Iron in the Southern Ocean: A model study of iron sources and their impact on the phytoplankton

In the iron limited Southern Ocean, the iron sources have a large impact on the spatial distribution and magnitude of the phytoplankton blooms, and thereby also on the net primary and export production (NPP and EP). In this thesis, the Southern Ocean input of iron from the sediments and from vertical supply to the mixed layer is investigated in a new Ocean General Circlation Biogeochemical Model (OGCBM). Focus is especially on the modelâ s representation of the iron sources, how they affect the biological production in the model and what can be learned from this about the iron sources in nature. The first part of the thesis documents the global performance of the Regulated Ecosystem Model (REcoM2) coupled to the Finite Element Sea-ice Ocean Model (FESOM). It shows that the model does a reasonable job reproducing the large scale surface patterns of the biogeochemical fields when compared to observations and other non-eddy resolving OGCBMs. The spatial fields of chlorophyll a and NPP are especially good. Further analysis of the Southern Ocean shows that the total net primary and export production here fits well with previous estimates. The second part of the thesis assesses the impact of the Ocean General Circulation Model (OGCM) on the vertical iron supply to the mixed layer of the Southern Ocean. Two similar model runs were carried out with REcoM2 coupled to two different ocean models; FESOM and MITgcm. The study shows that the modeled vertical iron fluxes, and thereby the NPP and EP, are strongly dependent on the ocean model. This is partly due to differences in the mixed layer dynamics, but is also affected by other mechanisms, such as the strength of the meridional overturning circulation. The study highlights the importance of considering the skill of the OGCM forcing the biogeochemistry. This is important for contemporary runs, but especially when predicting future changes to the biological pump using large scale OGCBMs. The rate of iron release from the sediments in nature is not well constrained, leading to large variations in its strength between biogeochemical models. The third part of the thesis focuses on the implications of changes in the modeled strength of the sediment source, and accompanying higher scavenging rates, on the phytoplankton composition and the opal export. The study shows, that while variations in the strength of the benthic iron flux can be counteracted by an appropriate scavenging rate, thereby producing very similar values for the total NPP in the Southern Ocean, it has a large impact on the spatial distribution of the NPP, on the diatoms and on the opal export. In order to optimize the sedimentary iron input to large scale OGCBMs, it is thus important to look beyond the modeled NPP and assess the biological production by diatoms, as well as the vertical export of opal, especially for the Southern Ocean. The general discussion focusses on the relative importance of the different iron sources in the Southern Ocean, how the different sources may change in the future and what factors modelers need consider when predicting these changes. Lastly further directions for work regarding the iron sources in the Southern Ocean as well as the new FESOM-REcoM2 model are described

In search of tipping points in the Arctic Ocean ecosystem

The Arctic Ocean is a sentinel for climate change as it warms more than twice faster than the global average. A long list of alterations have already been documented. The future implications for primary producers and consequently for the entire ecosystem and biogeochemical cycles are still uncertain. The objective of this project is to identify tipping points in the Arctic phytoplankton dynamics, their environmental drivers and their implications for biogeochemical cycles using biogeochemical modeling

The Pan-Arctic Continental Slope as an Intensifying Conveyer Belt for Nutrients in the Central Arctic Ocean (1985–2015)

Abstract Primary production in the Central Arctic Ocean (CAO) is limited by light and bioavailable nutrients. With the decline of the sea-ice cover in recent decades, and the resulting increase in light availability, nitrate limitation has been speculated to become more prominent. We used an eddy-permitting biogeochemical model simulation to estimate nitrate advective fluxes at different spatio-temporal scales (synoptic, mesoscale and sub-mesoscale) over the 1985–2015 period. We found that the pan-Arctic continental slope contributes disproportionately to the Dissolved Inorganic Nitrogen supply and that this supply is intensifying through two main processes: lateral eddy transport and upwelling. Despite this increasing supply in nitrate and an intensification of ocean dynamics, the nutrient supply is decreasing everywhere else in the central basins and the simulation indicates that the CAO is still shifting from light to nutrient limitation

Microbial Communities in the East and West Fram Strait During Sea Ice Melting Season

