Mesozooplankton, mainly comprised by a high number of copepod species, is influenced by high primary productivity enhanced by seasonal upwelling events. Although many copepod species have been traditionally considered as herbivores, they might feed on different food sources (e.g. microzooplankton or marine snow) when available. This diet plasticity can be tracked by changes in zooplankton isotopic composition, as their signature integrates both the changes in the nutrient sources (baseline) and their trophic position (TP). However, the extent to which changes in trophic dynamics of mesozooplankton are driven by any of these factors is poorly understood. With the aim of investigating whether the upwelling intensity affects the nitrogen baseline and trophic indicators of this group, the taxonomic composition of mesozooplankton samples and their nitrogen stable isotope composition (δ¹⁵N and CSIA-AA) were investigated during two years in a station at the NW Iberian Peninsula. As expected, higher phytoplankton availability during upwelling events induced zooplankton herbivory, resulting in lower TPs. However, variations in the microbial contribution index to TP, computed from CSIA-AA, revealed that the optimal coupling between metazoan and microbial food webs (i.e. 25% microbial contribution) was reached at moderate upwelling intensities, and decreased to low contributions (<15%) in both downwelling and strong upwelling situations. Therefore, enhanced phytoplankton productivity during moderate upwelling disruptions leads to a shorter and more efficient food web.

Primary Presenter: Inés Viana, Instituto Español de Oceanografía (ines.viana@ieo.csic.es)

Authors:

Rita García-Seoane,   ()

Inés G.Viana,   ()

M. Ángeles Louro,   ()

Antonio Bode,   ()