The ecology of communities dominated by Zannichellia taxa in western Europe. II. Distribution, synecology and productivity aspects in relation to environmental factors

The distribution of three Zannichellia taxa is discussed in relation to environmental factors such as chlorinity, size of the habitat, desiccation, water depth, water circulation and climatic conditions. Zannichellia palustris L. occurs in northern, central and western Europe, the coastal zone of the last area excepted. In the Baltic area it occurs in brackish- as well as freshwater habitats. The chlorinities recorded indicate a tolerance limit under field conditions of up to ±3.5%. To the south, in The Netherlands, it is confined to freshwater. The germination ecology indicates an obligate stratification period (at 4°C) of two months to achieve germination and this climatic condition is found in the area in which this taxon occurs. The growth-form (short but numerous vertical shoots) allows a development in very shallow and ephemeral freshwater habitats. Zannichellia pedunculata Rchb. reaches its optimum development in the coastal area of western Europe. Many of its habitats are subject to desiccation during the summer months and generally show chlorinities of less than 6–7%. In southern Europe chlorinities of 10% are tolerated. Its seeds germinate very well in summer without stratification. The seeds do not germinate in the dark; this inhibition disappears after a cold period and the optimum germination temperature then appears to be lowered. The drought-resistance is very high. Its distribution in temporarily dry habitats (±50% of the Zannichellia habitats in the Camargue, ±22% in The Netherlands) can be understood by these phenomena. Zannichellia major Boenn. is a perennial, and occurs only in northern Europe where it is confined to dynamic waters with strong water circulation and a rather stable chlorinity between 2.5–11%. A number of community types were distinguished on the basis of their floristic composition. In western Europe four Z. palustris communities were recognized in stagnant freshwaters and two in running waters. In the Baltic area (Finland, Denmark) five brackish-water communities with Zannichellia taxa were distinguished, in The Netherlands four brackish-water community types with Z. pedunculata and in the Camargue two brackish-water types with this taxon. Important and frequently encountered accompanying aquatic macrophytes are Potamogeton pectinatus L., Potamogeton pusillus L., Ranunculus baudotii Godr., Characeae and, in mesohaline waters, Ruppia maritima L. var. maritima. The macroscopic fauna inhabiting the phytocoenoses was studied qualitatively and quantitatively (numbers and biomass). The number of species in this fauna ranged from 30 (maximum of a community type) in isolated pools, up to 70 in semi-isolated permanent brackish waters in The Netherlands. Except for the situation in the Baltic area, the species composition is dominated by freshwater insects. The diversity index H′ (Shannon-Weaver) ranged from 0.61–2.37 for the numbers of animals, and from 0.86–2.71 for the biomass. The number of animals inhabiting the phytocoenoses ranged from 700–28,800/m2 and their biomass from 150 up to 22,900 mg ash-free dry weight/m2. The macrophyte biomass was also assessed. The maximum seasonal biomass recorded for a phytocoenosis (mixed vegetation stand) was between ±70–±300 g ash-free dry weight/m2. The highest biomass of a monospecific Zannichellia stand (Z. pedunculata) ever found was 246 g ash-free dry weight/m2. Generally, the Zannichellia-dominated communities have a more diverse fauna than the brackish Ruppia-dominated communities (except in the Baltic). According to this diversity, the Zannichellia-communities resemble freshwater communities. The number of species is generally higher than in the Ruppia-communities and often in the same order of magnitude as in freshwater. The number of specimens is lower than in the Ruppia communities but significantly higher than in many freshwater communities. The maximum biomass of the Zannichellia stands is about the same as in Ruppia stands but somewhat lower than in freshwater ecosystems and much lower than in Zostera communities.