Ja Craeymeersch

Large-scale spatial patterns in estuaries: estuarine macrobenthic communities in the Schelde estuary, NW Europe

Few macrobenthic studies have dealt simultaneously with the two major gradients in estuarine benthic habitats: the salinity gradient along the estuary (longitudinal) and the gradients from high intertidal to deep subtidal sites (vertical gradient). In this broad-scale study, a large data set (3112 samples) of the Schelde estuary allowed a thorough analysis of these gradients, and to relate macrobenthic species distributions and community structure to salinity, depth, current velocities and sediment characteristics. Univariate analyses clearly revealed distinct gradients in diversity, abundance, and biomass along the vertical and longitudinal gradients. In general, highest diversity and biomass were observed in the intertidal, polyhaline zone and decreased with decreasing salinity. Abundance did not show clear trends and varied between spring and autumn. In all regions, very low values for all measures were observed in the subtidal depth strata. Abundance in all regions was dominated by both surface deposit feeders and sub-surface deposit feeders. In contrast, the biomass of the different feeding guilds showed clear gradients in the intertidal zone. Suspension feeders dominated in the polyhaline zone and showed a significant decrease with decreasing salinity. Surface deposit feeders and sub-surface deposit feeders showed significantly higher biomass values in the polyhaline zone as compared with the mesohaline zone. Omnivores showed an opposite trend. Multivariate analyses showed a strong relationship between the macrobenthic assemblages and the predominant environmental gradients in the Schelde estuary. The most important environmental factor was depth, which reflected also the hydrodynamic conditions (current velocities). A second gradient was related to salinity and confirms the observations from the univariate analyses. Additionally, sediment characteristics (mud content) explained a significant part of the macrobenthic community structure not yet explained by the two other main gradients. The different assemblages are further described in terms of indicator species and abiotic characteristics. The results showed that at a large, estuarine scale a considerable fraction of the variation in abundance and biomass of the benthic macrofauna correlated very well with environmental factors (depth, salinity, tidal current velocity, sediment composition).

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.

The EBB tidal delta of the grevelingen: A man-made nursery for flatfish?☆

In order to investigate the impact of the closure of the Grevelingen estuary on the benthic life and the nursery function of its ebb tidal delta, a comparison was made between the present ebb tidal deltas of the Grevelingen and the Eastern Scheldt. Three groups of benthic animals were compared: macrobenthos, epibenthic animals (including demersal fish) and hyperbenthos. The ebb tidal delta of the Grevelingen is much richer for almost all animal groups discussed. The data suggest that this richness is a consequence of the changes that have occurred in the hydrodynamics since the closure of the Grevelingen estuary in 1971. The area now traps and retains mud, detritus, larvae of different groups of infaunal animals, larvae of epibenthic crustaceans, and eggs and larvae of fish. This change is reflected in the increased importance of the area as a nursery for flatfish, especially dab and sole. However, the increase in the nursery function of the ebb tidal delta does not make up for the loss of nursery areas in the estuary itself.