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Oxygen concentration and consumption at the entrance to the Baltic Sea from 1975 to 2000
Rasmussen, B.; Gustafsson, B.G.; Ærtebjerg, G.; Lundsgaard, C. (2003). Oxygen concentration and consumption at the entrance to the Baltic Sea from 1975 to 2000. J. Mar. Syst. 42(1-2): 13-30. dx.doi.org/10.1016/S0924-7963(03)00062-9
In: Journal of Marine Systems. Elsevier: Tokyo; Oxford; New York; Amsterdam. ISSN 0924-7963, more
Peer reviewed article  

Available in Authors 

Keywords
    Oxygen consumption; Oxygen depletion; Respiration; Marine

Authors  Top 
  • Rasmussen, B.
  • Gustafsson, B.G.
  • Ærtebjerg, G.
  • Lundsgaard, C.

Abstract
    The oxygen level in the deep layers (20–40 m) of the Baltic entrance is modelled as a function of an oxygen consumption rate constant and climate forcing. The oxygen level is calculated as median values of the observed oxygen concentrations. Climate forcing is expressed by water temperature and consumption period, representing the time during which the water was isolated below the halocline and experienced net respiration. First- and zero-order reaction terms are considered, and it is found that the first-order reaction yields an optimal representation of the natural range of oxygen concentration observations in the area. However, neither model performance nor conclusions are strongly dependent on the choice of reaction order. Using the first-order model, the modelled oxygen level declines exponentially with increasing consumption period, where the averaged rate constant for the oxygen consumption at 5 °C is estimated at 0.0045 day−1. The consumption rate increases exponentially by a factor of 3 with a 10 °C temperature increase. Climate forcing accounts for 64% of the total variation in oxygen level and 95% of its seasonal variation. The gross consumption rate is enhanced by 25% after the spring and autumn phytoplankton blooms, while it is reduced during winter, when surface-water primary production is light limited. This seasonal variation supports the argument that gross consumption can be limited by the availability of organic matter. The averaged seasonal variation in gross consumption rate is included in the model. In this way, we have accounted for both atmospheric forcing and seasonal variations in the bottom-water oxygen level at the Baltic entrance. The averaged error of the model estimate thus becomes 9%. This residual variation arises primarily from modelling uncertainties and variations in the gross consumption forced by interannual changes in the availability of organic matter for respiration. Our results support the hypothesis that the decline in oxygen conditions observed between 1960 and 2000 cannot be explained without considering changes in the export production in surface waters of the entrance to the Baltic Sea. The observed reduction in the oxygen level cannot be explained by the variability in atmospheric forcing on water exchange and bottom-water temperature as resolved by the present transport model.

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