THE SALPS OF SUMMER: GELATINOUS ZOOPLANKTON REPLACE HETEROTROPHIC PROTISTS AS MAIN PHYTOPLANKTON GRAZERS IN A TEMPERATE, COASTAL FOOD WEB

Identification of trophic transfer pathways in marine pelagic food webs is foundational to quantifying ecosystem processes, including production. We measured the relative grazing rates of 3 types of phytoplankton consumers: herbivorous protists, crustacean and gelatinous zooplankton in summer 2022 in the Northwest Atlantic, part of the Northeast Shelf LTER (NES-LTER). The system was oligotrophic with low phytoplankton biomass dominated by pico- and nanoplankton; conditions typical for microbial loop systems. Heterotrophic protists, krill (Euphausia krohnii), and a mixed assemblage of copepods did occasionally exert substantive grazing pressure while on average, salps (Thalia democratica) removed about an order of magnitude more phytoplankton biomass than crustacean zooplankton (normalized to biomass). Consumption of three flowcytometrically identified pico- and nanophytoplankton groups yielded similar differences between grazer types, with salp grazing exceeding those of other zooplankton. It is unclear whether recent rapid warming plays a role in the observed food web structure. Our results identify trophic pathways, dominant grazer types, and quantify their relative grazing impacts. Consumption of micron sized phytoplankton by salps is a clear demonstration that canonical predator-prey size ratios are insufficient. Together these data are foundational to the utility of model simulations and conceptional frameworks that require realistic scenarios to assess organic matter production and its fate in a rapidly changing and heavily utilized coastal ecosystems.

Presentation Preference: Oral

Primary Presenter: Susanne Menden-Deuer, University of Rhode Island (smenden@uri.edu)

Authors:

Susanne Menden-Deuer, University of Rhode Island  (smenden@uri.edu)

Deborah Steinberg, Virginia Institute of Marine Science  (debbies@vims.edu)

Pierre Marrec, University of Rhode Island  (pmarrec@uri.edu)

Patricia Thibodeau, University of New England  (pthibodeau1@une.edu)

Frankie Lopez, University of Rhode Island  (frankie.lopez@uri.edu)