POLYCHAETE DISTRIBUTION, DIVERSITY AND SEASONALITY RELATED TO SEAGRASS COVER IN SHALLOW SOFT BOTTOMS OF THE TYRRHENIAN SEA (ITALY)

Polychaete distribution, diversity and seasonality were studied in rel ation to covering of the small phanerogams Cymodocea nodosa and Zoster a noltii in shallow soft-bottoms off the island of Ischia (Tyrrhenian Sea, Italy). Samples were collected bimonthly from July 1988 to May 19 89 from four stations, selected for seagrass meadows of different phys iognomy and shoot density, and in a bare sandy bottom nearby, selected as a non-vegetated reference habitat. Two meadow compartments were co nsidered: the leaf stratum and the sediment. A hand-towed net was used to sample vagile organisms living in the leaf stratum, while PVC core rs were utilized for the sediment fauna. Stratification within the sed iment was studied considering three layers (0-5 cm, 5-10 cm and > 10 c m deep). On the whole a total of 4640 individuals of polychaetes, belo nging to 119 species. were collected; 4061 individuals and 115 taxa we re found in the core-samples, and 579 individuals and 35 taxa in the n et samples. Polychaete diversity and abundance was higher in both mead ow compartments and in all seasons in the station located in the meado w where more sheltered conditions occurred, high silt-clay and organic matter content in the sediment were observed. In addition, this is wh ere higher shoot density, plant Leaf Area Index (LAI) and Leaf Standin g Crop (LSC) were recorded. Whereas, lower diversity and higher patchi ness (differences among core replicates) were always recorded in the l ess vegetated stations, and on the bare sandy bottom. The polychaete p opulations of the sediment layers showed their lowest diversity and ab undances in summer. Whereas, the populations associated with the leaf stratum showed an opposite trend with higher development (number of sp ecies and individuals) in summer (July-September), consistent with the higher values of all the plant phenological parameters recorded (shoo t density, LAI and LSC). In all stations and seasons, the polychaetes were concentrated (84% of the whole abundance) in the upper sediment l ayer (0-5 cm) and decreased strongly in the deeper sediment layers. Fa unistic differences were recorded mainly between the leaf stratum and the sediment populations. The interstitial Syllidae Exogone naidina, P arapionosyllis elegans, Sphaerosyllis thomasi, dominated in the leaf s tratum; while endofaunal forms, such as Neanthes caudata, Peresiella c lymenoides, Heteromastus filiformis, Notomastus latericeus and Euclyme ne collaris, dominated at the sediment level. The differences in polyc haete community structure observed between the sites studied can be ex plained by the coupling of abiotic conditions (in particular the reduc ed hydrodynamics and sediment features) and meadow structure and habit at complexity characterized by plant phenological features (e.g., shoo t density, LAI, LSC). The seasonal and microdistributional (sediment a nd leaf stratum) patterns within each of the meadow stations are influ enced by ecology and life history of single species, and probably by t he higher predation pressure occurring in some seasons. These results suggest that at least for small-sized and less mobile organisms, like polychaetes generally, meadow structure, shoot density and other pheno logical plant features influence species composition and abundances bo th at large (between meadows) and smaller (within-meadow) spatial scal e.