Effects of biogeochemical processes on spatial distribution of iron, manganese, reactive phosphates and nitrates in the Black Sea

摘要

The aim of this study was to identify differences in the distribution of Mn (Manganese), Fe (Iron), NO3- (Nitrates) and reactive phosphates (RP) in the water columns of the eastern and western basin of the Black Sea region. Accordingly, the selected sampling area covered offshore waters from west to east including Istanbul (station KD01), Eregli (KD2A, 2B, and 2C), Inebolu (KD03), Sinop (KD04), Ordu (KD5A, 5B, 5C) and Trabzon (KD06). The B and C codes for stations 2 and 5 indicate that the stations are towards the basin centers of the Black Sea. At all the stations of the study area, the Mn values were below the limit of detection (5 mu g/L) up to 100 m depth. The Mn concentration reached maximum at 200 m depth of stations KD2B, KD5B, KD5C and KD06. This situation could be due to mobilization of Mn from the continental slope sediments (Mn pump). Also, it was observed that relatively high values of nitrates were associated with low RP concentration in the deep waters of the eastern basin. If nitrate is present in the suboxic zone, bacteria prefers nitrate and reduce it as N-2 (g). Then, the FeOOH=RP (ferric oxyhydroxides equivalent to reactive phosphate) particulates sink into deeper layers without reduction by bacteria and cause the upper layers to become poor in phosphates. In case of low values of nitrate in suboxic zone, FeOOH equivalent to RP particulates are reduced to dissolved Fe+2 by the bacteria. Significantly higher levels of nitrate concentrations were observed at station KD06. In conclusion, the eastern basin center is characterized by relatively higher nitrate concentrations in water columns compared to those in the western basin center. Therefore, the eastern basin contains lower RP concentrations and this limits the phytoplankton growth. It is concluded that where nitrate concentrations are higher than 2 mu M, the bacteria preferred to reduce the nitrates. On the other hand, particulate Fe can be reduced chemically with H2S in the deeper layers, where there is lower nitrate concentration.