Description: Contrasting models predict two different climate change scenarios for the Southern Ocean (SO), forecasting either less or stronger vertical mixing of the water column. To investigate the responses of SO phytoplankton to these future conditions, we sampled a natural diatom dominated (63%) community from today's relatively moderately mixed Drake Passage waters with both low availabilities of iron (Fe) and light. The phytoplankton community was then incubated at these ambient open ocean conditions (low Fe and low light, moderate mixing treatment), representing a control treatment. In addition, the phytoplankton was grown under two future mixing scenarios based on current climate model predictions. Mixing was simulated by changes in light and Fe availabilities. The two future scenarios consisted of a low mixing scenario (low Fe and higher light, low mixing treatment) and a strong mixing scenario (high Fe and low light, strong mixing treatment). In addition, communities of each mixing scenario were exposed to ambient and low pH, the latter simulating ocean acidification (OA). The effects of the scenarios on particulate organic carbon (POC) production, trace metal to carbon ratios, photophysiology and the relative numerical contribution of diatoms and nanoflagellates were assessed. During the first growth phase, at ambient pH both future mixing scenarios promoted the numerical abundance of diatoms (~75%) relative to nanoflagellates. This positive effect, however, vanished in response to OA in the communities of both future mixing scenarios (~65%), with different effects for their productivity. At the end of the experiment, diatoms remained numerically the most abundant phytoplankton group across all treatments (~80%). In addition, POC production was increased in the two future mixing scenarios under OA. Overall, this study suggests a continued numerical dominance of diatoms as well as higher carbon fixation in response to both future mixing scenarios under OA, irrespective of different changes in light and Fe availability.

Description: Contrasting models predict two different climate change scenarios for the Southern Ocean (SO), forecasting either less or stronger vertical mixing of the water column. To investigate the responses of SO phytoplankton to these future conditions, we sampled a natural diatom dominated (63%) community from today's relatively moderately mixed Drake Passage waters with both low availabilities of iron (Fe) and light. The phytoplankton community was then incubated at these ambient open ocean conditions (low Fe and low light, moderate mixing treatment), representing a control treatment. In addition, the phytoplankton was grown under two future mixing scenarios based on current climate model predictions. Mixing was simulated by changes in light and Fe availabilities. The two future scenarios consisted of a low mixing scenario (low Fe and higher light, low mixing treatment) and a strong mixing scenario (high Fe and low light, strong mixing treatment). In addition, communities of each mixing scenario were exposed to ambient and low pH, the latter simulating ocean acidification (OA). The effects of the scenarios on particulate organic carbon (POC) production, trace metal to carbon ratios, photophysiology and the relative numerical contribution of diatoms and nanoflagellates were assessed. During the first growth phase, at ambient pH both future mixing scenarios promoted the numerical abundance of diatoms (~75%) relative to nanoflagellates. This positive effect, however, vanished in response to OA in the communities of both future mixing scenarios (~65%), with different effects for their productivity. At the end of the experiment, diatoms remained numerically the most abundant phytoplankton group across all treatments (~80%). In addition, POC production was increased in the two future mixing scenarios under OA. Overall, this study suggests a continued numerical dominance of diatoms as well as higher carbon fixation in response to both future mixing scenarios under OA, irrespective of different changes in light and Fe availability.

Description: In some parts of the Southern Ocean (SO), even though low surface concentrations of iron (Fe) and manganese (Mn) indicate FeMn co-limitation, we still lack an understanding on how Mn and Fe availability influences SO phytoplankton ecophysiology. Therefore, this study investigated the effects of Fe and Mn limitation alone as well as their combination on growth, photophysiology and particulate organic carbon production of the bloom-forming Antarctic diatom Chaetoceros debilis. Our results clearly show that growth, photochemical efficiency and carbon production of C. debilis were co-limited by Fe and Mn as highest values were only reached when both nutrients were provided. Even though Mn-deficient cells had higher photochemical efficiencies than Fe-limited ones, they, however, displayed similar low growth and POC production rates, indicating that Mn limitation alone drastically impeded the cell's performance. These results demonstrate that similar to low Fe concentrations, low Mn availability inhibits growth and carbon production of C. debilis. As a result from different species-specific trace metal requirements, SO phytoplankton species distribution and productivity may therefore not solely depend on the input of Fe alone, but also critically on Mn acting together as important drivers of SO phytoplankton ecology and biogeochemistry.