Climate models project that the Arctic Ocean may experience ice-free summers by the second half of this century. This may have severe repercussions on phytoplankton bloom dynamics and the associated cycling of carbon in surface waters. We currently lack baseline knowledge of the seasonal dynamics of Arctic microbial communities, which is needed in order to better estimate the effects of such changes on ecosystem functioning. Here we present a comparative study of polar summer microbial communities in the ice-free (eastern) and ice-covered (western) hydrographic regimes at the LTER HAUSGARTEN in Fram Strait, the main gateway between the Arctic and North Atlantic Oceans. Based on measured and modeled biogeochemical parameters, we tentatively identified two different ecosystem states (i.e., different phytoplankton bloom stages) in the distinct regions. Using Illumina tag-sequencing, we determined the community composition of both free-living and particle-associated bacteria as well as microbial eukaryotes in the photic layer. Despite substantial horizontal mixing by eddies in Fram Strait, pelagic microbial communities showed distinct differences between the two regimes, with a proposed early spring (pre-bloom) community in the ice-covered western regime (with higher representation of SAR11, SAR202, SAR406 and eukaryotic MALVs) and a community indicative of late summer conditions (post-bloom) in the ice-free eastern regime (with higher representation of Flavobacteria, Gammaproteobacteria and eukaryotic heterotrophs). Co-occurrence networks revealed specific taxon-taxon associations between bacterial and eukaryotic taxa in the two regions. Our results suggest that the predicted changes in sea ice cover and phytoplankton bloom dynamics will have a strong impact on bacterial community dynamics and potentially on biogeochemical cycles in this region

Publisher Correction: Carbon dioxide sink in the Arctic Ocean from cross-shelf transport of dense Barents Sea water

In the version of this article initially published, author Cora Hörstmann was wrongly listed with a second affiliation with the Department of Ecoscience–Applied Marine Ecology and Modelling, Aarhus University rather than the Mediterranean Institute of Oceanography (MIO), Marseille, France. Furthermore, references 83–97, now found in the Supplementary Tables caption, were wrongly cited in the Data Availability section. The errors have been corrected in the HTML and PDF versions of the article

Microbial Communities in the East and West Fram Strait During Sea Ice Melting Season

Climate models project that the Arctic Ocean may experience ice-free summers by the second half of this century. This may have severe repercussions on phytoplankton bloom dynamics and the associated cycling of carbon in surface waters. We currently lack baseline knowledge of the seasonal dynamics of Arctic microbial communities, which is needed in order to better estimate the effects of such changes on ecosystem functioning. Here we present a comparative study of polar summer microbial communities in the ice-free (eastern) and ice-covered (western) hydrographic regimes at the LTER HAUSGARTEN in Fram Strait, the main gateway between the Arctic and North Atlantic Oceans. Based on measured and modeled biogeochemical parameters, we tentatively identified two different ecosystem states (i.e., different phytoplankton bloom stages) in the distinct regions. Using Illumina tag-sequencing, we determined the community composition of both free-living and particle-associated bacteria as well as microbial eukaryotes in the photic layer. Despite substantial horizontal mixing by eddies in Fram Strait, pelagic microbial communities showed distinct differences between the two regimes, with a proposed early spring (pre-bloom) community in the ice-covered western regime (with higher representation of SAR11, SAR202, SAR406 and eukaryotic MALVs) and a community indicative of late summer conditions (post-bloom) in the ice-free eastern regime (with higher representation of Flavobacteria, Gammaproteobacteria and eukaryotic heterotrophs). Co-occurrence networks revealed specific taxon-taxon associations between bacterial and eukaryotic taxa in the two regions. Our results suggest that the predicted changes in sea ice cover and phytoplankton bloom dynamics will have a strong impact on bacterial community dynamics and potentially on biogeochemical cycles in this region

Arctic Observatory FRAM

FRAM (FRontiers in Arctic Marine Monitoring) targets a modern vision of integrated underwater infrastructure. FRAM enhances sustainable knowledge for science, society and maritime economy as it enables truly year round observations from surface to depth in the remote and harsh arctic sea. Cutting edge technologies are being (further) developed and used to record essential ocean variables to improve our understanding of the Arctic and it’s ongoing processes. Data will be made freely available to the public via the AWI data portal