The mobile macro-invertebrate fauna of the Oosterschelde and the Westerschelde (SW Netherlands)
Hostens, K.; Mees, J.; Hummel, H. (2003). The mobile macro-invertebrate fauna of the Oosterschelde and the Westerschelde (SW Netherlands), in: Hostens, K. The demersal fish and macro-invertebrate assemblages of the Westerschelde and Oosterschelde estuaries (Southern Bight of the North Sea) = De demersale vis- en macro-invertebraten gemeenschappen van de Westerschelde en Oosterschelde estuaria (Zuidelijke Bocht van de Noordzee). pp. 87-103
In: Hostens, K. (2003). The demersal fish and macro-invertebrate assemblages of the Westerschelde and Oosterschelde estuaries (Southern Bight of the North Sea) = De demersale vis- en macro-invertebraten gemeenschappen van de Westerschelde en Oosterschelde estuaria (Zuidelijke Bocht van de Noordzee). PhD Thesis. Universiteit Gent. Faculteit Wetenschappen: Gent. XVI, 205, 1 cd-rom pp.
Abstract Both in the marine bay the Oosterschelde and in the Westerschelde estuary, the mobile macro- invertebrate fauna from sub tidal soft substrates was sampled with a three-metre beam trawl during 10 quarterly surveys between August 1999 and November 2001. A total of 35 species was recorded. In the Oosterschelde 33 species were found: 9 caridean prawns, 9 brachyuran crabs, 6 echinoderms, 3 caridean shrimps, 3 cephalopods, 2 anomuran crabs and 1 lobster. The dominating species in the Oosterschelde were brittiestar Ophiura species, brown shrimp Crangon crangon, starfish Asterias rubens, shore crab Carcinus maenas and hermit crab Pagurus bernhardus. Diversity in the Westerschelde was much lower, with only 8 species recorded (4 brachyuran crabs, 2 caridean prawns, 1 caridean shrimp, and 1 anomuran crab) and an overall dominance of Crangon crangon. The main abiotic factors influencing the macro-invertebrate diversity patterns were salinity, sediment/habitat type and temperature. In contrast to the Oosterschelde, the Westerschelde population of Carcinus maenas was not infected with the parasitic barnacle Sacculina carcini. In both systems introduced alien species (mainly crabs) occurred. Arch-fronted swimming crab Liocarcinus arcuatus outnumbered the local populations of flying crab Liocarcinus holsatus in the Oosterschelde. This is possibly related to the reproductive success of L. arcuatus, as seen from the difference in sex ratio between both species in winter (3: 1 females in L. arcuatus versus 2: 1 males in L. holsatus). The sex ratio in Carcinus maenas and Liocarcinus depurator did not differ from 1: 1, and respectively 30% and 100% of the females bore eggs in spring. In the near future these species might suffer the invasion of Japanese brush crab Hemigrapsus penicillatus. At the end of the 1990s, the total average density amounted to 3500 and 1600 individuals per 1000 square metres in the Oosterschelde and Westerschelde, respectively; total average biomass was 1700 and 240 gram ashfree dryweight per 1000 square metres. The seasonal distribution was related to the annual patterns in temperature and dissolved oxygen concentration. The (re)occurrence of Palaemon longirostris in the Westerschelde possibly reflects an improved water quality. A number of species stayed for a short period, while others were present in considerable densities throughout the year. Most of the smallest juveniles were recorded in spring and summer. Peak densities of 55 and 90% of the species were recorded during the warmer seasons in the Oosterschelde and Westerschelde, respectively. Only in the Westerschelde a spatial pattern in species composition was found to be related to the gradient in salinity and turbidity; three species reached higher densities in the eastern part of the estuary. Several macro-invertebrate species showed a high interannual variability. Higher densities were related to mild winters, the presence of abundant food resources, both sub- and intertidally, major engineering works, and presence of intertidal habitat.
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