MEPS 519:13-27 (2015)  -  DOI: https://doi.org/10.3354/meps11071

ABSTRACT: Human-induced stresses on the marine environment seem to favor some jellyfish species to the detriment of other competitors such as planktivorous fishes. In pristine ecosystems, trophic relationships among these consumers are poorly understood. We determined stable carbon and nitrogen isotope signatures of representative consumers in the relatively pristine ecosystem of the Cananéia Estuary, Brazil, in order to understand the food web structure. We described isotopic niche breadth, position, and overlaps between fish and jellyfish (including comb jelly) species. Most of the δ¹³C values suggest that phytoplankton is the major carbon source, especially for pelagic consumers. Sessile benthic invertebrates had enriched δ¹³C values, suggesting a contribution of microphytobenthic algae. Seasonal variation of values was significant only for ¹³C, with different patterns for pelagic and benthic organisms. Isotopic niche breadth of some jellyfishes was wider than those of fish species of the same trophic group, possibly as a consequence of their broad diets. Isotopic niche overlaps of fish and jellyfish species were related to: (1) trophic diversity, since planktivorous species occupied niches distinct from macroinvertebrate/fish feeders; and (2) life stages, since isotopic niche partitioning pattern can change during species ontogeny. Replacement of declining populations of fish by jellyfish competitors probably depends on the pool of other compensatory species, as well as on reproductive, growth, and feeding performance of other consumers. Description of isotopic niches provides a general picture of trophic roles, interactions and the degree of functional redundancy among species, allowing an evaluation of possible directions of community shifts resulting from the removal or proliferation of keystone consumers.

KEY WORDS: Gelatinous zooplankton · Trophic position · Forage fish · Dietary overlap · Stable isotopes