Environmental Monitoring Studies in the Coastal Lagoon of Cabras (Western Mediterranean, Italy): Hydrological and Sedimentary Features and Distribution of Benthic Macrofaunal Communities

• Paolo Magni^*, Giovani Fenzi and Giovanni De Falco
• IMC – International Marine Centre, Loc. Sa Mardini, 09072 Torregrande-Oristano, Italy.
  Email: ^* p.magni@imc-it.org
• The Regional Workshop on Marine Sciences & Natural Resources 25-26 May 2004.

Abstract

The Cabras lagoon is a large and shallow transitional system (area 20.5 km2, mean water depth 1.7 m) located in the Gulf of Oristano, on the west coast of Sardinia Island (Italy), western Mediterranean sea. The lagoon has a high economic rating due to fishery activities (e.g. Liza ramada, Mugil cephalus, Anguilla anguilla) involving about 300 fishermen, with a year fish catch in 1998 of about 400 tons and 3.5 million Euros income. However, there is a tendency to dystrophic events leading to massive losses of the biological resources both in the pelagic and the benthic environment. In order to assess the reasons of such environmental problems, monitoring programmes have been conducted on spatial and seasonal distribution of both environmental features and benthic macrofaunal communities of the lagoon.

Hydrological measurements were conducted through the water column using a multiprobe cast and sediment samples were collected for chemical analysis using gravity corers. In addition, grab sediment samples were collected for the determination of macrofauna (>0.5 mm). Univariate, graphical and multivariate analysis (e.g. PCA, clustering and nMDS-ordination techniques, assessment of rank similarity (R), Spearman rank correlation coefficients) were applied to assess the relationships between environmental variables, to describe patterns in community structure and species composition over seasons and to determine how species distribution was related to physical and chemical variables.

Results showed strong spatial and seasonal differences of both abiotic and biotic parameters. Hydrological features indicated mixed conditions in winter, while stratification was more apparent in other periods of the year. In summer, water temperature markedly decreased from the surface to the near bottom, while salinity showed an opposite trend both through the water column and from the inlet to the mouth of lagoon. In some areas of the lagoon, dissolved oxygen (DO) concentrations ranged from oversaturation (138% air saturation) at the surface, to hypoxia in near bottom water (5.8% air saturation).

Consistently, very high organic matter (OM) contents were found in the surface sediments of the lagoon, up to >20%, with annual means of 14-16%. Organic pollution-tolerant taxa, such as Tubificidae nc, Neanthes succinea and Polydora ciliata, dominated poor and heterogeneous macrofaunal communities. Results also indicated that macrofaunal communities in the Cabras lagoon tend to show a slight recovery during winter-spring, but are subjected to new regressions to an early stage of faunal succession in late-summer.

We suggest that the combination of high amounts of sedimentary organic matter, high water temperature and salinity, and/or the tendency to hypoxic conditions during the warm season, especially in relatively deeper waters and at sites of OM accumulation, may concur to episodic release to the water column of toxic hydrogen sulphide, with major environmental consequences at the basin scale.