Temporal variability of benthic macrofauna on Cassino beach, southernmost Brazil

Abstract

ABSTRACT. The temporal variability of benthic macrofauna on Cassino beach, southernmost Brazil, was studied for a period of one year (June 2004 to May 2005 based on monthly sampling. Three sites were selected distant 50m from each other. At each site, 3 transects were established, 2m equidistant from one another. Each transect extended from the base of the primary dunes to the inner surf zone at approximately 1m in depth, with 7 or 8 sampling levels. Within transects, the distance between the levels was 20m until the upper swash zone, from which distance was 10m until the 1-meter isobath. The temporal variation in the abundance of benthic macrofauna observed in the present study can be attributed to (1) the positive effects of the recruitment peaks and migration of particular species to the swash zone and (2) negative effects of the migration of some species to deeper waters, as well (3) as mortality through natural causes (stranding and action of predators) and (4) human causes (harvesting and vehicle transit). We attribute the expressive abundance increase of benthic macrofauna to recruitment. The stranding, that is, the trapping of the organisms on the upper parts of the beach, is likely the main cause of abrupt drops in benthic macrofauna abundance.KEYWORDS. Sandy beaches, benthic macrofauna, temporal variation, Cassino beach.RESUMO. Variabilidade temporal da macrofauna bentônica na praia do Cassino, extremo sul do Brasil. A variabilidade temporal da macrofauna bentônica na praia do Cassino, Rio Grande do Sul, Brasil, foi estudada durante o período de um ano (junho 2004 a maio de 2005) com base em coletas mensais. Escolheram-se três locais, distantes 50m um do outro, sendo que em cada local foram fundadas três transversais 2m eqüidistantes. Cada transversal se estendeu desde a base das dunas primárias até aproximadamente 1m de profundidade no infralitoral. A distância dos níveis de coleta em cada transversal foi de 20m até o limite superior da zona de varrido, a partir do qual a distância foi de 10m. A variação temporal da abundância da macrofauna bentônica, constatada no presente trabalho pode ser atribuída a (1) efeitos positivos decorrentes dos picos de recrutamento e migração de determinadas espécies para a zona de varrido e a (2) efeitos negativos como a migração de algumas espécies para águas mais profundas, a (3) mortalidade por causas naturais (embancamento e ação dos predadores) e (4) antrópicas (extrativismo e o trânsito de veículos). Ao recrutamento é atribuída a responsabilidade pela expressiva elevação da abundância da macrofauna bentônica. Já o embancamento, isto é, o aprisionamento dos organismos nas partes superiores da praia, provavelmente seja a principal causa das abruptas quedas nas abundâncias do macrozoobentos. PALAVRAS-CHAVE.Praias arenosas, macrofauna bentônica, variabilidade temporal, praia do Cassino.The distribution, abundance and diversity of beach macrofauna have been related to different physical factors, from which the most important are wave action, the size of sand grains and...

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