The objectives of this study are to study seasonal and spatial patterns in the hyperbenthos of the Oosterschelde and Westerschelde. In the Oosterschelde, a marine bay, samples were taken in the subtidal gullies, in the western (stations 5 and 10), central (stations 15 and 21), and eastern parts (station 29) and in the northern branch (station 38). In the Westerschelde, a true estuary with a marked salinity gradient, samples were taken at the subtidal border of the main channel: two samples were taken in the marine zone near the mouth of the estuary (stations 1 and 2), and two in the brackish water zone near the Dutch-Belgian border (stations 13 and 14). All samples were taken during daytime with a small hyperbenthic sledge ( opening 0.8 m² and mesh size 1 mm) at four campaigns between August 1999 and May 2000 (summer, spring autumn and winter) with the R.V. Luctor (CEMO, Yerseke, NL). The hyperbenthic community of both ecosystems are compared. Finally, the present data (1999-2000) of the hyperbenthos of the Westerschelde are compared with the data of 1990-1991 to reveal possible patterns on a mid-term temporal scale. In the past, the hyperbenthos of the Oosterschelde has been sampled only once in a limited pilot study (in the winter of 1988) at four stations. Therefore, this study is the first thorough assessment concerning the importance of the hyperbenthos in the Oosterschelde. Analysis of the hyper-benthic community structure was done using Hill's diversity indices, density data (number per surface and water volume sampled) and multivariate analyses in correlation with a limited selection of environmental variables. The hypotheses are: (1) the Oosterschelde harbours less hyperbenthic organisms than the Westerschelde because of its greater dominance of filter-feeding macrobenthic organisms (2) the water quality of the Westerschelde has improved over the last ten years (1990-2000), resulting in a more diverse and richer hyperbenthic fauna. A clear seasonal pattern can be observed in the hyperbenthos of the Oosterschelde. During summer, Anomura (Pisidia longicornis zoea larvae), Cnidaria (Eucheilota maculata) and Ctenophora (Pleurobrachia pileus) are the most dominant taxa. Also important during this season are Mysidacea (Mesopodopsis slabberi) and Brachyura (Macropodia rostrata zoea larvae), followed by Caridea (Crangon crangon zoea larvae), Gammaridea (Microdeutopus species) and Polychaeta (Nereis succinea). The highest number of species (59) are found in summer, most of them belonging to the merohyper-benthos. A strong correlation exists between the hyperbenthos and high temperatures and low oxygen concentrations during summer. Gammaridea (Microdeutopus species) and Ctenophora (Pleurobrachia pileus) are dominant in autumn. Also important are Mysidacea (Mesopodopsis slabberi), spat of Bivalvia and Polychaeta (Nereis succinea). The autumn community is correlated with high turbidity values. Lowest hyperbenthic densities are observed in winter. Only holohyperbenthic organisms occured, in particular Ctenophora (Pleurobrachia pileus) and Mysidacea (Schistomysis spiritus). Spring coincides with the highest hyperbenthic densities (on average 4000 ind./1000 m²), most of which are Ctenophora (Pleurobrachia pileus) and Brachyura (Carcinus maenas zoea larvae). Anomura (Pagurus bernhardus zoea larvae), Polychaeta (Lanice conchilega larvae) and Pisces (Pomatoschistus species postlarvae) are also common. As in summer, spring is dominated by merohyperbenthos. In spring, the water has high pH-values and high oxygen concentrations, which are correlated with a Phaeocystis bloom. The spatial patterns in the hyperbenthic community of the Oosterschelde are less pronounced in comparison with the seasonal patterns. Still, there are a few differences between the stations. All stations have high Ctenophor |
