Macrobenthic communities of an Amazonian estuary (Guajara Bay, Brazil): temporal and spatial changes

This study describes the spatial and temporal patterns of changes in the macrobenthic communities of Guajara Bay, an estuary in northern Brazil. Samples were collected at seven sites in the dry (December 2004 and September 2005) and rainy seasons (March and June 2005). On each occasion, four biological samples and one sediment sample were collected at each site using a Petersen grab (0.039 m(2)). Simultaneously, the pH of the surface water, salinity, electrical conductivity and temperature were measured. Water parameters, except temperature, showed strong seasonal variation. As a general rule, pH, salinity and electrical conductivity were higher in the dry season and increased toward the mouth of the estuary. Sediment characteristics varied little in space or time. Almost 99% of all the specimens collected were annelids. The abundance and diversity of the organisms were significantly higher in the rainy season, when there was a marked increase in the abundance of oligochaetes and insect larvae, whereas polychaetes predominated during the dry season. The results of the ANOSIM indicated significant differences in the composition of the macrofaunal communities between seasons. The groups formed by the CCA reflected the distance from the mouth of the estuary. At the innermost sites, characterized by lower electrical conductivity and rich, organic sandy sediments, oligochaetes predominated, whereas at the sites further downstream, where more saline conditions predominate, polychaetes were found in higher densities. The results indicated that the structure of the macrobenthic communities in this estuarine bay are structured primarily by salinity, and secondarily by seasonal changes in rainfall rates, which affect the characteristics of both the water and sediments.