My name is Clara Hoppe, and I work as an ecologist and biogeochemist at Alfred Wegener Institute, the Helmholtz Centre for Polar and Marine Research in Germany. My scientific aim is to understand what effects climate change has on the Arctic ecosystems and how these effects in turn affect the climate of our planet. MOSAiC is, for me, a really fantastic opportunity to broaden our understanding of these topics and I would join the campaign for two of the six legs, so from February-April, and then again from August-October, 2020. Today I hope to give you a little introduction into Arctic primary production and carbon export. When people think about important organisms in the Arctic, they tend to think about big animals such as the polar bear. But what most people are not aware of is the fact that Arctic marine ecosystems predicted depend on these tiny guys, the phytoplankton. Phytoplankton are unicellular photosynthetic organisms that grow in the surface of our oceans. Despite their smallest size, they're very important for several reasons. As all photosynthetic organisms, phytoplankton use sunlight, CO_2, and nutrients to produce the organic matter and biomass that sustains the ecosystems of our oceans. Via photosynthesis, phytoplankton also produce half of the oxygen in our atmosphere so that every second breath we take is produced by phytoplankton and not the land plants. Primary production also increases the uptake capacity of the oceans for CO_2 as photosynthesis remove CO_2 from its environment. Via this process together with the so-called biological carbon pump that exports the carbon from the surface to the deep ocean, the phytoplankton can contribute to climate on geological timescales and they're even thought to have contributed to the cycles of warm intervals in ice ages on our planet. During MOSAiC, the Ecosystem Team were for the first time measure primary production in high Arctic sea water and sea ice over the full annual cycle. The goal here is to really understand what controls productivity in the different seasons. We already know that in the Central Arctic, light is the main controlling factor for productivity. This is the case because firstly, the strong seasonality and the existence of the polar night, where the sun stays below the horizon for several months in a row, strongly reduce the time of the year when photosynthesis can even take place. Secondly, ice and snow cover on top of the ice strongly reduce the amount of light that actually gets through the ice into the water column and to the phytoplankton. Depending on the thickness of the ice and how much snow it has on top, sea ice can reduce the light availability to less than one percent of the incoming radiation. Even though the primary producers, and especially those that live in the sea ice, have adapted to and can survive in these conditions, their productivity is rather low. Nutrient concentrations are another important controlling factor for productivity because also their concentrations in the Arctic are rather low. Also, a strong solidification of the upper mixed layer reduces the nutrient supply to the surface from greater depth. Therefore, under conditions when there is enough light for phytoplankton to grow, nutrient concentrations become limiting quite quickly. Due to these constraints, primary production in the Arctic is tightly linked to sea ice concentrations, with low productivity in areas with very high sea ice cover, and highest productivity at the edges of the sea ice as well as in coastal areas. For the future, we expect, for example, that the decreased sea ice cover due to climate change, which increase the time when light is limiting productivity and therefore may increase the importance of nutrient limitation for Arctic productivity. During MOSAiC, we will hopefully develop a better understanding of the nutrient dynamics in the Central Arctic, as well as of the transmittance of light through snow and ice, which together will help us to understand our primary production data. As I mentioned before, one of the processes why primary production is so important is the so-called biological carbon pump. This term describes the fact that a part of the carbon that is fixed into biomass via photosynthesis sinks out of the surface into the deep ocean, where this remineralized and CO_2 is released back into the seawater. Even though only a very small fraction of the biomass ultimately reaches the sea floor and gets buried in the sediment, the biological carbon pump still leads to the transport of CO_2 from the surface to the deep ocean, thereby reducing the CO_2 in the surface. Because the surface is in equilibrium with the atmosphere, this actually leads to the ability of the ocean to take up more CO_2 than it would without biology. Why this is a mechanism that generally occurs, it is of particular interest if we want to understand how much of the anthropogenically emitted CO_2 the oceans will be taking up. Also, even the small fraction of about 0.1 percent of the organic matter that sinks out of the productive euphotic zone that really reaches the sea floor is so small, it still provides food for entire reach a deep sea ecosystems and our oceans, including the Arctic. The strengths and efficiency of the biological carbon pump. So how much of the organic matter that originates from photosynthesis sinks out of the surface ocean and how deep it is transported before it gets remineralized depends on different factors. Most importantly, it depends on the size and the composition of the sinking particles. Particles sink faster, the heavier and denser they are. Single phytoplankton cells, for example, depending on their size and composition, can sink between a few millimeters and two meters per day, giving bacteria plenty of opportunity to respire and remineralize the organic matter. Therefore, it is mainly the larger particles which contribute to the efficiency of the biological carbon pump. These larger particles are, for example, fecal pellets, so the excrement of zooplankton that graze on phytoplankton, or rather so-called marine snow aggregates. In sea ice covered areas such as the Central Arctic, sea ice algae are very important part of the biological carbon pump because they form dense aggregates that sink quickly into the deep ocean. During a plankton fuse in 2012 to the Central Arctic, for example, our colleagues observed in an area with dense coverage of the filamentous algae, Melosira arctica. The export efficiency was 85 percent. So 85 percent of the biomass actually sink all the way to the sea floor at 3,500 meter depth, and this is a much larger fraction than the usual one percent of the phytoplankton that reach the sea floor. Therefore, it seems that these algae aggregates in the Central Arctic are very important vector for carbon export and sustain ecosystems in the deep ocean. Therefore, changes in sea ice cover, sea ice characteristics, as well as species composition of primary producers, have really strong effects on not only deep sea ecosystems, but also the efficiency of the biological carbon pump and the storage of anthropogenic CO_2 in the oceans. Because the Arctic is so hard to reach and difficult to study, we currently have very little data on these processes from this quickly changing ecosystem. In MOSAiC, where we have several methods to study carbon export, we'll hopefully really increase our data coverage and understanding of this exciting system. With this, I hope I could give you a little introduction on why we care about Arctic primary production and carbon export and what exciting results we expect from MOSAiC.
