Seasonal variation of peracarid assemblages in natural and artificial marine environments of the Southwestern Atlantic Ocean

The diversity and seasonal variations of two assemblages of marine benthic peracarids were studied between a natural rocky shore and an artificial harbour area over a 12-month period. Samples were obtained monthly in La Estafeta, a rocky intertidal zone with low human impact, and Mar del Plata Harbour, a polluted environment, between March 2011 and March 2012. The two sites differed markedly in the composition and abundance of species across all seasons: the tanaid Tanais dulongii was most abundant in La Estafeta rocky shore, followed by the amphipods Monocorophium acherusicum, Hyale grandicornis, Ampithoe valida, the isopod Idotea balthica, the tanaid Leptochelia sp. and the isopod Sphaeroma serratum. In contrast, M. acherusicum was most abundant in the harbour area, followed by T. dulongii, S. serratum, Ericthonius punctatus, I. balthica, Caprella equilibra and C. dilatata. Total density of peracarids varied between months in La Estafeta rocky shore and Mar del Plata Harbour. In La Estafeta rocky shore mean density increased from March to May 2011 (autumn in the southern hemisphere; ca. 45,000 ind/m2), decreased sharply until August and then increased in January 2012. In Mar del Plata Harbour the mean density was lower from March to October (ca. 500,000 ind/m3), then increased and reached a maximum in January 2012 (more than 1,500,000 ind/m3), and decreased until the following March. This study suggests that the differences in peracarid assemblages, diversity and seasonality could be related to an effect of temperature, but we should not rule out a synergistic effect of other factors, such as pollution, food availability and hydrodynamic factors.