The planktonic food web in the Gulf of Naples based on the analysis of carbon and nitrogen stable isotope ratios

Plankton plays a key role in marine food webs by producing and transferring organic matter and energy to higher trophic levels. To define the trophic structure and interactions within the planktonic communities in the Gulf of Naples, we determined carbon and nitrogen stable isotope ratios in particulate organic matter (POM, <20 mu m), phytoplankton (20-200 mu m), and bulk (unsorted) and sorted mesozooplankton (200-2000 mu m) on a weekly basis in 2019. The stable isotope values of POM and, to a lesser extent, phytoplankton reflected the short-term (weekly) alternation between offshore and coastal waters within seasonal variability. Although the isotopic signatures of coastal and offshore sources were still detectable in bulk mesozooplankton, delta C-13 and delta N-15 of single groups remained almost unchanged throughout the year. The trophic niches of mesozooplankton groups defined using the Stable Isotope Bayesian Ellipses in R software showed a high degree of overlap among them, which was corroborated by the tendency of most groups toward omnivory, beyond their known feeding habits, as indicated by MixSIAR models. Collectively, our results highlight that the complex network of interactions within the planktonic community in the Gulf of Naples can buffer natural variability due to the specific hydrographic features of the system. This characteristic makes the trophic structure of this community an ideal model for monitoring the response of planktonic food webs to climate-and anthropic-driven changes.