The Role of Zooplankton Grazing and Nutrient Recycling for Global Ocean Biogeochemistry and Phytoplankton Phenology

• Published in: Journal of Geophysical Research: Biogeosciences
• Main Authors: Karakuş, Onur, Völker, Christoph, Iversen, Morten, Hagen, Wilhelm, Hauck, Judith
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: American Geophysical Union (AGU) 2022
• Subjects: Arctic Ocean; Phytoplankton; Southern Ocean; Zooplankton
• Online Access: https://epic.awi.de/id/eprint/57603/; https://epic.awi.de/id/eprint/57603/1/JGR%20Biogeosciences%20-%202022%20-%20Karaku%20-%20The%20Role%20of%20Zooplankton%20Grazing%20and%20Nutrient%20Recycling%20for%20Global%20Ocean.pdf; https://hdl.handle.net/10013/epic.29a171e3-5298-40c5-af52-9ba0fb8752e5

Zooplankton plays a notable role in ocean biogeochemical cycles. However, it is often simulated as one generic group and top closure term in ocean biogeochemical models. This study presents the description of three zooplankton functional types (zPFTs, micro-, meso- and macrozooplankton) in the ocean biogeochemical model FESOM-REcoM. In the presented model, microzooplankton is a fast-growing herbivore group, mesozooplankton is another major consumer of phytoplankton, and macrozooplankton is a slow-growing group with a low temperature optimum. Meso- and macrozooplankton produce fast-sinking fecal pellets. With three zPFTs, the annual mean zooplankton biomass increases threefold to 210 Tg C. The new food web structure leads to a 25% increase in net primary production and a 10% decrease in export production globally. Consequently, the export ratio decreases from 17% to 12% in the model. The description of three zPFTs reduces model mismatches with observed dissolved inorganic nitrogen and chlorophyll concentrations in the South Pacific and the Arctic Ocean, respectively. Representation of three zPFTs also strongly affects phytoplankton phenology: Fast nutrient recycling by zooplankton sustains higher chlorophyll concentrations in summer and autumn. Additional zooplankton grazing delays the start of the phytoplankton bloom by 3 weeks and controls the magnitude of the bloom peak in the Southern Ocean. As a result, the system switches from a light-controlled Sverdrup system to a dilution-controlled Behrenfeld system. Overall, the results suggest that representation of multiple zPFTs is important to capture underlying processes that may shape the response of ecosystems and ecosystem services to on-going and future environmental change in model projections.