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, delta C-13 and delta N-15, 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 delta C-13, delta N-15 and C/N molar ratio. The results showed mixed sources (autochthonous and allochthonous) of POM in all lagoon stations. Nevertheless, in autumn, the enriched C-13 and significantly depleted N-15 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 delta N-15 values (up to 18.2 parts per thousand). The results of PCA analysis evidenced a spatial distinction of POM load among polyhaline, euhaline not confined and euhaline confined areas, identified for the WFD accomplishment. Through 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. (C) 2013 Elsevier Ltd. All rights reserved.