Advances in Geosciences www.adv-geosci.net 1680-7340 1680-7359 6 1st Alexander von Humboldt International Conference 2006 10.5194/adgeo-6-161-2006 http://www.adv-geosci.net/6/161/2006/ http://www.adv-geosci.net/6/161/2006/adgeo-6-161-2006.html http://www.adv-geosci.net/6/161/2006/adgeo-6-161-2006.pdf 161 166 2006-01-31 Disentangling the effects of El Niño on a population of the polychaete <i>Sigambra bassi</i> in the Bay of Ancón, Peru T. S. Peña K. Johst V. Grimm W. E. Arntz J. Tarazona Facultad de Ciencias Biológicas, Universidad Nacional Mayor de San Marcos, Group DePSEA, Av. Venezuela s/n, Apartado 1898, Lima-100, Perú UFZ Centre for Environmental Research Leipzig-Halle, Department of Ecological Modelling, Permoserstr. 15, 04318 Leipzig, Germany Alfred Wegener Institute for Polar and Marine Research, Columbusstrasse, 27568 Bremerhaven, Germany The macrobenthic community in shallow soft-bottom areas in the Bay of Anc&#243;n, Peru, is characterized by low biodiversity due to low oxygen concentrations. During El Ni&#241;o (EN) events, higher water temperature and higher concentrations of dissolved oxygen induce a temporary increase in biodiversity. However, the structure and dynamics of the emerging macrobenthic community and populations, especially the polychaete <I>Sigambra bassi</I>, vary strongly among events. The reasons for this variation are poorly understood, in particular the relative influence of abiotic versus biotic factors. To disentangle how abiotic and biotic factors influence the different responses of the population of <I>S. bassi</I>, population models were developed based on detailed long-term monitoring data, which include four El Ni&#241;o events. The results show that <I>S. bassi</I> abundances are favored by abiotic environmental conditions during EN, namely high temperature and dissolved oxygen concentration, but these abiotic effects are modulated by different biotic processes. In two EN events with relatively similarly high temperature anomalies (EN 1982-1983 and 1997-1998) different biological interactions (competition, predation, facilitation) and different species composition of the community resulted in different responses of the population of <I>S. bassi</I>.