Inter-Research >  > Prepress Abstract

MEPS prepress abstract   -  DOI: https://doi.org/10.3354/meps13333

Resilience of plankton trophic structure to an eddy-stimulated diatom bloom in the North Pacific Subtropical Gyre

Moira D├ęcima*, Michael R. Landry

*Corresponding author:

ABSTRACT: We investigated the response of an open-ocean plankton food-web to a major ecosystem perturbation event, the Hawaiian lee cyclonic eddy Opal, using amino acid compound-specific isotopic analyses of amino acids (CSIA-AA) of individual zooplankton taxa. We hypothesized that the massive diatom bloom that characterized Opal would lead to a shorter food chain. Using CSIA-AA we differentiated trophic position (TP) changes that arose from altered transfers through protistan microzooplankton, versus metazoan carnivory, and assessed the variability at the base of the food web. Contrary to expectation, zooplankton TPs were higher in the eddy than in ambient control waters (up to 0.8 trophic level), particularly for suspension-feeders close to the food web base. Most of the effect was due to increased trophic transfers through protistan consumers, indicating a general shift up, not down, of grazing and remineralization in the microbial food web. Eucalanus sp., which were 15-fold more abundant in Opal than out, was the only taxa observed to be a true herbivore (TP = 2.0), consistent with a high phenylalanine (phe) δ15N value indicating feeding on nitrate-fueled diatoms in the lower euphotic zone. Oncaea sp., an aggregate-associated copepod, had the largest (1.5) TP difference, and lowest phe δ15N, suggesting that detrital particles were local hot spots of enhanced microbial activity. Rapid growth rates and trophic flexibility of protistan microzooplankton apparently allow the microbial community to reorganize to bloom perturbations, as microzooplankton remain the primary phytoplankton grazers, despite the dominance of large diatoms, and are heavily preyed on by the mesozooplankton.