Changes in the dynamics of shallow sandy-bottom assemblages due to sand extraction in the Catalan Western Mediterranean Sea

Abstract

Coarse to fine-sand sediments characterize shallow sublittoral soft bottoms in the Northwestern Mediterranean Sea. Within the framework of a wider research project on the littoral ecosystem of the Bay of Blanes (Catalan coast), the dynamics of shallow soft-bottom macroinfaunal assemblages have been followed since March 1992. These assemblages exhibited a highly predictive annual cycle. Abundance and biomass rose sharply during spring, followed by a striking drop through summer, and reaching the lowest values during winter. These cycles were consistent with the temporal variation in several key species. During the summer and autumn of 1994, shallow soft bottoms (10 to 30 m depth) off the Tordera River were dredged for beach nourishment. Recolonization in these dredged habitats was fast, and no changes in seasonal trends were detected after dredging. However, density values rose sharply during the following spring and autumn with exceptionally large numbers of Ditrupa arietina, Spisula subtruncata, and Branchiostoma lanceolatum. Dredging activities also led to rapid increases in biomass values, which were significantly higher than those obtained before dredging. After two years, densities were back to normal but biomasses were still high. Other species, such as the filter-feeder Callista chione and the carnivorous polychaetes Protodorvillea kefersteini and Glycera spp., were still clearly reduced after two years, suggesting that a longer period is needed to restructure dredged bottoms to their initial situation. Dredged habitats supported artisanal bivalve fisheries in the harbour of Blanes. The official catch data of bivalves (mainly C. chione, Acanthocardia aculeata, Donax trunculus, and D. variegatus) showed a decreasing yield since the end of dredging.