Spatial and seasonal changes of brackish peracaridan assemblages and their relation to some environmental variables in two tidal channels of the Ria de Aveiro (NW Portugal)

The shallow-water fauna of 2 main channels in the Ria de Aveiro (Canal de Ovar and Canal de Mira) was sampled between June 1995 and June 1996 using a suprabenthic sled. Samples were collected monthly at 5 stations and every third month at 11 other stations, covering both channels from the lower to the upper reaches. The channels differ in their hydrodynamics, in the distribution of the sediment environments and in the salinity gradient, thus providing an invaluable example for field studies. The results presented herein concern peracaridan crustaceans. Multivariate analysis (MDS, ANOSIM and UPGMA) of the faunal data showed significant differences between the 2 channels. Spatial and seasonal variations within the channels were also significant. The distinctive structural features were the higher number of species and overall dominance of mysids in the Canal de Mira as opposed to the dominance of amphipods and higher densities in the Canal de Ovar. In both channels there was a spatial pattern of high densities in areas with a rich sediment environment (fine sediments, high organic content, more diversified and stable food sources). Specific life cycle features, unfavourable temperature and salinity conditions, and probably also increased predation pressure induced a general impoverishment of the peracaridan assemblage during winter. Amphipods (mainly epibenthic and tube dwellers) and mysids (characteristic suprabenthic) showed defined spatial and seasonal patterns. Amphipods dominated the areas with more stable food sources, while the highly motile mysids succeeded in colonising and dominated in more dynamic and unpredictable environments. Density fluctuations of amphipods and mysids occurred seasonally in non-coincident cycles. On the whole, mysids were dominant during winter and spring and amphipods during summer and autumn.