Impact of UV radiation and nutrients on the elemental composition of zooplankton in a Mediterranean high mountain lake

摘要

Homeostatic elemental composition implies that the internal elemental composition of an organism resists changes despite external variability, and the process of regulation is known as stoichiometric homeostasis. While algae are a clear example of non-homeostatic organisms, zooplankton represents an inter-mediate degree of homeostasis (non-strict homeostasis) (STERNER & ELSER 2002). Previous studies showed that the de-gree of homeostasis in zooplankton is a function of different biotic features, including their ontogenetic development (VILLAR-ARGAIZ et al. 2002), reproduction events (VENTURA & CATALAN 2005) and the elemental composition of their food source (DEMOTT et al. 1998). However, other studies reported more subtle responses of zooplankton elemental composition to abiotic factors. Thus, ELSER et al. (2000) described a higher P content in Daphnia pulicaria from Alaska than in Daphnia pulex from a temperate region, suggesting latitudinal effects on body elemental com-position. While temperature has been considered a key abiotic factor explaining variations in the elemental composition of terrestrial organisms (REICH & OLEKSYN 2004), several studies have described changes in elemental composition produced by ultraviolet radiation (UVR) in algae (e.g. XENOPOULOS et al. 2002). Although the influence of food on the homeostatic na-ture of zooplankton has been examined (DEMOTT et al. 1998), the relative importance of this factor with respect to other non-biotic factors such as UVR has been neglected. The purpose of this work was to assess in situ the effects of UVR and food quality and quantity.on biomass, carbon (C) and phosphorus (P) content and C/P ratio of the calanoid copepod Mixodiaptomus laciniatus across an experimentally generated trophic gradient.