Global

Seasonal variation in the relative contributions of littoral and pelagic food sources to the diets of open-water zooplankton and subsequent changes in their trophic

The seasonal variation of seston stable isotopes signature of carbon (δ13C) in Lake Maggiore during 2008 was related to seasonal variation of the lake phytoplankton community, investigated in terms of phytoplankton taxonomic groups, morpho-functional groups (MBFG), cell size classes and cell shape classes. Three open water stations were selected to reflect truly pelagic, influenced by littoral and riverine carbon sources; phytoplankton samples were collected from two water depths, 0-20 m and 25-50 m. Among stations differences in δ13C signatures of seston were statistically non significant, confirming that allochthonous input may become important only after exceptional rainfall events. Nonparametric multiplicative regression (NPMR) was utilized to identify among the phytoplankton parameters (taxonomic groups, MBFG, cell shapes, cell sizes) which were the best predictors of the δ13C variation. Bacillariophyceae and two morpho-functional groups, Group 6 (non-flagellated organisms with siliceous exoskeletons) and Group 3 (large filamentous algae with aerotopes) were statistically significant. Non-metric multidimensional scaling (NMS) ordination was used to investigate differences in phytoplankton samples and their relationship with δ13C variation. When morphological and functional phytoplankton traits were superimposed, the NMS ordination showed that cylinder cells (for cell shape classes), class 500-2000 mm3 (for cell size classes) and Group 6 (for MBFG) were the most related to δ13C variation. Our study confirms that the variation in the pelagic δ13C seston most probably reflects changes in phytoplankton carbon isotopic signature, consequent to changes in availability of carbon sources depending on the season and due to different isotopic fractionation of phytoplankton taxonomic groups. Statistical investigations have allowed us to investigate the potential role of phytoplankton morphological and functional traits in the seasonal variation of δ13C and to suggest their importance in the interpretation of natural stable isotopes studies in lake ecosystems

While the role of littoral food sources in shallow lakes has been widely investigated, uncertainties still exist about the relevance of such sources for deep lakes. Here we report quantitative estimates for the contribution of littoral versus pelagic sources in supporting the three most important planktivorous fish of a deep, large, subalpine lake in Italy. Contributions of pelagic (p) and littoral (q) signatures of δ13C and δ15N stable isotopes were detected in fish muscular tissue by applying a dynamic baseline mixing model. This model integrates tissue-specific metabolic turnover (m) and fish growth (k) rates over baselines δ13C seasonality. Annual fluctuations for both pelagic and littoral baselines were not negligible (ΔC = 10‰ and 8‰, respectively). We calculated that they could not be ignored, since contributions of pelagic and littoral signatures would be largely underestimated (up to 30% p and 13% q for whitefish (Coregonus lavaretus) and roach (Rutilus rutilus), respectively). When fish relied upon pelagic consumers, stepwise 15N-enrichment (E) of pelagic preys linearly decreased with prey-size-specific predation pressure. Therefore, longer food webs would be proportionally less stepwise 15N-enriched than shorter ones