To improve the water quality of a habitat or to stop eutrophication driven by internal loading of phosphorus from anoxic bottoms, the deepwater in inshore and offshore sea basins may be oxygenated using sea-based measures. The minimum oxygen concentration DOmin in periodically stagnant basins is controlled by three factors, namely (i) the rate of oxygen depletion dDO/dt. (ii) The residence time Te of the stagnant water. (iii) The oxygen concentration DOstart in the basin water at the beginning of a stagnation period. A theoretical framework is presented, suitable or evaluation of sea-based measures to improve the oxygen conditions in periodically stagnant basins. Five different sea-based measures to improve the oxygen conditions are recognized and discussed. The application of sea-based measures might be particularly attractive for habitat improvements in basins with poor oxygen conditions despite land-based measures are fully implemented. The sea-based measures are illustrated by hypothetical applications to the By Fjord on the west coast of Sweden, a salt-stratified fjord with periodically stagnant anoxic basin water confined behind the shallow Sunninge Strait in the mouth. An interesting result of one of the hypothetical applications is that dredging of the Sunninge Strait in 1958 and 1976 reduced tidal currents by a factor of 4. This reduced the tidal power supply to vertical mixing in the basin water, which led to prolonged residence time Te, from 2-3 years to 3–5 years, and deteriorated oxygen conditions in the basin water.
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