Description: The Southern Ocean is considered to be a major player in the climate system of our planet while being extremely sensitive to climate change itself. The pelagic Southern Ocean is limited by the bioavailability of iron. Zooplankton has a large impact on the remineralization of iron in the water column and thereby an important influence on primary production. Indications exist that due to increasing water temperatures in the course of climate change, vast areas of the Southern Ocean might shift from a krill to a salp-dominated community. Since the degree of iron remineralization is dependent on the taxonomic group of zooplankton, we investigated the different impacts that salp and krill fecal pellets have on iron chemistry and its bioavailability to Southern Ocean phytoplankton, during a Polarstern cruise in spring 2018. We incubated salp and krill fecal pellet material in Antarctic low-iron water without phytoplankton. In a second step, a concentrated natural phytoplankton community was added into the thusly preconditioned water and for the first time ever the iron uptake into the living cells, in respect to the fecal pellet type that acted as an iron source, was determined. Our results indicate that iron released from salp fecal pellets into the seawater was significantly more bioavailable to phytoplankton than iron from krill fecal pellets, since phytoplankton picked up 0.28 nmol Fe L-1 d-1 from water treated with salp fecal pellets and 0.16 nmol Fe L-1 d-1 from water treated with krill fecal pellets. These results demonstrate that salps might actually play a role in stimulating phytoplankton growth in the Southern Ocean, thusly influencing the biological carbon pump.

Description: The Southern Ocean is considered to be a major player in the climate system of our planet while being extremely sensitive to climate change itself. The pelagic Southern Ocean is limited by the bioavailability of iron. Zooplankton has a large impact on the remineralization of iron in the water column and thereby an important influence on primary production. Indications exist that due to increasing water temperatures in the course of climate change, vast areas of the Southern Ocean might shift from a krill to a salp-dominated community. Since the degree of iron remineralization is dependent on the taxonomic group of zooplankton, we investigated the different impacts that salp and krill fecal pellets have on iron chemistry and its bioavailability to Southern Ocean phytoplankton, during a Polarstern cruise in spring 2018. We incubated salp and krill fecal pellet material in Antarctic low-iron water without phytoplankton. In a second step, a concentrated natural phytoplankton community was added into the thusly preconditioned water and for the first time ever the iron uptake into the living cells, in respect to the fecal pellet type that acted as an iron source, was determined. Our results indicate that iron released from salp fecal pellets into the seawater was significantly more bioavailable to phytoplankton than iron from krill fecal pellets, since phytoplankton picked up 0.28 nM Fe d-1 from water treated with salp fecal pellets and 0.16 nM Fe d-1 from water treated with krill fecal pellets. These results demonstrate that salps might actually play a role in stimulating phytoplankton growth in the Southern Ocean, thusly influencing the biological carbon pump.

Description: The Southern Ocean (SO) is an important sink for anthropogenic carbon dioxide (CO2). Climate change will cause changes in various environmental parameters, which in turn can affect growth and productivity of SO phytoplankton. Due to global warming, sea surface temperatures will increase and lead to a more stratified and shallower mixed layer resulting potentially in higher light availability and enhanced primary production of iron-limited phytoplankton. On the other hand, ocean acidification may reduce the bioavailability of iron to phytoplankton. To examine the influence of iron availability in combination with current and future higher CO2 concentrations under low and high irradiance on SO phytoplankton physiology, bottle manipulation experiments with a natural phytoplankton assemblage from the Drake Passage were conducted. Ocean acidification led to lowered abundances of Pseudo-nitzschia species at both irradiances. While higher irradiance stimulated daily particulate organic carbon production, this stimulating effect, however, was reduced under high pCO2, but only under iron-limitation. Moreover, the ratio of biogenic silicate to particulate organic carbon remained unchanged by high pCO2 for both iron treatments under high light, but declined under low light. Gaining more insight on the complex interplay of multiple environmental factors is valuable to predict future responses of SO phytoplankton to climate change.

Description: In many regions of the Southern Ocean, surface concentrations of the trace metal iron are very low. Iron is an essential nutrient, required for numerous metabolic pathways in phytoplankton cells. Atmospheric dust is an important source for iron input into the ocean. An insufficient supply of iron can lead to reduced growth and alterations in the photophysiology. Therefore, iron is a key factor in controlling Antarctic phytoplankton productivity and species composition. However, in experiments looking at the effects of iron on phytoplankton physiology, iron is commonly added as iron chloride and not in the form of dust. This PhD project will focus on the effects of inorganic iron in comparison to iron-containing dust as iron sources in combination with current and future elevated CO2 concentrations on Southern Ocean phytoplankton ecology and physiology. Rising CO2 concentrations in the atmosphere will reduce the pH of the world’s oceans. Ocean acidification will affect Southern Ocean phytoplankton by potentially altering the availability of iron. In order to study the impact of different climate change scenarios on Southern Ocean phytoplankton, laboratory experiments with selected species as well as shipboard experiments with natural phytoplankton assemblages during an expedition to the Southern Ocean will be conducted.