Aquatic deoxygenation associated with resuspension of anthropogenic organic matter.

Aquatic environments are highly polluted due to the anthropogenic pressures associated with large populations. Among these ecosystems, coastal lagoons are particularly sensitive to anthropogenic disturbances. The Mar Menor is an example of such an ecosystem, with an extremely degraded status with high levels of anthropogenic eutrophication. As a consequence, reduced dissolved oxygen levels have been observed in this lagoon following episodic wind events that induce mixing and resuspension of bottom sediments, leading to catastrophic consequences for fish populations. Therefore, understanding the processes that control the oxygen depletion in lagoon waters is thus essential to appropriately manage the ecosystem. This study analyses oxygen consumption in the water column associated with sediment resuspension events induced by wind. The study was carried out in a laboratory, where mixing was induced using an oscillating grid device. The range of mixing intensities were selected to be close to the turbulence encountered in the lagoon's field conditions. The resuspended sediment used for the experiments had been taken from the bed of the Mar Menor at three sampling sites. The results of this study indicate that oxygen depletion in the water column during mixing increases with the amount of sediment resuspended, the mean diameter of the particles and the organic matter content in the sediment bed. Data on oxygen depletion in coastal lagoons, estuaries, seas, and reservoirs were then used to develop a predictive model for oxygen depletion, based on the concentration of resuspended sediment, its characteristic particle diameter, and the organic matter content of the sediment bed. Organic matter content was found to be the most significant factor contributing to oxygen depletion. The resulting model determines the oxygen depletion associated to sediment resuspension events with sediments affected by anthropogenic eutrophication.