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Bendtsen J.,VitusLab. | Hansen J.L.S.,University of Aarhus
Ecological Modelling | Year: 2013

Hypoxic conditions (O2<2mgL-1) are frequently observed in the relatively shallow and stratified North Sea-Baltic Sea transition zone. Inter-annual variability with more extensive hypoxia has been observed in years with calm weather conditions during late summer. A future warmer climate may increase hypoxia in the area due to combined effects from decreased oxygen solubility and increased respiration rates. Feedbacks from climate change can, therefore, amplify negative effects from eutrophication, such as hypoxia. Here we apply a high resolution three-dimensional ocean circulation model with a simple pelagic and benthic oxygen consumption model (OXYCON), based on the seasonal organic carbon budget in the area, and demonstrate that the model is able to simulate the temporal and spatial variability of the observed oxygen concentration in the bottom waters during a three-year period. The potential impact from a warmer climate was analysed in a sensitivity study with a 3°C warmer climate, and showed a significant increase of hypoxic bottom areas compared to present day conditions. The relative role of increased respiration and decreased oxygen solubility in the inflowing bottom water and at the surface was analysed and it was found that decreased solubility accounted for about 25% of the simulated decrease in bottom water oxygen concentration in the centre of the area in the early fall. A sensitivity study showed that the simulated effect from a 3°C temperature increase on the bottom water oxygen concentration could be compensated by a 30% reduction in the export production. The model simulations of the North Sea-Baltic Sea transition zone indicate a significant expansion of the hypoxic areas and a lengthening of the hypoxic period under a warmer climate. © 2012 Elsevier B.V. Source


Hilligsoe K.M.,University of Aarhus | Richardson K.,University of Aarhus | Richardson K.,Copenhagen University | Bendtsen J.,VitusLab. | And 4 more authors.
Deep-Sea Research Part I: Oceanographic Research Papers | Year: 2011

Data collected at open water stations (depth>400m) in all major ocean basins in 2006-2008 are used to examine the relationship between the size structure of the phytoplankton community (determined by size fractionated chlorophyll filtration), temperature and inorganic nutrient availability. A significant relationship (p<0.0005) was found between community size structure and temperature, with the importance of large cells in the community decreasing with increase in temperature. Although weaker than the temperature relationship, significant relationships were also noted between community cell size and DIN (nitrate, nitrite and ammonium: p=0.034) and phosphate (p=0.031) concentrations. When the data were divided into two groups of equal size, representing the samples with the highest and lowest DIN/phosphate concentrations, respectively, no difference could be identified between the slopes of the lines representing the relationship between size structure and temperature. There was, however, a difference in the intercepts between the two groups. If the relationship between size and temperature was only a response to nutrient availability, we would expect that the response would be the strongest in the groups with the lowest nutrient concentrations. These results suggest that, in addition to a nutrient effect, temperature may also directly influence the size structure of phytoplankton communities. The size structure of the phytoplankton community in this study correlated to the magnitude of primary production, export production (determined after Laws et al., 2000) and integrated water column chlorophyll. Significant relationships were also found between mesozooplankton production (determined using the proxy of calanoid+cyclopoid nauplii abundance as a percentage of the total number of these copepods) and both temperature and phytoplankton size, with production being the lowest in the warmest waters where phytoplankton were the smallest. In the North Atlantic, export production and community size structure appear to be related to ocean uptake of CO2 from the atmosphere. The reported results suggest that ocean warming may directly alter plankton community structure. This, in turn, may alter the structure of marine food webs and impact the performance of the open ocean as a natural carbon sink. © 2011 Elsevier Ltd. Source


Bendtsen J.,VitusLab. | Gustafsson K.E.,VitusLab. | Lehtoranta J.,Finnish Environment Institute | Saarijarvi E.,Vesi Eko Oy | And 2 more authors.
Boreal Environment Research | Year: 2013

Mechanical oxygenation has been suggested as means of reducing unwanted effects from hypoxia but the near-field dynamics and thereby the potential impact from large scale applications is poorly understood. We present results from a field experiment and model study where the dynamics close to an oxygenator in a shallow coastal area in the Gulf of Finland were studied. The oxygenator created a buoyant plume by pumping surface water through a pipe into the bottom water at a rate of about 1 m3 s-1. A passive tracer (rhodamine) was added to the inlet of the pump and its lateral spreading was subsequently observed in a relatively thin layer below the pycnocline. The dispersion of rhodamine was applied to parameterize the entrainment rate in a plume model and the total outflow was increased by a factor of about 7. A sensitivity study with different pump rates and cross flow velocities were analysed. © 2013. Source

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