Macrofauna of the lower reach and estuary of Biobio river (Chile): changesassociated to seasonal changes of the river flow

The Biobio river (ca. 37 degreesS) is one of the most important Chilean lot ic systems with regard to their water flow. Its estuary is reduced, especia lly during maximum flow. With the purpose of testing the hypothesis that du ring summer, the decrease of river flow results in a higher deposition of f ine particles. and that the increase in river flow during winter results in the washing and transport of deposited particles, and consequently in a de crease of the benthic macrofauna. Sediment samples were collected for sedim entological and faunistical analysis during August 1993 and March 1994, The sedimentological analyses indicate that mean size of sediment corresponded to coarse sands. During summer, the deposition of suspended particles is f acilitated by the decrease in the river flow and results in higher percenta ge of fine particles and organic matter in the sediment. The benthic macrof auna was composed by 16 taxa during winter and 42 taxa during summer. Durin g winter, that macrofauna was represented by just a few individuals. During summer, the species richness and macrofaunal abundances increased. The mos t abundant taxa were an archiannelid species and the polychaete Perinereis gualpensis. The increase of river flow during winter determines the transpo rt of the deposited material toward the sea and consequently, the washing o f benthic organisms. Even though, values of macrofaunal abundance and bioma ss during summer were about one order of magnitude higher than during winte r, they are still lower than those mentioned for other estuaries of south-c entral Chile (ca. 39 degrees S). That confirms the atypical character of th e Biobio river estuary, which due to its seasonal changes, significantly af fects the community structure of the benthic macrofauna.