Inputs of biogenic carbonate sediment from Posidonia oceanica seagrass meadows to four beaches of the Sinis peninsula (Sardinia, western Mediterranean) were evaluated.Beach and continental shelf sediment samples were analysed for grain size distribution and composition, biogenic vs. siliciclastic,
in order to identify the provenance of beach sediments and sediment transport pathways. Seabed mapping was carried out in order to identify the distribution of meadows and sediment deposits offshore. Shelf sediments were collected in unvegetated sites and in P. oceanica meadows. Sediments from unvegetated sites were coarse
sands and gravel, mainly siliciclastic (biogenic carbonate content is 3–7%). Sediments from P. oceanica meadows were coarse sand, mainly biogenic (carbonate contents varying between 60 and 90%). Beach sediments showed bimodal grain size distribution (59% of samples) resulting from mixing of coarser siliciclastic with finer
biogenic materials in variable proportions. Biogenic carbonate contents in beach sediments range from 0 to 90%, reaching the highest values in offshore samples. Analysis of grain size and compositional trends from shelf to beach sediments highlighted that the latter originate from two
different sources: erosion of granitic outcrops, providing the siliciclastic component, and export of sediments from P. oceanica meadows, providing biogenic material. P. oceanica meadows also influence shore by contributing towards maintaining the beach
sediment budget.
Pinna nobilis is an endemic bivalve of the Mediterranean Sea. In the last decades P. nobilis populations have declined drastically due to increasing anthropogenic pressure and it has been declared a protected species since 1992. Despite the need for conservation, knowledge of the ecology and monitoring of the main populations of P. nobilis are limited. This study considered a population living within a Posidonia oceanica meadow in the Gulf of Oristano (western Mediterranean, Italy). The study area, about 150 hectares, part of which is included within a Marine Protected Area and a Site of Community Importance, was subdivided in 3 sub-areas. The percentage cover of different habitat types (P. oceanica, dead matte and sand) in each sub-area was measured and meadow features (substrate coverage, matte compactness and shoot density) characterized. The hypotheses of differences in density, percentage of dead individuals, population structure, shell burial level and orientation of P. nobilis, were investigated according to sub-areas and to habitat type. The spatial distribution was patchy, and the habitat type resulted a key factor in determining both density and distribution. A strong edge effect was demonstrated: more than half of the observed individuals colonized the P. oceanica border. Matte compactness and shoot density were found to affect the density and distribution of P. nobilis. Shell burial level and percentage of dead individuals varied with sub-areas and habitat types. Size distribution was bimodal and common shell orientation was observed in two sub-areas. These results contribute to increase the knowledge of population ecology of this species and to provide useful information for implementing conservation policies
Abstract:
An opportunistic macrobenthic assemblage was studied from 2001 to 2003 in a central area of the Cabras
lagoon (western Sardinia, Italy), known to be affected by environmental disturbances (i.e. organic overenrichment
of sediments, and episodic events of hypoxia/anoxia and sulphide development). We
identified recurrent seasonal changes in this macrobenthic assemblage, with a general impoverishment
in summer and a recovery in winter/spring. The nereids Neanthes succinea and Hediste diversicolor were
found to replace the spionid Polydora ciliata as the most dominant species in the summer for 3
consecutive years. Occasional, unsynchronized appearances of small-sized deposit feeders, such as
Tubificidae, Capitella cf. capitata, chironomid larvae and Hydrobia spp., were observed in winter/spring.
We suggest that these changes are driven by the interplay of environmental conditions (worse in
summer) with numerous biotic factors. This includes different tolerance levels of taxa to low oxygen
concentrations and sulphides, variability in larval supply and post-larval transport, as well as competition
for space and food between and within different functional groups, and facilitation through animal
bioturbation and sediment reoxidation. A conceptual model is proposed to demonstrate how environmental
conditions and biotic interactions may control the benthic assemblage in such a harsh lagoon
environment.
