At the Dogger Bank a few species showed a remarkable decrease. The polychaete Nephtys cirrosa shows a decrease from 2000 onwards, the sand star Acrocnida brachiata and the bivalve Mysella bidentata from 1999 and the polychaete Aricidea minuta decreased in 1997 and has no longer been found since 1999. On the other hand the population density of the gastropod Euspira nitida (formerly called Natica alderi) had recovered from a dip in the period 1998-2000. The occurrence of Ensis phaxoides is new for the Dogger Bank. Further, living specimens of the bivalve Gouldia minima and the gastropod Turbonilla pusilla have not been found on the DCS before.4. In the Oyster Ground the decreasing trend that has been observed in previous years in the brittle star Amphiura filiformis and the polychaete Nephtys hombergii had turned into au increase in 2001. However, densities of A. filiformis were still low at the Frisian Front. There was a number of species that have not been found before during the BIOMON programma. The occurrence of the polychaete Nephtys assimilis is new for the Oyster Ground. The records of the polychaete Sabella penicillus, the bivalves Montacuta tenella and Gari costulata and the gastropods Turbonilla pusilla and Roxania utriculus are probably new to the DCS.5. In the offshore area, a slightly increasing trend that has been observed in the sea urchin Echinocardium cordatum in the preceding years has come to an end in 2001. In contrast, the gastropod Euspira nitida showed a recovery of populations from 2000 onwards. Station OFF 33 that in 2000 had shown a rich fauna that was clearly different from the other stations in the offshore area was still rich in 2001, but the fauna composition did not show 'exotic' elements.6. In the Coastal area there seemed to be a recovery of the populations of the gastropod Euspira nitida. After an 8 year period of very low densities the species returned at 5 stations, albeit in low numbers. The bivalve Tellina fabula also showed a steady increase, after a dip in the second half of the nineties. At stations where banks of Spisula subtruncata or Ensis americanus occurred, biomass values were very high. |
