Study of post-extraction ecological effects in the Kwintebank sand dredging area (SPEEK)
Vanaverbeke, J.; Bellec, V.; Bonne, W.; Deprez, T.; Hostens, K.; Moulaert, I.; Van Lancker, V.; Vincx, M. (2007). Study of post-extraction ecological effects in the Kwintebank sand dredging area (SPEEK). Belgian Science Policy: Brussel. 92 pp.
This study investigated (1) the long-term changes in benthic metazoan organims of different size classes (macrobenthos and meiobenthos) and different taxa within a size class (nematodes and harpacticoid copepods within the meiobenthos) on the Kwintebank where sand was extracted at high rates and (2) the possible sedimentological and biological recovery of the central depression of the Kwintebank after its closure for extraction in February 2003.The long term trends were assessed by integrating available historical data on the different benthic taxa. Baseline data, describing the characteristics of the benthic communities before sand extraction was initiated were absent, since the first sampling on the Kwintebank was performed in 1978, whereas sand was already extracted since 1976. However, the extraction rates during the first years of exploitation were very low compared to the amount extracted at the end of the 1990’s and the beginning of the 21st century. Therefore we compared community characteristics from low-extraction periods with those obtained during intense extraction.Results on the macrobenthos suggest that macrobenthic life was indeed impacted in the areas where intense extraction occurred. However, this impact was not dramatic since the main species living on the Kwintebank are adapted to a highly dynamical environment (eg. a sandbank).Nematode communities did change drastically since 1978 and were unstable during the extraction period. Larger predatory nematodes disappeared in favour of more opportunistic deposit feeding species. Nematode biomass spectra revealed a shift towards smaller nematodes. These changes are likely due to physical disturbances induced by the exploitation of the sandbank, but the absence of long-term data from an undisturbed area prevents to deduce definitive conclusions.Harpacticoid copepods in the central depression were affected by the extraction activities when analysed at the species level whereas the use of an ecotype approach remains problematic due to the high spatial variation in copepod distribution since the relative abundance of large species fluctuates and can increase in the sand extraction disturbance situations as well. In addition, the spatial variation of the big epi- and endobenthic species is naturally high in the sandbanks sediments.The spatial concentration of the extraction activities on the Kwintebank has clearly impacted the morphological, sedimentological and biological characteristics of the sandbank. Biological assessment should be primarily done by assessing changes at the species level, but our results suggest that nematode biomass spectra could be a valuable tool as well. However, this should be confirmed by more research on a global scale.Baseline data on the original characteristics and data providing information on the natural evolution of non-impacted communities are absolutely necessary to deduce sound evidence on the possible impacts of extraction activities on the benthic environment.Short-term recovery of the central depression after its closure for extraction activities was not observed. The area evolved into a new environment with sediment characteristics and an evolution in mean grain size similar to the trends observed in the adjacent swale.The poor macrobenthic community which was present in the central depression during the exploitation evolved to a community with higher densities, species richness and diversity comparable to the communities encountered at other sandbanks on the Belgian Continental Shelf with similar grain size characteristics. Nematode communities were very different from the communities present during the extraction period and showed a higher stability. This stability was not observed at the northern part of the sandbank were extraction is still ongoing. In addition, nematode biomass spectra revealed that larger nematodes were again present in the absence of dredging activities. However, the nematode communities were very different from the communities observed in 1978, when only small volumes of sand were extracted from the Kwintebank.It cannot be concluded that recovery has taken place for the copepod ecotype distribution in the central depression after cessation of dredging, since temporal and spatial variability may make the observation of recovery at ecotype level quite difficult and because as an additional problem, the ecotype distribution not consistently reflected a disturbance situation as was presumed. A general increase in harpacticoid copepod densities after the cessation of the exploitation of the central area was observed only in autumn. This trend was absent in the winter situation.Some general recommendations towards the assessment of the effects of sand extraction activities on the benthic environment include amongst others:
assessment of the natural situation before the start of the exploitation
sampling a reference site on a regular basis to deduce natural trends
incorporating all benthic groups in monitoring programmes since they reveal different aspects of the disturbance effects
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