POPULATION-DYNAMICS OF DIPLOLAIMELLOIDES-BRUCIEI, A NEMATODE ASSOCIATED WITH THE SALT-MARSH PLANT SPARTINA-ANGLICA

The halophyte Spartina anglica, which occurs abundantly in the lower parts of Western European salt marshes, contains large amounts of standing dead plant material year round, supporting a rich faunal community. The population dynamics of Dipolaimelloides bruciei, one of the dominant nematode species present on S. anglica, was studied in relation to the decomposition stage of the plant material. S. anglica vegetations are regularly flooded at high tide, reducing the nematode population density on the plant material as nematodes are flushed from the plants. The extent of population reduction by flooding was studied in a laboratory experiment using litter of 2 different decomposition stages. D. bruciei was present throughout the year on all types of plant material, including live green plant parts. The population densities were highest on the older plant material, reaching 1000 to 2000 ind. g-1 in late summer and autumn. In the laboratory the rate of removal by flooding was 4.4 times higher on brown leaves than on yellow leaves, while the birth rates were almost identical. As a result the population on yellow leaves increased at a much higher rate than on brown leaves and reached much higher densities. The total number and biomass of D. bruciei formed in the S. anglica vegetation were calculated, assuming that the birth rate of the species depended only on temperature, to be 9 million ind. M-2 yr-1 accounting for 114 mg C m-2. The total amount of carbon ingested by D. bruciei as bacterial biomass accounted for 7.5 % of the total bacterial biomass produced. The dominant bacterivorous nematodes together may remove over 20 % of the total bacterial biomass.