Advances in Geosciences www.adv-geosci.net 1680-7340 1680-7359 6 1st Alexander von Humboldt International Conference 2006 10.5194/adgeo-6-57-2006 http://www.adv-geosci.net/6/57/2006/ http://www.adv-geosci.net/6/57/2006/adgeo-6-57-2006.html http://www.adv-geosci.net/6/57/2006/adgeo-6-57-2006.pdf 57 61 2006-01-09 ENSO and sandy beach macrobenthos of the tropical East Pacific: some speculations T. Vanagt E. Beekman M. Vincx S. Degraer Gent University, Department of Biology, Marine Biology Section, B-9000 Gent, Belgium The influence of the ENSO cycle on marine fauna and flora has only recently been given the attention it deserves. The very strong 1997–1998 El Ni&#241;o and its obvious effects on marine biota was a key point in ENSO research, but unfortunately few quantitative data about the 1997–1998 El Ni&#241;o itself are available. <P style="line-height: 20px;"> To gather information about the effect of ENSO on the macrobenthos, we performed a bi-weekly transect monitoring on an Ecuadorian sandy beach in 2000–2001, during the strong La Ni&#241;a following the 1997–1998 El Ni&#241;o, and in the normal period of 2002–2004. <P style="line-height: 20px;"> In this paper, intertidal macrofaunal densities at higher taxonomic level are used to compare a La Ni&#241;a phase with the 'normal' situation. The few existing documents about El Ni&#241;o and sandy beach macrobenthos, and scattered data from previous and current research, were used to complete the picture. <P style="line-height: 20px;"> Total macrobenthos densities were 300% lower during the La Ni&#241;a phase compared with equal months in the normal phase. Especially Crustacea and Mollusca showed a marked increase in densities towards the normal situation (94% and 341% respectively). Polychaeta and Echinodermata, however, showed higher densities during the La Ni&#241;a phase (22% and 73% respectively). <P style="line-height: 20px;"> Two possible explanations are proposed. (1) Low densities during the La Ni&#241;a could be due to the very strong preceding El Ni&#241;o, suggesting the populations were still recovering. This hypothesis is supported by previous work done in the south of Peru. This is, however, a cold water system, compared to the Ecuadorian warm water system. (2) The second hypothesis states that a La Ni&#241;a will have a very severe impact on the intertidal macrofauna of a warm water system like the Ecuadorian coast.