Stable carbon and nitrogen isotope ratios as tools to evaluate the nature of particulate organic matter in the Venice lagoon

Complex environments, such as transitional waters, are characterized by several sources of particulate organic matter (POM). The analyses of stable isotopes, δ13C and δ15N, are usually used to discriminate between allochthonous and autochthonous POM. This study aimed to characterize the nature of POM through the stable isotopic analysis for the first time in the Venice lagoon. Water samples were seasonally collected during 2011 and 2012 at 28 stations during the monitoring program for the accomplishment of the objectives of the Water Framework Directive (WFD, Directive, 2000/60/CE). Dissolved nutrient concentrations, chlorophyll a, total suspended solids were determined and POM was characterized through δ13C, δ15N and C/N molar ratio. The results showed mixed sources (autochthonous and allochthonous) of POM in all lagoon stations. Nevertheless, in autumn, the enriched 13C and significantly depleted 15N POM suggested the major influence of terrestrial contribution, probably due to the increase of freshwater inputs and land runoff. Differently, in the summer period sewage discharges and biological processes as well as primary production and microbial degradation prevailed, giving a strong seasonal increase in δ15N values (up to 18.2‰). sults of PCA analysis evidenced a spatial distinction of POM load among polyhaline, euhaline not confined and euhaline confined areas, identified for the WFD accomplishment. h the application of multiple sources mixing model (IsoSource) by means of two variables and five different POM sources, a prevalence of allochthonous sources (82%) over autochthonous was estimated in the Venice lagoon.