Sedimentary Records Of Hypoxia Status In The Red-tide Zone Of The East China Sea

Hypoxic zones in the coastal oceans have been spreading since the1960s,many of which were proved to be related with biological production processesdriven by coastal eutrophication. To the end of2012, more than400systems,including the sea areas adjacent to the Changjiang River estuary, were reported to behypoxic. So far researches on hypoxia in the East China Sea were mainly focused onthe sea area northeastern to the Changjiang River estuary, while little attention waspaid on the “red-tide zone” south to the estuary, where hypoxia was also found.More studies were demanded due to the lack of scientific recognition on thetemporal change of hypoxia in the “red-tide zone”.In the current study, a sediment core named2950was collected from thered-tide zone. Some redox sensitive elements (RSE), degree of pyrite (DOP) andgreen-sulfur bacteria related pigments were detected on the210Pb-dated sedimentcore. Status of dissolved oxygen (DO) in the bottom water of the “red-tide zone”over past decades was reconstructed. And the relationship between massive theHABs and variation of DO level of the bottom water was discussed.In the current study, it was found that Mo, V and U of sediment core2950shown obvious enrichment since the1990s, and the concentrations of Mo, V and Uincreased about70%,40%and30%, respectively. It was suggested that depositionalenvironment had become more reduced in the “red-tide zone”, which reflecteddecreased DO level in the bottom water. Values of DOP in sediment core2950were0.11~0.39, which were less than the critical value of hypoxia (0.46), suggesting thatno hypoxia status was recorded in the sediment core through DOP. The down-coredecrease of DOP values further confirmed this idea. Pigments in sediment core2950were analyzed with HPLC and no pigments related to green-sulfur bacteria (GSB) were detected. It was implied that the hypoxic water column in the studied area wasnot serious enough to reach anoxic condition.Based on these findings, it was suggested that DO level started to decreasesince the1990s in the red-tide zone of the East China Sea, which might beexacerbated by the large-scale red tides since then. At the same time, the emerginghypoxia promoted the accumulation of organic matter and led to theaccumulation oforganic matters in this area from the1990s. Expanding hypoxia could be expected inthe foreseeable future and it was nessesary to employ more indicators of hypoxia tomake a well depiction fo the hypoxia status and its long-term changes.