During the years 1968-1970 a study has been made of the soft-bottom macrofauna (Coelenterata, Mollusca, Echiuroidea, Polychaeta, Archiannelida, Crustacea Malacostraca) of the estuaries of the rivers Rhine, Meuse, and Scheldt in the southwestern part of The Netherlands and of the adjacent part of the North Sea. 1751 samples were taken with 0.1 and 0.2 m² Van Veen grabs; in the intertidal zone several hundreds of samples were dug out, whereas further samples were taken for the analysis of interstitial polychaetes and archiannelids. Of the environmental factors especially salinity, sediment composition, current velocity, water temperature, oxygen saturation, turbidity, water pollution, exposure to waves, and the influence of ice have been investigated. Salinity shows a gradient from over 19.0 promille Cl in the offshore waters to below 0.3 promille Cl in the freshwater tidal area. Sediments mostly consist of medium, fine and very fine sands, but coarse sands, muds, and peat banks also occur. Current velocity is in general below 1.5 m/sec., whereas water temperatures range between about 1 and 22°C in the course of the year. Oxygen saturation is low in the rivers (0-70%) and shows in the brackish water areas a gradient to values over 100% reached in the marine parts of the estuaries. Water pollution shows the reverse pattern, but turbidity is highest in the brackish parts of the rivers. The larger part of the shores of the estuaries is sheltered against wave-action, but the beaches along the North Sea may be considered as semi-exposed. The influence of ice is most severe in the freshwater tidal area. Food is abundant in the estuaries, but less in the offshore parts of the North Sea. Taxonomical remarks have been made on, among others, Polinices polianus, Venus gallina striatula, Eteone foliosa, Nereis virens, Nephtys caeca, Aricidea minuta, Spio martinensis, Streblospio shrubsolii, Tharyx marioni, and Ophelia borealis. For 2 coelenterates, 19 gastropods, 44 lamellibranchs, 1 echiurid, 76 polychaetes, 4 archiannelids, 6 leeches, 15 decapod crustaceans, 2 mysids, 8 cumaceans, 1 tanaid, 9 isopods, 31 amphipods, 8 echinoderms, and 1 species of Branchiostoma the distribution is compared with the environmental data mentioned above or earlier papers treating this have been enumerated. For many species this comparison was carried out by means of a maximum-likelihood method. The following general trends in the distribution of the species studied over the estuarine area were observed. The biological limit of the estuarine environment at the seaward side could be placed where the level sea-bottom changes into a topography of tidal channels and shallows. At the landward side no clear-cut limit could be demonstrated. The brackish-water minimum in the species distribution demonstrated first by Remane (1934) could be confirmed for the estuaries of the Delta area. It appears that nearly all species occurring in the brackish water area also live or may live in either the more marine parts of the estuaries or in fresh water. Hence, nearly all may probably be considered as euryhaline marine or freshwater species. The fauna investigated could be divided in a relatively stenotherm component occurring only offshore and a relatively eurytherm component occurring offshore as well as in the estuaries. Below low water level no species could be demonstrated to be restricted by hydrostatic pressure to a certain depth range. Only three species could be shown to be restricted to the intertidal zone. Several species proved to be restricted to a certain sediment range. Of the species occurring in the brackish water area most appeared to prefer muddy sediments. Therefore, species considered as brackish-water species probably often are really euryhaline inhabitants of mud-bottoms. No species could be shown to be governed directly in its distribution by the exposure to wave-action, but many proved to be so indirectly by the nature of the sediment. Some species probably were inhibited in their occurrence by a high turbidity. Pollution and low values for the oxygen saturation could be shown to be inhibiting for several species in the freshwater tidal area, but for only very few in the brackish tidal area. Shortage of food probably is the reason for the absence from the offshore waters of some species common in the seaward parts of the estuaries. It proved to be impossible to apply any biological brackish water subdivision to the estuaries of the Delta area. The subdivisions proposed earlier could be correlated to hydrographical irregularities of the estuaries. The species characteristic for the brackish water stretch of the estuaries of the Delta area are demonstrated to occur in nearly all estuaries and brackish waters along the Atlantic coasts of Europe. Some species also occur in the western part of the Mediterranean. This European estuarine brackish-water fauna, however, is nearly completely different with regard to species composition from similar brackish water faunas in South Africa, S.E. Asia, New Zealand, and N. Siberia, and along the Pacific as well as along the Atlantic and Gulf coasts of N. America. Hence, a similarity of brackish-water faunas all over the world does not exist. The species diversity proved to be lowest in the brackish parts of the estuaries and in the intertidal zone of the freshwater tidal area. Especially intertidal sand-bottoms show a gradient from a high number of species in the beaches along the North Sea to nearly no species in the sand flats of the freshwater tidal area; intertidal mud bottoms and subtidal sand bottoms show a comparable pattern. Because the brackish water areas lodge a relatively high number of species introduced from other parts of the world, they are supposed to be undersaturated with regard to the number of species occurring. Omnivores are relatively abundant ,in the brackish parts of the estuaries, so the few species occurring apparently occupy wide niches. The percentage of species with pelagic larvae decreases strongly from the sea to the river, whereas the number of species with a non-pelagic development shows the reverse pattern. Pollution, turbidity, size of brackish waters and the critical salinity hypothesis of Khlebovich are ruled out as an explanation for the species minimum in brackish waters. Instability of temperature and salinity partially explain this phenomenon, but a satisfactory explanation for the exceptions only is provided by the time-stability hypothesis of Sanders. It is shown that the history of the N. W. European estuaries during the Pleistocene glaciations sufficiently explains the poverty of brackish water species in these estuaries. Also for other brackish water faunas, geological history probably forms a reasonable explanation for their poverty. |
