Biogeochemical cycling in a subarctic fjord adjacent to the Greenland Ice Sheet

Temperatures in the Arctic have increased rapidly in recent years resulting in the melting of sea ice and glaciers at unprecedented rates. In 2012, sea ice extent across the Arctic reached a record minimum and the melt extent of Greenland Ice Sheet reached a record maximum. The accelerated mass loss of the Greenland Ice Sheet has resulted in increased meltwater input to Greenland’s fjords and coastal waters. While the impact of changes in sea ice cover on the marine ecosystem has been well documented, the effect of meltwater runoff on Greenland’s ecosystems remained largely unstudied. By linking the complex physical oceanography to biogeochemistry in Greenland fjords, this thesis aimed to increase our understanding of the annual carbon dynamics in high latitude fjord systems and specifically identify the impact of melting of the Ice Sheet on Greenland’s fjord ecosystems. In Chapter 2, the environmental factors that control the timing and intensity of the spring bloom in Godthåbsfjord are described. In high-latitude fjord ecosystems, the spring bloom generates a major part of the annual primary production and thus provides a crucial energy supply to the marine food web. A combination of out-fjord winds and dense coastal inflows drive an upwelling in the inner part of Godthåbsfjord during spring (April-May), which supplies nutrient-rich water to the surface layer that is subsequently transported downstream. The upwelling results in strong biogeochemical gradient in fjord with absence of blooming close to the tidewater glaciers where the upwelling occurs but the development of an intense and prolonged spring bloom in the central region of the fjord from mid-March to May. Weakening of the upwelling and changes in the dominant wind direction in late May, reversed the surface water transport, so that warmer water was transported towards the inner outlet glacier terminus, and a bloom was now observed close to the glacier. Our results suggest that the timing, intensity and location of the spring bloom in Godthåbsfjord are controlled by a combination of upwelling strength and wind forcing. These physical processes hence play together with sea ice cover a crucial role in structuring food web dynamics of the fjord ecosystem. During summer, the Greenland Ice Sheet releases large amounts of freshwater, which strongly influences the physical and chemical properties of the adjacent fjord systems and continental shelves (Chapter 3 and 4). Freshwater runoff itself influences circulation patterns and stratification in Greenland fjords. Observations from different meltwater rivers around Greenland show that the meltwater is not an important source of inorganic nitrate and phosphate, and the direct surface input of meltwater will consequently not stimulate primary production within the fjords (Chapter 3). However the input of glacial meltwater does strongly impact the fjord circulation and consequently the marine ecosystem productivity although this is very differently regulated in fjords with either land-terminating or marine-terminating glaciers (Chapter 4). Rising subsurface meltwater plumes originating from marine-terminating glaciers entrain large volumes of deep water, and the resulting nutrient upwelling sustains high phytoplankton productivity in the inner fjord throughout summer. In contrast, fjords with land-terminating glaciers lack this upwelling mechanism, and hence, are characterized by substantially lower productivity. Data on commercial halibut landings confirms that coastal regions under the influence of large marineterminating glaciers are hotspots of marine productivity. As the shrinking of the Greenland Ice Sheet will induce a switch from marine-terminating to land-terminating glaciers, our results suggest that ongoing climate change can drastically alter the productivity in the coastal zone around Greenland with large socio-economic implications. Furthermore Chapter 3 shows that glacial meltwater leads to high input of dissolved silica as glacial activity stimulates rock weathering. Up-scaled to the entire Greenland Ice Sheet, the export of dissolved silica to adjacent coastal areas equals 22 ± 10 Gmol Si yr-1, and this value could increase 160% by the year 2100 following projections of accelerated mass loss from the Greenland Ice Sheet. This increased silica export may substantially affect phytoplankton communities as silica is an essential element for diatoms. When this silica-rich meltwater mixes with upwelled deep water, we also observed that growth of diatoms is stimulated relative to other phytoplankton groups, thus providing a high quality food source for higher trophic levels. In Chapter 5, the impact of meltwater on the carbonate dynamics of these productive coastal systems is quantified. Our data reveal that the surface layer of the entire fjord and adjacent continental shelf are undersaturated in CO2 throughout the year. This results in an average annual CO2 uptake of 65 g C m-2 yr-1, indicating that the fjord system is a strong sink for CO2 compared to other coastal areas. The largest CO2 uptake occurs in the inner fjord near to the Greenland Ice Sheet and high glacial meltwater input correlates strongly with low pCO2 values. Model simulation of the impact of meltwater on the carbonate system revealed that around a quarter of the CO2 uptake can be attributed to the non-conservative behavior of pCO2 during the mixing of fresh water and saline fjord water. This result in a CO2 uptake of 1.8 mg C per kg of glacial ice melted implying that glacial meltwater is a driver for CO2 uptake in Greenland fjords. The largest part of the high CO2 sink is however due to the strong biological activity both during spring and summer. The fate of this organic matter determines the carbon sink in the fjord system in the end. The POC export from the photic zone followed the seasonality of the primary production both in Kobbefjord and Godthåbsfjord (Chapter 6 and 7). But the strong seasonality in pelagic productivity was not reflected in the sediment biogeochemistry, showing only moderate variation. The largest fraction of the sedimented organic material is buried in the sediment while ~ 38 % is mineralized in the sediment, mainly through sulfate reduction (69% of the benthic mineralization). Both studies highlight a discrepancy between POC flux and primary production, with higher export of carbon compared to local production. My findings demonstrate that glaciers have a fundamental impact on hydrographic circulation and consequently on biogeochemical cycling in Greenland’s fjords

