Mesozooplankton grazing minimally impacts phytoplankton abundance during spring in the western North Atlantic

Francoise Morison; James Joseph Pierson; Andreas Oikonomou; Susanne Menden-Deuer

doi:10.7717/peerj.9430

Mesozooplankton grazing minimally impacts phytoplankton abundance during spring in the western North Atlantic

PeerJ. 2020 Jul 17:8:e9430. doi: 10.7717/peerj.9430. eCollection 2020.

Authors

Francoise Morison¹, James Joseph Pierson², Andreas Oikonomou^{1

3}, Susanne Menden-Deuer¹

Affiliations

¹ Graduate School of Oceanography, University of Rhode Island, Narragansett, RI, United States of America.
² Center for Environmental Science, University of Maryland, Cambridge, MD, USA.
³ Institute of Balances of internal water, Hellenic Centre for Marine Research, Athens, Greece.

Abstract

The impacts of grazing by meso- and microzooplankton on phytoplankton primary production (PP) was investigated in the surface layer of the western North Atlantic during spring. Shipboard experiments were performed on a latitudinal transect at three stations that differed in mixed layer depth, temperature, and mesozooplankton taxonomic composition. The mesozooplankton community was numerically dominated by Calanus finmarchicus at the northern and central station, with Calanus hyperboreus also present at the northern station. The southern station was >10 °C warmer than the other stations and had the most diverse mesozooplankton assemblage, dominated by small copepods including Paracalanus spp. Microzooplankton grazing was detected only at the northern station, where it removed 97% of PP. Estimated clearance rates by C. hyperboreus and C. finmarchicus suggested that at in-situ abundance these mesozooplankton were not likely to have a major impact on phytoplankton abundance, unless locally aggregated. Although mesozooplankton grazing impact on total phytoplankton was minimal, these grazers completely removed the numerically scarce > 10 µm particles, altering the particle-size spectrum. At the southern station, grazing by the whole mesozooplankton assemblage resulted in a removal of 14% of PP, and its effect on net phytoplankton growth rate was similar irrespective of ambient light. In contrast, reduction in light availability had an approximately 3-fold greater impact on net phytoplankton growth rate than mesozooplankton grazing pressure. The low mesozooplankton grazing impact across stations suggests limited mesozooplankton-mediated vertical export of phytoplankton production. The constraints provided here on trophic transfer, as well as quantitative estimates of the relative contribution of light and grazer controls of PP and of grazer-induced shifts in particle size spectra, illuminate food web dynamics and aid in parameterizing modeling-frameworks assessing global elemental fluxes and carbon export.

Keywords: Grazing; Mesozooplankton; Microbial food web; Microzooplankton; North Atlantic; Phytoplankton; Plankton; Primary production; Spring bloom.

Grants and funding

This work was conducted within the NASA supported North Atlantic Aerosols and Marine Ecosystems Study (NAAMES, grant NNX15AL2G). Further support for data analysis and synthesis was provided by the NASA EXport Processes in the global Ocean from RemoTe Sensing (EXPORTS) field campaign (grant 80NSSC17K0716 to Susanne Menden Deuer and Tatiana Rynearson). There was no additional external funding received for this study. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.