Seasonal hypoxia in the Amur Bay (Japan Sea)
Seasonal variability of hypoxia conditions in the Amur Bay of the Japan Sea is described on the base of detailed hydrochemical surveys conducted in four seasons of 2008. Hypoxia is caused by microbiological mineralization of "excessive" organic matter, mainly diatom cells, which sank in bottom layer of the sea in conditions of low photosynthetic active radiation and weak water dynamics. The biomass is "excessive" because the phytoplankton bloom in the Amur Bay is too intensive due to eutrophication of the bay. There are two main sources of nutrients: wastes of Vladivostok city and terrestrial water discharge by the Razdolnaya River. The river delivers more phosphates and three times more inorganic nitrogen than the waste waters. The phytoplankton blooms might be caused by strong short-term input of nutrients into the upper layer of the sea during the river floods. Discharge of the Razdolnaya River reached 800 m3/s in the early June of 2008 when estuarine waters enriched by suspended matter and nutrients covered a vast area in the Bay where good conditions for phytoplankton bloom were formed by combination of strong stratification, high nutrients at the sea surface and low abundance of zooplankton. After the bloom, the phytoplankton died and sank on the bottom. Dissolved oxygen from the bottom layer was consumed by microbiological decay of died diatoms in conditions of light deficiency (at the depth > 15 m) and phosphates, ammonium, silicates and dissolved inorganic carbon were produced - the hypoxia was formed at the sea bottom. The value of hypoxia changed seasonally with the lowest dissolved oxygen content in the late summer. Recently, hypoxia is observed regularly in the Amur Bay that is a prominent signal of negative changes in its ecosystem which may have serious consequences. Thus, many died small fishes were found in the Amur Bay in the middle September, 2008, possibly because of direct influence of hypoxia. Another possible mechanism of hypoxia influence on fish is a sulfate reduction in the bottom water layer or/and in bottom sediments and spreading of this poisoned water to shallows during autumn upwelling happened usually in September. From the other hand, upwelling is the main process that destroys the hypoxia. In winter, the oxygen content in the bottom layer of the Amur Bay is completely recovered by convective mixing and high primary production.