Systematics and ecology of the Isefjord marine fauna (Denmark): With a Survey of the Eelgrass(Zostera)Vegetation and its Communities

Abstract

This paper represents the results of more than 30 years field observations and research work carried out from the Isefjord Laboratory, Vellerup Vig (Zealand, Denmark), which is owned and financed by the author. The Isefjord complex, 36 km long, including the main fjord and the Roskilde Fjord, and covering about 400 km2, is a shallow (7–10 m, max. 30 m) area dominated by substrata of fine sand to mud with a high content of organic matter. As a guidebook for future research the first part gives information about distribution, systematics, ecology, and breeding of the majority of the 477 animal species described. Nearly 400 species are invertebrates and of the chordates 68 species are true fishes. 63% of all animal species have not previously been recorded from the area, 27 species are new to Danish seas, and one (Polychaeta, Capitellidae) is new to science. The hydrography is treated mainly on the basis of literature. The fjord system may be classified as mixohaline: the Isefjord proper is polyhaline (18 ± − 30 ± ‰), and the Roskilde Fjord is mesohaline (5 ± − 18 ± ‰). From April to August the Isefjord interior can be classified as a hypersaline estuary due to reduced precipitation and evaporation in connection with high water temperature. It is concluded that the water exchange between the fjord and the Kattegat is insignificant apart from that caused by wind pressure in winter and autumn. The tidal range (about 20 cm) is only of local importance. The water temperature essentially follows that of the air (shallow area), and normally fluctuates from negative temperatures with ice layer in the winter to summer temperatures often far above 20°C. The recent climatic changes in the North Atlantic are discussed, and it is shown that the surface water temperature increased considerably 1930ℰ50 not only in the Isefjord complex but in practically all interior Danish waters, while salinity changes have been of minor importance. The annual gross production of phytoplankton in the Isefjord is exceptionally high in comparison with that of other larger Danish seas (results of Steemann Nielsen, 1951, 1958), and the primary production reaches values of a eutrophic freshwater lake. The eelgrass (Zostera marina) vegetation is treated in a special chapter, and the extensive literature concerning the so-called disease and destruction of the eelgrass 1933ℰ34 in northern Atlantic waters is discussed. Temperature and eelgrass conditions in the Isefjord suggest that not disease or parasitic attack but the recent climatic change (increased temperature) is the primary reason for the decline. The effect of the disappearance of the Zostera on environmental factors and fauna is demonstrated, and the importance of the eelgrass as shore protection is shown. The final chapters give a broad ecological outline of the area and treat species composition and distribution, specific morphological variation, reproduction and larval development (for 152 invertebrates), and the influence of environmental factors, especially temperature and salinity, on the fauna. Finally, pollution problems are discussed.