Jan Seys

Spatial distribution of oligochaetes (Clitellata) in the tidal freshwater and brackish parts of the Schelde estuary (Belgium)

The benthic fauna of the Belgian part of the polluted Schelde estuary, called the Zeeschelde, was studied in September–October 1996 at 143 stations along 35 transects. This study is part of the OMES research program, funded by the Flemish Government, to build an ecosystem model of the Zeeschelde in order to help policy makers to decide upon the future of this unique estuarine system. Of all organisms retained on a 250 μm mesh-size, oligochaetes were co-dominant in the mesohaline part and the only dominant group in the tidal freshwater zone. The oligochaete fauna of the ß-mesohaline zone consisted of two brackish water tubificids, Heterochaeta costata and Tubificoides heterochaetus, and the euryhaline naidid Paranais litoralis. The freshwater species Limnodrilus hoffmeisteri penetrated as far as Boerenschans. Abundance in this zone is rather low with numbers increasing with exposure. The part of the oligohaline zone that coincides with the maximum turbidity zone is extremely poor in benthos, due to high physical, chemical and biological stress, with very low numbers of L. hoffmeisteri, Tubifex tubifex and P. litoralis. In the tidal freshwater section, mass populations of the tubificids L. hoffmeisteri and T. tubifex occur (peak densities of almost 3.106 ind m−2, maximum biomass: 25.7 g ADW m−2) with lower numbers of Limnodrilus claparedeianus, Limnodrilus udekemianus and Limnodrilus profundicola mainly in the part of the Zeeschelde close to Gent where better oxygen conditions are found. Oligochaetes are more abundant in finer sediments, resulting in a clear vertical gradient with low mean values in the deeper subtidal coarse sediments and highest numbers in the upper intertidal where fluid muds consolidate. In comparison with L. hoffmeisteri, T. tubifex was more abundant in the finest sediments. Vorticellidae are more common on tails of oligochaetes in the oxygen-enriched part (4–9%) than in the oxygen-deficient zone (< 0.5%). Regeneration rates, being considered as a suitable measure of predation pressure, increase with salinity and size of the worms and are highly specific. In comparison with other temperate estuarine systems, the Zeeschelde has impoverished communities with oligochaetes as the dominant taxa. Though a future improvement of the water quality will definitely result in a less productive but more varied benthic community in the freshwater tidal zone, oligochaetes will probably continue to play an important role due to sustained harsh conditions.

Spatial and temporal patterns of intertidal macrobenthic populations in the Oosterschelde: are they influenced by the construction of the storm-surge barrier?

The construction of a storm-surge barrier in the mouth of the Oosterschelde caused important hydrodynamical and morphological changes that could influence the macrobenthic populations. This paper is one in a series of five all dealing with the effects of the storm-surge barrier on macrozoobenthos and analyses the spatial and temporal distribution of macrozoobenthos in the Oosterschelde and its relationship with some environmental parameters, based on two large scale sampling campaigns, one before and one after the completion of the barrier.

Long-term changes (1979-89) in the intertidal macrozoobenthos of the Oosterschelde estuary

To evaluate the effects of the construction of a storm surge barrier in the Oosterschelde, long-term patterns (1979–89) in abundance and biomass of the intertidal macrozoobenthos were studied at 14 permanent stations. Additionally, data of a large-scale survey in late summer 1985 and 1989 were analysed. In this paper, patterns in general parameters are discussed.Late summer values of total biomass, total density, species richness, diversity and abundance- and biomass ratio show no overall significant trend during the study period. The changes in the hydrodynamics and the morphology of the Oosterschelde after the completion of the storm surge barrier do not seem to have influenced the normal patterns in benthic populations. The observed patterns are determined by the occurrence of severe versus mild winters, rather than by hydrodynamic changes caused by the construction of the barrier. Low biomasses, high densities (particularly of opportunistic species) and higher “stress-values” (abundance- and biomass ratio) in 1985(−87) indicate a temporal disturbance by severe winter weather. At the elevated COST-station 27, total biomass decreased sharply in 1985, due to a short-term increase in exposure time, caused by the manipulation of the storm surge barrier.

Long-term changes (1979–89) in the intertidal macrozoobenthos of the Oosterschelde estuary: are patterns in total density, biomass and diversity induced by the construction of the storm-surge barrier?

To evaluate the effects of the construction of a storm surge barrier in the Oosterschelde, long-term patterns (1979–89) in abundance and biomass of the intertidal macrozoobenthos were studied at 14 permanent stations. Additionally, data of a large-scale survey in late summer 1985 and 1989 were analysed. In this paper, patterns in general parameters are discussed.

Spatial and temporal patterns of intertidal macrobenthic populations in the Oosterschelde

The construction of a storm-surge barrier in the mouth of the Oosterschelde caused important hydrodynamical and morphological changes that could influence the macrobenthic populations. This paper is one in a series of five all dealing with the effects of the storm-surge barrier on macrozoobenthos and analyses the spatial and temporal distribution of macrozoobenthos in the Oosterschelde and its relationship with some environmental parameters, based on two large scale sampling campaigns, one before and one after the completion of the barrier.The sediment of the sampling stations was fine, well sorted sand, with an average mud content of about 2.5 %. Only in the Krabbenkreek the sediment was coarser in 1989. The tidal elevation of the sampling sites decreased significantly in 1989.The density of macrozoobenthos was significantly lower, the biomass higher in 1989. The density was dominated by deposit feeders, the biomass by filter feeders. The difference in biomass between both years was mainly due to a substantial increase of the biomass of filter feeders in 1989. The number of species per station was significantly smaller in 1989 than in 1985. Between 1985 and 1989, frequency of occurrence decreased in 34 versus 13 which increased, density increased in 13 species and decreased in 34 species, biomass increased in 18 species and decreased in 29 species.Based on TWINSPAN several clusters of stations, each with a different faunal composition, were identified. These clusters did not form distinct zones on the tidal flats but were dispersed widely.The relationship between density and biomass of different trophic groups and the mud content of the sediment and the depth was analysed. This relationship sometimes differed clearly between years. The correlation coefficient of a multiple regression between density and biomass of individual species and environmental factors, although significant in most cases, was very low, indicating that only a small proportion of the species variability was explained. The relationship between benthos and environmental factors was further analysed by canonical correlation analysis and multivariate discriminant analysis that gave different results for the 1985 and 1989 data. This is probably due to the broad tolerance of the species to the range of the environmental variables found in our study area. From a TWINSPAN of the density data of 1985 and 1989 together we could conclude that, although the environmental parameters in a group of stations, showing a large faunal similarity in one year, did not change, the faunal composition did. This indicates that faunal changes are not necessarily linked to changes in the measured environmental parameters.In the discussion the different factors affecting macrobenthic populations are situated and it is suggested that the macrobenthic populations are probably more towards the nonequilibrium end of the continuum between nonequilibrium and equilibrium communities as defined by Wiens (1984).Although the impact of the construction of the barrier on the macrobenthic community seems at present to be rather small this does not mean that on the long-term there will be no effect.