The role of krill grazing in Southern Ocean nutrient cycles

One of the major problems mankind is facing in this century is an increasing number and intensity of natural disasters (e.g. hurricanes, floods, melting ice sheets). Many of these disasters are indicators of global climatic changes related to the ever-increasing amount of CO2 humans release into the atmosphere. So far, a large proportion of these CO2 emissions has been absorbed by the oceans and tucked away for centuries, but to predict the future, we need to understand the mechanisms involved. This proposal tackles one possible mechanism within the Southern Ocean. We hypothesize that a small crustacean - Antarctic krill / helps the drawdown of carbon (C) from the atmosphere into the deep ocean. Three facts about krill lead to this suggestion: Firstly, krill are very abundant, with a total mass greater than that of the human population. Secondly, krill have very high feeding rates and feed mainly on phytoplankton, the algae, which build their own organic C from the CO2 dissolved in the water. Thirdly, krill faeces sink as compact pellets towards the seafloor. Thus, krill mediate the carbon transition from small floating algae to large sinking fecal pellets, a process known as the 'biological C pump'. Unfortunately, it is slightly more complicated than this. With the sinking pellets, krill might also export other elements from the surface layer, e.g. iron (Fe) and silicon (Si) that are essential for the algae to grow and often in limiting concentrations. Even though krill are a key species in the Southern Ocean food web and commercially fished for, little is known about their role in biogeochemical cycles. None of the essential processes has been measured before in detail, thus, we need more information to test our hypothesis: 1. How much C, Si and Fe are in the krill fecal pellets? 2. Do the elements dissolve out of the pellets before sinking to depth? 3. Do krill accumulate Fe into their bodies? 4. How much Fe and Si do krill release in dissolved form when feeding? The last question is especially important, because a fast regeneration of particulate Fe into the dissolved form via krill might stimulate algal growth and therefore a further uptake of CO2. Our plan is to tackle these questions during a cruise in the Southern Ocean. We will collect krill and incubate them on board to measure the rates of pellet production and release of dissolved nutrients. We will sample their pellets from different water depths, to compare the total numbers and the content of C, Fe and Si. These measurements will be related to water column profiles of Fe and Si, both in dissolved form as nutrients and in particulate form in algal cells. We will sample at a range of stations within different environments / some with lots of algae, others with few, some with sufficient Fe and Si, others with too little. This will enable us to make simple equations that relate the various rates in krill (see 4 questions above) to their available food and nutrient situation. With help of these equations, we can scale up the results from our sampling sites to answer our overall question: Do krill support the biological C pump by exporting C and recycling nutrients, or do they stop the pump by removing Fe and Si from surface water? Our ship-time bid is for 11 days in the Scotia Sea during 2009/2010. To increase our seasonal and regional coverage, we will supplement the data set with a range of frozen samples from previous cruises. Both measuring Fe and handling krill are non-trivial tasks; therefore the proposal combines expertise across two institutes, the British Antarctic Survey and the National Oceanography Centre Southampton. The scientists involved supply all the essential skills in locating, catching and experimenting with krill (Atkinson, Fielding, Schmidt), in trace metal clean work (Achterberg, Rijkenberg), in oceanography (Venables) and in marine chemistry (M. Whitehouse).