The results of a study on spatial and temporal distribution of macro-invertebrates in the soft sediments of the entire Zeeschelde, i.e. the Belgian part of the Schelde estuary are presented in this report. This study, part of the OMES-program (Study on Environmental Impact of the Sigma plan), has been carried out at the University of Gent (Prof. M. Vincx) in close cooperation with the Institute of Nature Conservation (Prof. P. Meire), in the period November 1995 – June 1998. It focussed on those aspects that were insufficiently understood from previous studies on benthic invertebrates of this area, such as the distribution of faunal elements in the freshwater tidal area, with special attention for the dominant taxon of Oligochaeta.Material was derived from a single extensive sampling campaign at 143 stations on 35 transects along the entire salinity- and depth gradient of the Zeeschelde in September/October 1996 and from monthly campaigns at four selected permanent stations: two in freshwater intertidal area (Ballooi, Appels), one in the shallow subtidal freshwater zone (Mariekerke) and one at a mesohaline intertidal flat (Groot Buitenschoor: GBR). A first step towards experimental work on the bioturbating activity of Oligochaeta in the freshwater sediments of the Zeeschelde, was taken.From these data it is shown that the Zeeschelde sediments are characterised by three clearly distinct benthic communities, all dominated or codominated by oligochaetes. Oligochaetes are codominant (0-30 % of total density subtidally, 60-90 % intertidally) in the b-mesohalinicum (Belgian-Dutch border up to Antwerpen) with other taxa (particularly Polychaeta, Amphipoda and Mollusca). They are the only dominant benthic taxon (> 95 %) in the freshwater tidal area.The b-mesohaline zone is characterised by a species assemblage comparable to the one found in other West-European estuaries. Oligochaeta are represented here by the tubificids Heterochaeta costata and Tubificoides heterochaetus and the naidids Paranais litoralis and Amphichaeta sannio. The freshwater species Limnodrilus hoffmeisteri and Tubifex tubifex were found only once at the GBR-station, i.e. in November when peak numbers are counted in the freshwater tidal zone, and at a surprisingly great depth with respect to their size (escape behaviour high salinity values).Total densities of oligochaetes are much lower here than in the freshwater tidal area (mean density: 20,000 m-2) and increase with increasing exposition. Also the other soft-bottom invertebrate taxa are much more abundant on the intertidal flats than in the strongly dynamic gullies. The opposite is true for species such as Nereis succinea, Balanus improvisus, Polydora ligni and Boccardia redeki, most of them being inhabitants of hard substrates. The oligohaline zone (Rupelmonde-Antwerpen), coinciding with the maximum turbidity zone, is very poor in terms of invertebrate life, with only minor numbers of oligochaetes (freshwater species L. hoffmeisteri and T. tubifex and the euryhaline P. litoralis) and polychaetes (B. redeki). In the freshwater tidal area, very dense populations of tubificids occur, with peak densities amounting to 3.106 m-2 and maximal biomasses of 25.7 g ash-free dry weight m-2. Those “mass populations” of oligochaetes constitute mainly of the ubiquists L. hoffmeisteri and T. tubifex, and smaller numbers of L. claparèdeianus, L. udekemianus and L. profundicola. The latter three become increasingly more important where oxygen conditions are somehow improved (Gent-Dendermonde). Two naidids (Nais elinguis and Paranais frici) and one tubificid (Potamothrix hammoniensis) were found in spring – early summer only, when algal mats emerge on the tidal flats (Appels) and oxygen conditions are most favourable (March-May). Other freshwater species rarely found in the Zeeschelde are: Dero digitata, Spirosperma ferox, Potamothrix bavaricus, Quistadrilus multisetosus, Ilyodrilus templetoni and Enchytraeidae spec. The total number of oligochaete species identified from the Zeeschelde so far amounts to 18.Oligochaetes were detected in 85 % of all freshwater stations, with as dominant species: L. hoffmeisteri (87 %), T. tubifex (65 %), L. claparèdeianus (62 %), L. udekemianus (45 %) and L. profundicola (37 %). More stations without Oligochaeta were found in the brackish part and the mean frequency of occurrence was much lower here (43 %), with each of the dominant species found in less than one third of the stations: T. heterochaetus (28 %), H. costata (19 %) and P. litoralis (6 %).The relative importance of the smaller fraction ( > 250 µm and < 1 mm) – neglected in most macrobenthic surveys – is demonstrated in this study. From the non-Oligochaeta of the freshwater tidal area, oligohalinicum and mesohalinicum 95 %, 100 % and 74 % respectively of the total density were counted in the ‘small fraction’. Values for Oligochaeta are 75 %, 86 % and 97 % respectively. In terms of biomass, the ‘small fraction’ of Oligochaeta can not be neglected and was found to be as large as 59 %, 79 % and 95 % respectively.Numbers of oligochaetes increase with decreasing median grain size, with peak numbers in the finest sediments i.e. on the high intertidal flats. Although very often L. hoffmeisteri