Polychate distribution, diversity and seasonality related to seagrass cover in shallow bottoms of the Tyrrenian Sea (Italy)

Abstract

Polychaete distribution, diversity and seasonality were studied in relation to covering of the small phanerogams Cymodocea nodosa and Zostera noltii in shallow soft-bottoms off the island of Ischia (Tyrrhenian Sea, Italy). Samples were collected bimonthly from July 1988 to May 1989 from four stations, selected for seagrass meadows of different physiognomy and shoot density, and in a bare sandy bottom nearby, selected as a non-vegetated reference habitat. Two meadow compartments were considered: the leaf stratum and the sediment. A hand-towed net was used to sample vagile organisms living in the leaf stratum, while PVC corers were utilized for the sediment fauna. Stratification within the sediment was studied considering three layers (0-5 cm, 5-10 cm and > 10 cm deep). On the whole a total of 4640 individuals of polychaetes, belonging to 119 species, were collected; 4061 individuals and 115 taxa were found in the core-samples, and 579 individuals and 35 taxa in the net-samples. Polychaete diversity and abundance was higher in both meadow compartments and in all seasons in the station located in the meadow where more sheltered conditions occurred, high silt-clay and organic matter content in the sediment were observed. In addition, this is where higher shoot density, plant Leaf Area Index (LAI) and Leaf Standing Crop (LSC) were recorded. Whereas, lower diversity and higher patchiness (differences among core replicates) were always recorded in the less vegetated stations, and on the bare sandy bottom. The polychaete populations of the sediment layers showed their lowest diversity and abundances in summer. Whereas, the populations associated with the leaf stratum showed an opposite trend with higher development (number of species and individuals) in summer (July-September), consistent with the higher values of all the plant phenological parameters recorded (shoot density, LAI and LSC). In all stations and seasons, the polychaetes were concentrated (84% of the whole abundance) in the upper sediment layer (0-5 cm) and decreased strongly in the deeper sediment layers. Faunistic differences were recorded mainly between the leaf stratum and the sediment populations. The interstitial Syllidae Exogone naidina, Parapionosyllis elegans, Sphaerosyllis thomasi, dominated in the leaf stratum; while endofaunal forms, such as Neanthes caudata, Peresiella clymenoides, Heteromastus filiformis, Notomastus latericeus and Euclymene collaris, dominated at the sediment level. The differences in polychaete community structure observed between the sites studied can be explained by the coupling of abiotic conditions (in particular the reduced hydrodynamics and sediment features) and meadow structure and habitat complexity characterized by plant phenological features (e.g., shoot density, LAI, LSC). The seasonal and microdistributional (sediment and leaf stratum) patterns within each of the meadow stations are influenced by ecology and life history of single species, and probably by the 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 phenological plant features influence species composition and abundances both at large (between-meadows) and smaller (within-meadow) spatial scale.