Seasonal variability of stable isotope values and niche size in tropical calanoid copepods and zooplankton size fractions

The seasonal variability of stable isotope values (δ15N and δ13C), ∆15N values, and niche width of calanoid copepod species, three zooplankton size fractions and seston were assessed from samples obtained monthly between January and December 2012 at a coastal sampling site in the Eastern Tropical Pacific off Mexico (19°09′03″ N, 104°44′50″ W). Twenty calanoid copepod species were analyzed, 15 of which were sampled in both the cool, productive ‘mixed’ period and the warm, oligotrophic ‘stratified’ period. While copepod δ15N and ∆15N values were higher during the mixed period, seston δ15N values did not differ between periods. The δ13C values indicated a seasonal difference between a priori-defined groups of oceanic and coastal copepod species. The seasonal variations of both the isotopic niche (Bayesian ellipses) values and Layman metrics in tropical copepods and zooplankton fractions suggested an opportunistic feeding strategy responding to the local coupled physical–biological seasonal processes. During the productive mixed period, the trophic web was more robust and stable, and a wider niche was observed. Contrary to commonly reported responses to environmental conditions, we found a more complex mesozooplankton food web during the productive period and a shorter web during the oligotrophic period, highlighting the importance of researching trophic dynamics in coastal tropical ecosystems.