Impact of global change on coastal oxygen dynamics and risk of hypoxia

Climate change and changing nutrient loadings are the two main aspects of global change that are linked to the increase in the prevalence of coastal hypoxia - the depletion of oxygen in the bottom waters of coastal areas. However, it remains uncertain how strongly these two drivers will each increase the risk of hypoxia over the next decades. Through model simulations we have investigated the relative influence of climate change and nutrient run-off on the bottom water oxygen dynamics in the Oyster Grounds, an area in the central North Sea experiencing summer stratification. Simulations were performed with a one-dimensional ecosystem model that couples hydrodynamics, pelagic biogeochemistry and sediment diagenesis. Climatological conditions for the North Sea over the next 100 yr were derived from a global-scale climate model. Our results indicate that changing climatological conditions will increase the risk of hypoxia. The bottom water oxygen concentration in late summer is predicted to decrease by 24 mu M or 11.5% in the year 2100. More intense stratification is the dominant factor responsible for this decrease (58 %), followed by the reduced solubility of oxygen at higher water temperature (27 %), while the remaining part could be attributed to enhanced metabolic rates in warmer bottom waters (15 %). Relative to these climate change effects, changes in nutrient runoff are also important and may even have a stronger impact on the bottom water oxygenation. Decreased nutrient loadings strongly decrease the probability of hypoxic events. This stresses the importance of continued eutrophication management in coastal areas, which could function as a mitigation tool to counteract the effects of rising temperatures

An annual cycle of diatom succession in two contrasting Greenlandic fjords : from simple sea-ice indicators to varied seasonal strategists

We recorded diatom species succession over one full year (May 2017-May 2018) using automated sediment traps installed in two contrasting Greenlandic fjords: the seasonally ice-covered Young Sound in high-arctic Northeast Greenland and the nearly sea-ice free Godthabsfjord in subarctic Southwest Greenland. The traps were positioned at differing water depths (37m in Young Sound vs. 300m in Godthabsfjord). Distinct differences between the study sites were observed in both sediment and diatom fluxes. In Young Sound, total diatom flux was extremely seasonal and as high as 880 x 10(6) valves m(-2) d(-1) in the spring. In Godthabsfjord, total diatom flux was more stable throughout the year, with a maximum of 320 x 10(6) valves m(-2) d(-1) in the summer. The diatom assemblage in Young Sound was dominated by the sea-ice species Fragilariopsis oceanica, Fragilariopsis reginae-jahniae and Fossula arctica, which exhibited pulse-like deposition in the trap during and after the ice melt. In Godthabsfjord, the fluxes were dominated by Chaetoceros (resting spores), while the remaining assemblage was characterised by the cold-water indicator species Detonula confervacea (resting spores) and Thalassiosira antarctica var. borealis (resting spores) together with Fragilariopsis cylindrus. Our data show that, F. oceanica, F. reginae-jahniae and F. arctica exhibit similar seasonal behaviour and a clear link to sea ice. Fragilariopsis cylindrus seems to have a more flexible niche, and based on our study, cannot be considered an unequivocal ice indicator. Taking into account these ecological and seasonal preferences of individual diatom species is crucial when reconstructing past sea-ice conditions both qualitatively and quantitatively.Peer reviewe

Glacier retreat alters downstream fjord ecosystem structure and function in Greenland

