Quantifying the contribution of sympagic versus pelagic diatoms to Arctic food webs and biogeochemical fluxes (SYM-PEL)

At the base of the Arctic food web, there are three major primary producers: small flagellates, diatoms living in open water (pelagic) and diatoms growing in sea ice (sympagic). The role of the sea ice diatoms is perceived differently across the research community. For ecologists they are central to the polar ecosystem, while those looking at global ocean scales consider them less important and have not incorporated them into their models projecting climate change feedbacks. This may reflect their minor (<10%) contribution to the total primary production in Arctic waters. However, two newly developed trophic marker approaches that can trace diatoms from sea ice and open water within the food web, consistently find a strong ice algae 'signal' in polar consumers. Even in whales, seals and polar bears, as much as 80% of their body fat reserves are from carbon originally fixed by ice algae. How is this possible? How will this change in a warming Arctic? Our project aims to answer this puzzle and to bridge the gap between the contrasting perceptions of ice algae.

We propose to quantify the relative importance of ice algae vs. open water diatoms for consumers living in the high Arctic - considering different species, regions and times of the year. We will also look at material that sinks to the seabed, and is collected in sediment traps. Our first hypothesis is that the input of ice algae to Arctic food webs and to export fluxes is disproportionately higher than their contribution to total primary production. Our second hypothesis examines the mechanisms behind these energy transfers, focussing on the more subtle concept of food benefit. It is not just the total annual amount of food that matters; it also has to arrive at the right time, be accessible and be nutritious.

To test these hypotheses, we have developed a method based on "Highly Branched Isoprenoids" (HBIs). These lipid molecules are specific to a series of diatom species specific either to sea ice or open water. Using the ratio of ice-versus water column-derived HBIs, we can now trace the relative roles of these energy inputs to the food web. The chemical stability of these molecules as they pass through the food web is a key advantage of this tracer method, as previously it has been very difficult to follow the fate of ice- or water column derived algae.

We propose to take part in an ice drift across the Central Arctic Ocean (MOSAiC) that will give the opportunity to sample the foodweb and material from sediment traps for subsequent HBI analysis in our lab in Plymouth. We will also determine the body condition of various consumers as an integrator of net benefit derived from each food type over the season. The cruise data set will be complemented with data from other Arctic expeditions and those estimated with a second, independent diet method by our Project partners. This will give a pan-Arctic overview of the importance of ice algae to the lipid stores of key consumers.

Then, simulation model outputs of future climate projection will allow scaling up to the whole Arctic Basin. First, we will work with Project partners modelling life cycles of key zooplankton species, to estimate their potential to colonise a future, more ice-free central Arctic Ocean. Second, we will use NEMO-MEDUSA - the oceanic component of the UK's Earth system model (UKESM1) - to determine whether projected increases in pelagic primary production could compensate for loss of ice algae as a food source for zooplankton. Our findings, and those of other participants in MOSAiC, will be used to initiate a "roadmap" for the incorporation of ice algae into NEMO-MEDUSA. By helping to bridge between the physical, biogeochemical and ecological functions of sea ice and requirements of large-scale modelling, we aim to improve our understanding of the changing Arctic and its provision of services to mankind.

This section briefly summarises who are the main beneficiaries and how they may benefit. The Attachment "Pathways to Impact" expands on this summary with more detail on how we plan to achieve this.

Major non-academic beneficiaries include:

Those involved with industry, particularly developing fishery interests.
There is a range of established, developing and trial fisheries in the Arctic and sub-Arctic, and these will benefit from our research. Substantial stocks of pelagic fish exist in Arctic- and subarctic regions, some of which are becoming more accessible to fishing with changing sea ice or economic drivers. Recent work involving Project Partner Flores has shown that the lipids in polar cod comprise substantial proportions of ice-derived algae, posing questions, for example, over the trajectories of stocks as sea ice continues to decline. Likewise there is an exploratory Norwegian fishery targeting lipid rich Calanus spp. for omega-3 fatty acid food supplements. Our focus on lipids, food quality and the role of sea-ice derived food will be relevant to long-term investment and development planning for fisheries such as these.

Those involved in management and conservation in the Arctic
In the same way that our work is relevant to fishing industries as described above, it is relevant to bodies involved with management and conservation of polar food webs. This includes inter-govermental-, governmental and non-governmental organisations. As one example the World-Wildlife Fund is active in the Arctic with involvement in the Arctic Council, for stewardship and conservation in the Arctic. Our aim to understand food web reliance on ice algae helps to provide an evidence base for the ongoing responsible management and conservation of Arctic resources.

The general public, including the next generation
Our proposal is based around the role of sea ice and its biota in a rapidly changing Arctic. This subject is highly topical, well publicised and photogenic; melting ice is an iconic symbol of climate change. The visual appeal is especially strong for the next generation who are already witnessing the effects of rapid climate change. Our Pathways to Impact has a special focus on the use of visual education media to engage with this latter group. This engagement is through the general topic of climate change in the Arctic, as well as the specifics of our own results.