Grazing and egg production by Calanus finmarchicus across the Fram Strait: Implications for a changing Arctic Ocean

Abstract

Copepods of the genus Calanus are fundamental to the biogeochemical and ecological functioning of the Arctic Ocean. They are an energy-rich food source for higher trophic levels, including commercially important fish stocks. They accumulate lipid stores and sequester them at depth during an overwintering period. Recent climatic warming has shifted the range of the typically Atlantic Calanus finmarchicus northwards into the Arctic. Rapid warming has also changed the timing, composition, distribution, and magnitude of the phytoplankton blooms on which Calanus spp. rely, altering the quantity and composition of the food available and potentially shifting towards flagellate-dominated communities.
To better understand the effects of the food environment on C. finmarchicus and the limits to its production, this thesis presents the composition of the plankton assemblage with rates of grazing and production of C. finmarchicus females across a spatiotemporal scale. Metabolic carbon budgets were created to investigate the relationships of ingestion and production. Chapter 2 presents rates of ingestion, rates of production and gonad maturation in C. finmarchicus in spring 2018, finding a disconnect between ingestion and production and surplus carbon in the budget. The surplus carbon and highly variable grazing rates were attributed to the energetically expensive process of gonad maturation. Chapter 3 presents rates of ingestion, rates of production and gonad maturation in C. finmarchicus in summer 2019 during post-bloom conditions. The microplankton assemblage was low in biomass and dominated by ciliates and flagellates. The animals were food limited, thus suggesting the project shift to a flagellate dominated system in the future Arctic may have negative consequences for Calanus. Chapter 4 compares plankton taxonomic composition of the Fram Strait across seasons, revealed through metabarcoding to microscopic identification and to high-throughput flow imaging. The results provide evidence for the future shift to a plankton assemblage dominated by the smaller taxa. The flexible and diverse diet C. finmarchicus consumed in the high biomass plankton assemblage exceeded metabolic demand. However, when the plankton shifts towards its projected future composition, there are negative consequences for C. finmarchicus. The rates of grazing and production in C. finmarchicus presented here are vital measurements that form the basis of our understanding of phytoplankton-zooplankton interaction and will constrain future biogeochemical models.

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ORCID for Holly Elizabeth Jenkins: orcid.org/0000-0002-1055-642X

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