The melting of the Greenland Ice Sheet is accelerating, with glaciers shifting from marine to land termination and potential consequences for fjord ecosystems downstream. Monthly samples in 2016 in two fjords in southwest Greenland show that subglacial discharge from marine-terminating glaciers sustains high phytoplankton productivity that is dominated by diatoms and grazed by larger mesozooplankton throughout summer. In contrast, melting of land-terminating glaciers results in a fjord ecosystem dominated by bacteria, picophytoplankton and smaller zooplankton, which has only one-third of the annual productivity and half the CO2 uptake compared to the fjord downstream from marine-terminating glaciers.publishedVersio

Validation of pop-up satellite archival tags (PSATs) on Atlantic cod (Gadus morhua) in a Greenland fjord

Traditional tagging techniques are simple and cost-effective, but inferences require recaptures and data on movement/migration are limited to a start and end position at unpredictable intervals. Pop-up satellite archival tags (PSATs) offer other opportunities, as they provide positions at pre-programmed times and collect on-route data, which can be used to describe position, behavior, and habitat preferences. Species suitability should, however, be documented prior to large-scale studies using PSATs. We deployed PSATs on six relatively large (total length 84–125 cm) Atlantic cod (Gadus morhua) in inshore West Greenland waters. Three tags were physically recovered, providing high-resolution data on depth and temperature (readings every 3 s), while three tags did not report (recovery rate = 50 %). To evaluate the tag’s applicability on Atlantic cod, we made a detailed behavioral analysis by defining swimming behavior, occupied water types and depth phases, which were cross-evaluated in relation to depth, temperature and water stratification to identify behavioral patterns. Distinct and shared patterns in swimming behavior were evident and we found no signs of impaired swimming behavior except for an adaptation period lasting up to 39 h after release. Generally, the three cod were pelagic and preferred waters ranging 2–5 °C. When encountering colder water masses these were avoided. During late summer/early autumn, increased vertical activity could in some cases be linked to darkness and a high-activity event could be linked to possible predator avoidance. All combined, we conclude that PSATs are suitable to monitor natural behavior on large specimens of Atlantic cod for periods of at least four months.publishedVersio

An Interdisciplinary Perspective on Greenland’s Changing Coastal Margins

Greenland’s coastal margins are influenced by the confluence of Arctic and Atlantic waters, sea ice, icebergs, and meltwater from the ice sheet. Hundreds of spectacular glacial fjords cut through the coastline and support thriving marine ecosystems and, in some places, adjacent Greenlandic communities. Rising air and ocean temperatures, as well as glacier and sea-ice retreat, are impacting the conditions that support these systems. Projecting how these regions and their communities will evolve requires understanding both the large-scale climate variability and the regional-scale web of physical, biological, and social interactions. Here, we describe pan-Greenland physical, biological, and social settings and show how they are shaped by the ocean, the atmosphere, and the ice sheet. Next, we focus on two communities, Qaanaaq in Northwest Greenland, exposed to Arctic variability, and Ammassalik in Southeast Greenland, exposed to Atlantic variability. We show that while their climates today are similar to those of the warm 1930s­–1940s, temperatures are projected to soon exceed those of the last 100 years at both locations. Existing biological records, including fisheries, provide some insight on ecosystem variability, but they are too short to discern robust patterns. To determine how these systems will evolve in the future requires an improved understanding of the linkages and external factors shaping the ecosystem and community response. This interdisciplinary study exemplifies a first step in a systems approach to investigating the evolution of Greenland’s coastal margins

The Case for a Sustained Greenland Ice Sheet-Ocean Observing System (GrIOOS)

Rapid mass loss from the Greenland Ice Sheet (GrIS) is affecting sea level and, through increased freshwater and sediment discharge, ocean circulation, sea-ice, biogeochemistry, and marine ecosystems around Greenland. Key to interpreting ongoing and projecting future ice loss, and its impact on the ocean, is understanding exchanges of heat, freshwater, and nutrients that occur at the GrIS marine margins. Processes governing these exchanges are not well understood because of limited observations from the regions where glaciers terminate into the ocean and the challenge of modeling the spatial and temporal scales involved. Thus, notwithstanding their importance, ice sheet/ocean exchanges are poorly represented or not accounted for in models used for projection studies. Widespread community consensus maintains that concurrent and long-term records of glaciological, oceanic, and atmospheric parameters at the ice sheet/ocean margins are key to addressing this knowledge gap by informing understanding, and constraining and validating models. Through a series of workshops and documents endorsed by the community-at-large, a framework for an international, collaborative, Greenland Ice sheet-Ocean Observing System (GrIOOS), that addresses the needs of society in relation to a changing GrIS, has been proposed. This system would consist of a set of ocean, glacier, and atmosphere essential variables to be collected at a number of diverse sites around Greenland for a minimum of two decades. Internationally agreed upon data protocols and data sharing policies would guarantee uniformity and availability of the information for the broader community. Its development, maintenance, and funding will require close international collaboration. Engagement of end-users, local people, and groups already active in these areas, as well as synergy with ongoing, related, or complementary networks will be key to its success and effectiveness

