Date

2014-07-01

Author

Tuğrul, Süleyman
Murray, James W.
Friederich, Gernot E.
Salihoglu, Ilkay

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The Black Sea, a land-locked deep basin with sulfide bearing waters below 150-200 m, has been subject to anthropogenic pressures since the 1970s. Large inputs of nutrients (nitrate - N, phosphate - P, silicate - Si) with high N/P but low Si/N ratios and subsequent development of intensive eutrophication over the basin have changed vertical distributions and inventories of nutrients and redox-sensitive metals in the oxic, suboxic and anoxic layers. Chemical data sets obtained between 1988 and 2010, and older data from before 1970 were evaluated to assess spatial/temporal variations of the dissolved oxygen (O-2), nutrients and dissolved/particulate manganese (Mn-d, Mn-p) in the water column from the lower salinity, oxygenated surface waters through the SubOxic Layer (SOL; O-2 50). The surface waters over the basin were rich in silicate (25-70 mu M), but poor in nitrate (500) but very low N/P (<1.0) ratios. After the mid 1970s, construction of dams, especially on the Danube River, resulted in lower Si concentrations. At this time the increased loads of anthropogenic nitrate and phosphate by the major rivers resulted in lower Si/N, but still high N/P molar ratios, which enhanced eutrophication (production of particulate organic matter, POM) drastically in the coastal waters. This led to reductions in the surface Si/N ratio by up to 500-fold in the western basin while the N/P ratio increased. The enhanced POM export increased the nitrate inventory and thus N/P ratios of the NW shelf waters spreading over the whole basin. The increased export of POM decreased the Si inventory of the upper layer down to the boundary of sulfidic waters. This export also increased O-2 consumption and removal of nitrate to N-2 form by denitrification in the oxic/suboxic interface, leading to seasonal/decadal changes in the boundaries of the nitracline and main oxycline and changes in the slopes of the nitrate-phosphate and Apparent Oxygen Utilization (AOU)-nitrate regressions in the steep oxycline down to the SOL. These slopes are much smaller than those observed in the lower layer of Marmara Sea fed by the Black Sea outflow. The enlargement of SOL by similar to 15-20 m after the 1970s modified the vertical features of nitrate, phosphate and manganese (Mn-d, Mn-p) species in the redox gradient zone.

Subject Keywords

Aquatic Science, Ecology, Evolution, Behavior and Systematics, Oceanography

URI

https://hdl.handle.net/11511/56830

Collections

Graduate School of Marine Sciences, Article