and T. tubifex are found in mixed populations (60 % of the former, 30 % of the latter species and 10 % other Limnodrilus species), T. tubifex appears to become particularly dominant in the finest, elevated sediments. Whereas the species spectrum is predominantly determined by the salinity regime (and to a lesser extent by oxygen conditions), the abundance of Oligochaeta in the Zeeschelde is mainly linked to sediment characteristics, with the highest values associated with “fluid muds” in intertidal sedimentation areas. The benthic fauna in the freshwater tidal zone of the Zeeschelde is comparable to those of other hypertrophic, oxygen deficient freshwater systems. The mesohaline part shows similarities with brackish parts of other European estuaries.More than 95 % of all oligochaetes were found in the top 10 cm of the sediment and more than 90 % in the top 7 cm. Naididae that often swim and rely on algae and diatoms, are restricted to the top 1-2 cm. In Tubificidae, we could demonstrate a correlation of the depth of occurrence (of the heads) with the length of the animal and hence with the reproductive age and species: juveniles dominate in the upper layers, mature specimens on average at about 5 cm depth and all other stages in between. That explains why species reproducing the year round such as L. hoffmeisteri and L. claparèdeianus (with always reasonable numbers of small immatures and/or juveniles), occur on average shallower than for instance T. tubifex and L. udekemianus, species with more delimited reproductive periods. At the brackish water station GBR there are indications that tubificids move vertically as a response to variations in temperature or oxygen conditions.The temporal pattern in numbers of oligochaetes differs according to the salinity regime. At the brackish GBR-station, populations of the dominant tubificids H. costata and T. heterochaetus do not fluctuate strongly in terms of biomass but show reproductive density peaks in July-December. The naidids A. sannio and P. litoralis are common in spring and dissappear completely in late summer-early autumn. In contrast, the freshwater stations show a uniform pattern with an increasing population size from May-June onwards, a peak in late summer (September/October) and a sudden population crash in the first half of the winter. Possible explanations for this are discussed, with special attention to the potential role of oligochaetes as food for thousands of ducks. The very fast recolonisation of the Ballooi tidal flat after a period of severe frost in December 1996 – January 1997 is linked to the strong dispersion potentials of oligochaetes and the restocking from deeper waters. Population-dynamics of the dominant species of oligochaetes are discussed and reproductive periods determined: H.costata (April-July), T.heterochaetus (May-August), A.sannio (December and May), P. litoralis (April-June). Although the limnetic species T. tubifex and L. hoffmeisteri are sexually active throughout the year, both species appear to have two ‘generations’ per year, with the peak in reproduction 1-2 months later in L. hoffmeisteri (September-October and spring) than in T. tubifex (July-August and late winter).Vorticellidae are frequently encountered as epizoic organisms on tails of limnetic oligochaete species, with a preference for large specimens of L. udekemianus and T. tubifex, and for relatively elevated, silty stations in the upper part of the Zeeschelde. Increasing rates of regeneration in oligochaetes suggest an increase in fish predation towards the Belgian-Dutch border. Tail-cropping appeared to be species- as well as size dependent, and increases with exposure time. This supports the hypothesis that the most elevated parts of the Zeeschelde intertidal flats are visited by fish during flood, taking advantage of the mass occurrence of oligochaete (tails). Alternatively waterfowl might be responsible for the tail-cropping, although research on the birds indicated that the cumulative foraging intensity is not higher on the elevated parts of the flats than deeper down.The impact of oligochaetes on many processes at the sediment-water interface is discussed and the results of some initial experiments with limnetic Zeeschelde tubificids are described. It is shown that densities as found in this part of the Zeeschelde are capable of actively displacing organically enriched layers to deeper regions (at a rate of ca. 1 cm per month) resulting in thicker, looser ‘bands’ of organic material (increase in thickness: 1 cm per two months). Remarkably high values of exopolymers (EPS) found in the experimental sediments could not be linked to worm activity.Finally we made an attempt to predict the changes in benthic communities with improved water conditions and a substantial decline in organic input. An increase in number of oligochaete species, the appearance/increase in other limnetic taxa (f.i.: Chironomids, Pisidium,…) and a reduction in the productivity of the oligochaetes is expected in the freshwater tidal area. In the oligohaline zone that is currently almost devoid of life due to oxygen deficiency, potentials are available for the development of benthic communities with representatives of the brackish and the freshwater area. |
