The ecology of Ruppia-dominated communities in western Europe. III. Aspects of production, consumption and decomposition

In this final paper in the series on Ruppia-dominated communities several functional aspects are treated, most attention being paid to the cycle of the macrophyte material. The productivity of the Ruppia stands was studied by regular samplings of the macrophyte biomass and estimations of the annual turnover. Biomass was measured at the moment of the seasonal maximum in a large number of localities in the Camargue, France, in the costal area of The Netherlands and in the coastal archipelago near Tvarminne, Finland. Seasonal maximum biomass of the Ruppia stands proved to be rather similar in the three study areas (mostly around 150 g m−2 ash-free dry weight), whereas the annual turnover rate showed distinct differences between the localities in the Camargue (1.5–2.5) and those in The Netherlands and Finland (1.2–1.4). The productivity of Ruppia is only moderate compared with the productivity of aquatic macrophytes in freshwater and marine habitats. The decline of Ruppia stands as a result of prolonged turbidity brought about by excessive phytoplankton growth was studied in detail in two eutrophicated brackish ponds in The Netherlands. The transparency in these ponds proved to be continually low (10–20 cm Secchi) due to extremely dense phytoplankton populations (0.3–1.7 mg chlorophyll a l−1). The results of an experiment in which Ruppia stands were artificially shaded in clear water suggest that the Ruppia decline in the two turbid ponds is directly caused by shade. The consumption of Ruppia cirrhosa (Petagna) Grande by a dense population of coot (Fulica atra L.) in a brackish pond on Texel, The Netherlands, was quantified in the autumn of 1976. By daily counts of the numbers of feeding birds the number of coot-days could be estimated. Three different methods to determine the amount of R. cirrhosa daily consumed by the coots, i.e., biomass differences between Ruppia stands in exclosures and those in the rest of the pond, feeding experiments in an aviary and theoretical calculations based on the standard metabolic rate, delivered results in the same order of magnitude. Twenty percent of the maximum biomass of Ruppia (100 g m−2) proved to have been consumed by coots in the period September–November. In the Camargue, France, an attempt was made to delimit the feeding grounds of three quantitatively important waterfowl species, the flamingo (Phoenicopterus ruber L.), the wigeon (Anas penelope L.) and the coot. Large numbers of flamingo foraged in R. cirrhosa localities, this species generally consuming invertebrates and seeds. For the wigeon, only the localities used as feeding grounds during the day could be studied; these all proved to be inhabited by Ruppia. However, the wigeon feed intensively by night, and their nocturnal feeding grounds in the Camargue are unknown. There are many indications that the wigeon in the Camargue feeds mainly on R. cirrhosa. Theoretical calculations showed that, for the maintenance of the complete hibernating wigeon population, 1200 ha of Ruppia-covered lagoon would be needed (10% of the area actually covered by R. cirrhosa in the Camargue in 1977). The coot proved to feed mainly in localities inhabited by Potamogeton pectinatus L. The decomposition of R. cirrhosa was studied quantitatively in a field experiment with litter bags. The breakdown of the plant material in the bags took place within one year. Experiments in which amphipod and isopod species were fed with senescent R. cirrhosa showed that Gammarus spp. and Idotea chelipes (Pallas) can consume considerable amounts of dying and dead plant material and that the mineralization is markedly increased when their feeding activity is high. Finally the results of the complete study on Ruppia communities are brought together and discussed. The approach used in this study is critically reviewed and desiderata for further research are given.