Problems and perspectives in the use of C and N Stable Isotope Analysis for exploring lacustrine food webs

Ten years ago, the Institute of Ecological Studies was asked by the limnological subcommittee of CIPAIS (Commissione Internazionale per la Protezione delle Acque Italo-Svizzere) to present innovative projects to improve the knowledge deriving from the monitoring of Lake Maggiore and its catchment area. The occasion was pivotal, starting a characterization of the pelagic food web of the lake and for identifying zooplankton trophic relationships, together with their seasonal changes in relation to abiotic and biotic variables. Studies for the characterization of biotic relationships, including predation and competition dynamics within the zooplankton community, dependence on phytoplankton food sources and predatory pressure from fish had been conducted in the past using statistical analyses on zooplankton population dynamics. These early studies combined with laboratory expertise and with zooplankton elemental and microscopic analyses (see Manca & Tognota 1993; Manca et al. 1994, 1995, 1997), to provide the background of the newly established project. Our interest in the molecular approach to investigate trophic interactions was also stimulated by observations of a strong population decline of the cladoceran Daphnia in Lake Maggiore during the period 1989-1996, and by the need to disentangle the roles of predation vs. competition pressure and food limitation. On this issue, and particularly on the possibility of tracing Daphnia food limitation, the late Annie Duncan had already developed a methodology based on elemental analysis and seasonal changes in relation to the quantity and quality of food resources (Duncan 1985). Applying this method allowed us to have an insight into zooplankton taxa dynamics and developmental stages in different seasons, coupled with laboratory zooplankton elemental analysis. Carbon and nitrogen stable isotopes analyses were successfully applied to an alpine lake, providing a useful contribution to the debate on isotopic nitrogen signal dynamics in fishless lakes (Cattaneo et al. 2004). Over a period of ten years, the project has been enriched with other aspects connected to the analyses of persistent pollutants and to biomagnification estimates, providing a fundamental contribution to the understanding of pathway flows of persistent pollutants in aquatic food webs, and of the crucial role of zooplankton in this (cfr. Bettinetti and Manca 2017). Further investigations were promoted by the ad-hoc subcommittee within the CIPAIS, after recent DDT pollution in Lake Maggiore was detected (in June 1996, cfr. http://www.cipais.org/html/lago-maggiore-pubblicazioni.asp for further information).