|Biogeochemical cycling in a subarctic fjord adjacent to the Greenland Ice Sheet|
Meire, L. (2016). Biogeochemical cycling in a subarctic fjord adjacent to the Greenland Ice Sheet. PhD Thesis. Universiteit Gent/NIOZ: Gent. 199 pp.
Biogeochemical cycle; Ice sheets; AN, Greenland [Marine Regions]; Marine
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.