The ecology of Ruppia-dominated communities in Western Europe. II. Synecological classification. Structure and dynamics of the macroflora and macrofauna communities

In this paper the structure and dynamics of macroflora and macrofauna communities dominated by the genus Ruppia are treated. The aquatic plant communities were classified according to the methods of the Zurich-Montpellier School. In the mediterranean region as well as in N.W. Europe, two plant associations were distinguished, one dominated by Ruppia maritima L., the other by R. cirrhosa (Petagna) Grande. In the mediterranean region the communities in temporary ponds and marshes were classified in the Rupitum brevirostris, that could be subdivided into three subassociations occurring under different salinity conditions, whereas the communities in the permanent ponds and in the extensive lagoons were classified in the Cladophoro-Ruppietum cirrhosae, in which four subassociations were distinguished. In N.W. Europe, two subassociations of the Ruppietum maritimae and two of the Ruppietum cirrhosae have a local distribution along the coasts of the inner Baltic, whereas a number of other subassociations of both the Ruppietum maritimae and the Ruppietum cirrhosae are more generally distributed in ponds and lakes along the N.W. European coasts. The structural features of the plant communities are described in detail. The parvopotamid growth form proved to strongly dominate in nearly all Ruppia stands. Hence, horizontal and vertical spatial patterns are remarkably uniform in the communities studied. Periodicity, long-term fluctuations and succession in the plant beds are treated extensively; as these temporal patterns are strongly influenced by competition between the most important plant species, much attention was paid to such competitive relationships. The competition between Ruppia cirrhosa and Potamogeton pectinatus L. is crucial in the extensive mesohaline lagoons. R. cirrhosa completely ousts P. pectinatus at mean salinities above 9% Cl−, the opposite happens at mean salinities below 4% Cl− and mixed stands of both species occur at intermediate salinities. The species composition of the macrofauna in the Ruppia beds and their structural relation to the plants were investigated. Of the 139 species encountered, only 30 were generally abundant in the majority of the localities studied. These were mainly ubiquitous ‘true’ brackish-water species. The Ruppia-dominated communities were classified according to their macrofauna composition and the result was compared with the phytosociological classification, resulting in a combined classification of biocoenoses. The Ruppia stands generally proved to be poor in macrofauna species. This is probably related to the general poverty of macrofauna species in the N.W. European brackish waters. The macrofauna communities in the Ruppia stands in the Finnish Baltic are slightly richer in species than those in The Netherlands and in the Camargue, S. France, due to the fact that the latter are isolated. With respect to the structural relations between the macrofauna and the plants, it must be pointed out that ca. 60 species were found predominantly in the aquatic vegetation, but most of them also occurred on other (hard) substrates. Thirteen species were ‘attached epiphytes’, 29 ‘creeping epiphytes’, four egg-depositors and 15 canopy-dwellers; ca. 45 species did not show any structural relation to the vegetation (18 benthic infauna species, six creeping and 22 swimming species). A quantitative sampling programme was carried out in the Ruppia habitats studied that provided information on the numbers and biomass of the macrofauna species in the plant beds. Total fauna numbers ranged from 2000–44 000 ind./m2 and total biomass from 1.1–22.9 g ash-free dry weight/m2. Species diversity and evenness were calculated; H′ was in the range 0.13–2.12, and J′ ranged from 0.15 to 0.76. The macrofauna communities in the Ruppia beds are characterized by a strong dominance of only a few species (e.g. Nereis diversicolor O.F. Mull, Chironomus salinarius Kieffer, Tubifex costatus Claparede, Hydrobia spp.). Consequently, they have a low species diversity compared to marine and freshwater aquatic plant-dominated communities. The possible causes for this low diversity are discussed.