Community Structure of the Subtidal Macroinfauna in an Estuarine Mussel Bed in Southern Chile

The mussels Choromytilus chorus and Mytilus chilensis are the most distinctive bivalves of the subtidal soft bottoms of Queule River estuary, southern Chile (ca. 40 °S). In some artificially farmed areas of this estuary, the semi-buried mussels reach densities as high as 250–300 adult indiv. m-2. We carried out seasonal (1983) and fortnightly samplings (August 1984 to December 1985) to compare abiotic and biotic characteristics (i. e., macroinfaunal community structure) on bottom areas with and without mussels. Biodeposition within the mussel bed area did not result in organic enrichment of the sediments, a situation which is discussed in relation to the breakdown of mussel biodeposits. During most of this study, organic matter (%) was higher in the sediments of this area versus that without mussels. While the sediments of the mussel and control area were dominated by the same macroinfauna (i.e., polychaetes), that of the mussel area generally had lower abundances, number of species, and diversity indices. Several explanations are presented to account for diminished abundance and diversity values here. These include insufficient substrate for settling larvae, space limitation, and negative effects of sediment characteristics. Summary Mussel biodeposition in a mussel bed area at the Queule River estuary did not result in organic enrichment of the sediments. This may be attributed to the high rate of biodeposit disintegration. During most of the study, the percentage of combustibles (i. e., organic matter) was higher in the mussel bed sediments. This was not necessarily the effect of mussel biodeposit accumulation, since field studies (between-site comparisons during flood and ebb tides) showed no significant differences between the amount of organic matter in the near fluid, fecal-rich sediments surface (i. e., 0.5 cm) of the mussel and control area. The macroinfauna of both areas was dominated by the spionid Minuspio chilensis and one capitellid worm. The abundance of the whole macroinfauna and that of the most characteristic taxa peaked during summer-autumn months. No changes were detected in the community structure of either area (e. g., no changes in the rank of dominant species). The abundances, number of species, and diversity indices were generally lower in the mussel bed area. This condition may be related to negative effects of some sort of mussel-macroinfauna interaction such as space reduction, adult-larval interactions, or changes in sediment characteristics.