The ecology of Ruppia-dominated communities in western Europe. I. Distribution of Ruppia representatives in relation to their autecology

Abstract

This paper contains the first of three parts of a study of the ecology of Ruppia-dominated communities. After introductory remarks on the problematic subdivision of the genus Ruppia and on its distribution in the world, a detailed picture is given of the distribution of the European Ruppia taxa in three study areas at different latitudes (Finland, The Netherlands, S. France), in relation to important environmental factors, e.g. salinity, size and permanence of the water body, substrate type. Ruppia cirrhosa (Petagna) Grande appeared to be characteristic for medium and large permanent water bodies with an annual mean salinity between 2 and 35‰ Cl − (total range 1.5–60‰ Cl −). Ruppia maritima var. maritima L. proved to occur mainly in small and medium-size permanent water bodies with an annual mean salinity between 0.5 and 8‰ Cl − (total range 0.3–15‰ Cl −), whereas R. maritima var. brevirostris (Agardh) Aschers. Graebn. proved to be characteristic for temporary waters of various sizes (salinity range 1–42‰ Cl −). An examination of the ionic content of the water of 8 Ruppia habitats in The Netherlands revealed that the minor components of salinity (K +, Ca 2+, Mg 2+, SO 4 2−) showed considerable variations, resulting in conspicuous differences in proportional ionic compositions in different waters. The Na, K, Mg and Ca contents of Ruppia taxa were measured in material from several natural habitats. The metal contents of Ruppia cirrhosa proved to be similar to those found for Zostera marina L. R. maritima var. brevirostris differed greatly from the other two Ruppia taxa regarding the metal content and showed similarities to fresh water species in this respect. The development of the Ruppia taxa in their natural habitats is described in detail. The switch from winter quiescence to exponential growth is temperature-determined in both Ruppia species. In contrast, the moment of biomass decrease is determined by plant properties: after 4 months of exponential growth, long stems decompose near their base resulting in easy detachment of above-ground parts of the vegetation. In The Netherlands, a further decay is followed by quiescence, whereas in S. France another period of exponential growth can start from the remaining horizontal runners. Further, the biomass decrease is strongly influenced by mechanical damage due to wind action and by bird and macrofauna grazing. Ruppia maritima var. brevirostris is capable of completing its entire life cycle in a few months and is therefore well-adapted to temporary environments. In Finland, it also occurs on exposed shores in moving sandy sediments. The results of outdoor culture experiments carried out in The Netherlands with the three Ruppia taxa are discussed. Both Ruppia species had a better development on soft mud than on sand and showed a decreased growth rate in water containing 18.5‰ Cl − compared with 3.5‰ Cl −. In 28‰ Cl −, only Ruppia cirrhosa survived, but little growth was observed. The similar reaction of the two species was rather surprising. The taxonomical characters proved to be very constant in the cultures. A principal difference in pollination mechanism between R. cirrhosa and R. maritima proved to exist: the pollination of R. cirrhosa takes place at the water surface; consequently, the peduncles have a length significantly related to water depth. R. maritima has an under-water pollination mechanism in which an air-bubble envelops the pistils; the pollen freed from the same inflorescence floats on the surface of the bubble and causes self-pollination. The characteristics of R. maritima var. brevirostris proved constant even after 2 subsequent generations cultivated from seeds. In a final evaluation, the importance of plant properties to the distribution and the strategy for survival of the Ruppia taxa is discussed.