Biogeochemical parameters of sea water and bottom sediments near the Curieuse Island (Seychelles Islands)

Biogeochemical reef studies carried out in 1981 and 1984 found low concentration of total natural and anthropogenic hydrocarbons in inshore waters. Detection of lignin in marine and bottom sediments indicates that the land has major effect on makeup of organic matter there. Comparison of compositions of organic matter in sea water, suspended matter and bottom sediments indicated that it was altered rapidly by the reef community. Thus, in the inshore zone of the island, runoff from the land is important in supplying nutrients to the reef ecosystem alongside with transport of nutrients by deep waters. Concentrations of nutri¬ents (N, P) in the inshore zone are higher than in waters of the tropical part of the ocean. Nitrogen is the limiting element in development of phytoplankton in the inshore zone.

(Table 1) Lithological and organic-geochemical compositions of bottom sediments from the Southwestern Indian Ocean (Mozambique Basin)

Geochemical changes in organic matter of bottom sediments from the Mozambique Basin at the river-sea barrier from the mouths of the Zambezi and Limpopo rivers toward the pelagic zone are discussed. Changes in bitumen, hydrolyzable material, humic acids, amino acids, n-alkanes, and polycyclic aromatic compounds resulting from genetic and diagenetic factors are described. This information is significant for paleoceanology reconstructions and for knowing ways of organic matter transformation into fossil forms.

Carbon dioxide assimilation rate, gas and ion composition in samples, and bacterial production of organic matter within and above the Broken Spur and TAG hydrothermal fields, Mid-Atlantic Ridge

Rate of CO2 assimilation was determined above the Broken Spur and TAG active hydrothermal fields for three main ecosystems: (1) hydrothermal vents; (2) 300 m near-bottom layer of plume water; and (3) bottom sediments. In water samples from warm (40-45°C) vents assimilation rates were maximal and reached 2.82-3.76 µg C/l/day. In plume waters CO2 assimilation rates ranged from 0.38 to 0.65 µg C/l/day. In bottom sediments CO2 assimilation rates varied from 0.8 to 28.0 µg C/l/day, rising up to 56 mg C/kg/day near shrimp swarms. In the most active plume zone of the long-living TAG field bacterial production of organic matter (OM) from carbonic is up to 170 mg C/m**2/day); production of autotrophic process of bacterial chemosynthesis reaches about 90% (156 mg C/m**2/day). Thus, chemosynthetic production of OM in September-October is almost equal to that of photosynthetic production in the oceanic region. Bacterial production of OM above the Broken Spur hydrothermal field is one order lower and reaches only 20 mg C/m**2/day.