Effects of coastal upwelling on the structure of macrofaunal communities in SE Brazil

Abstract

The effects of coastal upwelling on the structure of macrofaunal communities were investigated in two shallow bays in SE Brazil. Water, sediment and fauna samples were collected at four time-points corresponding to austral summer, fall, winter and spring, respectively. Water column temperature and salinity profiles indicated that upwelling occurred in summer–spring (December and November), but not in fall–winter (April and August). The structure of macrofaunal communities differed consistently between these periods. The sediment content of labile organic matter did not vary as a function of upwelling and could not explain the changes in macrofaunal communities. Rather it appeared that macrofaunal community structure was determined by organic matter quality (i.e. phytoplankton composition), physical disturbance regimes and bottom-water temperature. Physical disturbance caused by S–SE winds, warm water temperatures (up to 26°C) and resuspension-driven phytoflagellate blooms during non-upwelling were associated to higher density (2511–2525ind m−2) and dominance of small opportunistic species such as spionid, paraonid and capitellid polychaetes. In contrast, stable hydrodynamic conditions, diatom blooms and lower water temperatures (down to 18°C) during upwelling resulted in lower density of macrofauna (796–1387ind m−2) and a shift in species composition to relatively large-sized magelonids and carnivorous polychaetes. Therefore, organic matter quality, physical disturbance regimes, and bottom-water temperature were the major factors regulating the life-cycles, composition and density of macrofaunal communities in these less productive subtropical upwelling systems.