Relationships Between Chemical Characteristics of Sediments and Macrofaunal Communities in the Cabras Lagoon (Western Mediterranean, Italy)

The Cabras lagoon is a large and shallow transitional system which periodically suffers from dystrophic events leading to massive losses of its biological resources. Excessive organic matter (OM) and sulphur compounds in sediments are often claimed to be major environmental co-factors triggering these events. However, still limited information is available on their relationships with benthic macrofaunal communities in coastal lagoons. In order to assess these relationships, we conducted monthly field investigations between 2001 and 2002, at three regularly-spaced stations set between the inlet and the mouth of the Cabras lagoon. Results showed reduced conditions of sediments, worsening during the warm season. This was consistent with very high OM contents, up to >‰20%, with annual means of 14–16%. Organic pollution-tolerant taxa, such as Tubificidae nc, Neanthes succinea and Polydora ciliata, dominated poor macrofaunal communities. Results of multivariate analysis indicated significant differences in both environmental variables and macrofaunal community structure and species composition between stations and through different periods. This study documented for the first time in a coastal lagoon that inorganic reduced sulphur pools (acid volatile sulphide and chromium reduced sulphur) of sediments are leading (‚best-matching’) environmental factors influencing the spatial and temporal distribution of macrofaunal abundance and biomass. We conclude that impoverished macrofaunal communities in the Cabras lagoon may show a slight recovery during winter–spring, but tend to regress to an early stage of faunal succession in late summer. We suggest that the combination of excessive amounts of sedimentary organic matter and high temperatures tends to lead to anoxic conditions especially in relatively deeper and more saline waters. These events may concur to periodic increase of production rates of toxic hydrogen sulphide in sediments, which rapidly diffuses to the water column with deleterious effects both on the benthic and pelagic components of the lagoon ecosystem.