Resistance and reconfiguration of natural flexible submerged vegetation in hydrodynamic river modelling

In-stream submerged macrophytes have a complex morphology and several species are not rigid, but are flexible and reconfigure along with the major flow direction to avoid potential damage at high stream velocities. However, in numerical hydrodynamic models, they are often simplified to rigid sticks. In this study hydraulic resistance of vegetation is represented by an adapted bottom friction coefficient and is calculated using an existing two layer formulation for which the input parameters were adjusted to account for (i) the temporary reconfiguration based on an empirical relationship between deflected vegetation height and upstream depth-averaged velocity, and (ii) the complex morphology of natural, flexible, submerged macrophytes. The main advantage of this approach is that it removes the need for calibration of the vegetation resistance coefficient. The calculated hydraulic roughness is an input of the hydrodynamic model Telemac 2D, this model simulates depth-averaged stream velocities in and around individual vegetation patches. Firstly, the model was successfully validated against observed data of a laboratory flume experiment with three macrophyte species at three discharges. Secondly, the effect of reconfiguration was tested by modelling an in situ field flume experiment with, and without, the inclusion of macrophyte reconfiguration. The inclusion of reconfiguration decreased the calculated hydraulic roughness which resulted in smaller spatial variations of simulated stream velocities, as compared to the model scenario without macrophyte reconfiguration. We discuss that including macrophyte reconfiguration in numerical models input, can have significant and extensive effects on the model results of hydrodynamic variables and associated ecological and geomorphological parameters

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

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

Structure and dynamics of the NO sensing domain of the human soluble Guanylate Cyclase

Soluble guanylate cyclase (sGC) is a heterodimeric heme-protein composed of two subunits called α and β [1-5]. The most common heterodimeric form is the combination of α1 with β1 subunits [1]. The α1 (80KDa) and β1 (70 KDa) subunits are 690 and 619 amino acids in length respectively, and are encoded by the genes, GUCY1A2 and GUCY1A3 respectively [3].(...

Land use/cover change from 1868 to 2000 in the north Ethiopian highlands: integration of satellite imagery and terrestrial photography

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