Tracing Glacial Meltwater From the Greenland Ice Sheet to the Ocean Using Gliders

The Greenland Ice Sheet (GrIS) is experiencing significant mass loss and freshwater discharge at glacier fronts. The freshwater input from Greenland will impact the physical properties of adjacent coastal seas, including important regions of deep water formation and contribute to global sea level rise. However, the biogeochemical impact of increasing freshwater discharge from the GrIS is less well constrained. Here, we demonstrate the use of bio-optical sensors on ocean gliders to track biogeochemical properties of meltwaters off southwest Greenland. Our results reveal that fresh, coastal waters, with an oxygen isotopic composition characteristic of glacial meltwater, are distinguished by a high optical backscatter and high levels of fluorescing dissolved organic matter (FDOM), representative of the overall colored dissolved organic matter pool. Reconstructions of geostrophic velocities are used to show that these particle and FDOM-enriched coastal waters cross the strong boundary currents into the Labrador Sea. Meltwater input into the Labrador Sea is likely driven by mesoscale processes, such as eddy formation and local bathymetric steering, in addition to wind-driven Ekman transport. Ocean gliders housing bio-optical sensors can provide the high-resolution observations of both dissolved and particulate glacially derived material that are needed to understand meltwater dispersal mechanisms and their sensitivity to future climatic change

Review article: How does glacier discharge affect marine biogeochemistry and primary production in the Arctic?

Freshwater discharge from glaciers is increasing across the Arctic in response to anthropogenic climate change, which raises questions about the potential downstream effects in the marine environment. Whilst a combination of long-term monitoring programmes and intensive Arctic field campaigns have improved our knowledge of glacier-ocean interactions in recent years, especially with respect to fjord/ocean circulation, there are extensive knowledge gaps concerning how glaciers affect marine biogeochemistry and productivity. Following two cross-cutting disciplinary International Arctic Science Committee (IASC) workshops addressing the importance of glaciers for the marine ecosystem, here we review the state of the art concerning how freshwater discharge affects the marine environment with a specific focus on marine biogeochemistry and biological productivity. Using a series of Arctic case studies (Nuup Kangerlua/Godthäbsfjord, Kongsfjorden, Kangerluarsuup Sermia/Bowdoin Fjord, Young Sound and Sermilik Fjord), the interconnected effects of freshwater discharge on fjord-shelf exchange, nutrient availability, the carbonate system, the carbon cycle and the microbial food web are investigated. Key findings are that whether the effect of glacier discharge on marine primary production is positive or negative is highly dependent on a combination of factors. These include glacier type (marine- or land-terminating), fjord-glacier geometry and the limiting resource(s) for phytoplankton growth in a specific spatio-temporal region (light, macronutrients or micronutrients). Arctic glacier fjords therefore often exhibit distinct discharge-productivity relationships, and multiple case-studies must be considered in order to understand the net effects of glacier discharge on Arctic marine ecosystems

Highly variable iron content modulates iceberg-ocean fertilisation and potential carbon export

Marine phytoplankton growth at high latitudes is extensively limited by iron availability. Icebergs are a vector transporting the bioessential micronutrient iron into polar oceans. Therefore, increasing iceberg fluxes due to global warming have the potential to increase marine productivity and carbon export, creating a negative climate feedback. However, the magnitude of the iceberg iron flux, the subsequent fertilization effect and the resultant carbon export have not been quantified. Using a global analysis of iceberg samples, we reveal that iceberg iron concentrations vary over 6 orders of magnitude. Our results demonstrate that, whilst icebergs are the largest source of iron to the polar oceans, the heterogeneous iron distribution within ice moderates iron delivery to offshore waters and likely also affects the subsequent ocean iron enrichment. Future marine productivity may therefore be not only sensitive to increasing total iceberg fluxes, but also to changing iceberg properties, internal sediment distribution and melt dynamics