Mechanistic understanding of the role of diatoms in the success of the Arctic Calanus complex and implications for a warmer Arctic

Copepod species of the genus Calanus (Calanus hereafter) are rice grain-sized crustaceans, distant relatives of crabs and lobsters, that occur throughout the Arctic Ocean consuming enormous quantities of microscopic algae (phytoplankton). These tiny animals represent the primary food source for many Arctic fish, seabirds and whales. During early spring they gorge on extensive seasonal blooms of diatoms, fat-rich phytoplankton that proliferate both beneath the sea ice and in the open ocean. This allows Calanus to rapidly obtain sufficient fat to survive during the many months of food scarcity during the Arctic winter. Diatoms also produce one of the main marine omega-3 polyunsaturated fatty acids that Calanus require to successfully survive and reproduce in the frozen Arctic waters. Calanus seasonally migrate into deeper waters to save energy and reduce their losses to predation in an overwintering process called diapause that is fuelled entirely by carbon-rich fat (lipids). This vertical 'lipid pump' transfers vast quantities of carbon into the ocean's interior and ultimately represents the draw-down of atmospheric carbon dioxide (CO2), an important process within the global carbon cycle. Continued global warming throughout the 21st century is expected to exert a strong influence on the timing, magnitude and spatial distribution of diatom productivity in the Arctic Ocean. Little is known about how Calanus will respond to these changes, making it difficult to understand how the wider Arctic ecosystem and its biogeochemistry will be affected by climate change.

The overarching goal of this proposal is to develop a predictive understanding of how Calanus in the Arctic will be affected by future climate change. We will achieve this goal through five main areas of research:

We will synthesise past datasets of Calanus in the Arctic alongside satellite-derived data on primary production. This undertaking will examine whether smaller, more temperate species have been increasingly colonising of Arctic. Furthermore, it will consider how the timing of life-cycle events may have changed over past decades and between different Arctic regions. The resulting data will be used to validate modelling efforts.

We will conduct field based experiments to examine how climate-driven changes in the quantity and omega-3 content of phytoplankton will affect crucial features of the Calanus life-cycle, including reproduction and lipid storage for diapause. Cutting-edge techniques will investigate how and why Calanus use stored fats to reproduce in the absence of food. The new understanding gained will be used to produce numerical models of Calanus' life cycle for future forecasting.

The research programme will develop life-cycle models of Calanus and simulate present day distribution patterns, the timing of life-cycle events, and the quantities of stored lipid (body condition), over large areas of the Arctic. These projections will be compared to historical data.

We will investigate how the omega-3 fatty acid content of Calanus is affected by the food environment and in turn dictates patterns of their diapause- and reproductive success. Reproductive strategies differ between the different species of Calanus and this approach provides a powerful means by which to predict how each species will be impacted, allowing us to identify the winners and losers under various scenarios of future environmental changes.

The project synthesis will draw upon previous all elements of the proposal to generate new numerical models of Calanus and how the food environment influences their reproductive strategy and hence capacity for survival in a changing Arctic Ocean. This will allow us to explore how the productivity and biogeochemistry of the Arctic Ocean will change in the future. These models will be interfaced with the UK's Earth System Model that directly feeds into international efforts to understand global feedbacks to climate change.

The major beneficiaries will be fisheries policy makers, biogeochemical modellers, climate scientists and political bodies concerned with the health and management of the Arctic ecosystem. Improved understanding of Arctic fisheries is important for understanding the food security and provision of protein and nutrition for many nations, including the UK. Our work will also be of benefit to academics interested in understanding trophic interactions and the biogeochemistry of marine ecosystems. The involvement of PhD students and early career scientists will ensure that we contribute to training the next generation of research scientists.

Understanding how the abundance and lipid content of Calanus is likely to change is central to understanding how the productivity of fish and higher trophic levels will be affected by future environmental change. This understanding is therefore of central importance to fisheries and ecosystem managers throughout the Arctic. Knowledge of zooplankton mediated carbon sequestration is required for accurate model parameterisation and validation, and thus of benefit to biogeochemical and climate modellers: New parameterisation of the spatio-temporal distribution of copepod abundance will be ready for coupling to MEDUSA, the biogeochemical model adopted by NERC and the UK Met Office to provide marine biogeochemistry in UKESM1, the UK community Earth System Model being used in phase 6 of the Coupled Model Intercomparison Project (CMIP6) that will feed into IPCC Assessment Report 6. More accurate predictions of Calanus distributions will provide more detailed insight into the knock-on effects for the composition and abundance of Arctic fish communities. In turn, this will provide more realistic assessments of food security and the provisioning of marine protein and essential omega-3 polyunsaturated fatty acids.

We will maximize the visibility and accessibility of our research to Arctic policy makers and fisheries scientists by presenting our program and its findings at Arctic Frontiers conferences (http://www.arcticfrontiers.com/), the annual gathering of academics, politicians and business with interests in the Arctic. Our program has been designed with the involvement of ecosystem and biogeochemical modelers from the outset. On-going, 2-way dialogue between the observational/experimental and modeling components of our work will enable the effective uptake of our data products. Involvement of NEMO-MEDUSA in CMIP6 will ensure that our work contributes to the broader political process via the reports of the IPCC. We are also working directly with US and Norwegian modelling groups that are developing seasonal and multidecadal forecasts for the benefit of management and policy regarding high-value fisheries like Alaska pollock and marine mammal conservation. This simultaneous integration of multiple, well-tested copepod models into three internationally prominent oceanographic models will, to our knowledge, constitute the most complete ensemble experiment in understanding and predicting the future of Arctic zooplankton to date.