CO2-ASSIMILATION RATE AND BACTERIAL PRODU CTION OF ORGANIC-MATTER ABOVE HYDROTHERMAL FIELDS AT 26-DEGREES-N AND 29-DEGREES-N, MID-ATLANTIC RIDGE

The rate of (CO2)-C-14-assimilation was estimated above the Broken Spu r and TAG active hydrothermal fields in 3 main locations: (1) near the openings of black smokers; (2) in the 300-m near bottom water layer p lume; (3) bottom sediments. In the water samples from warm (40-45 degr ees C) vents the assimilation rates were maximal and reached 2.82-3.76 mkgC/l day. In the plume waters, CO2-assimilation rates ranged from 0 .38 to 0.65 mkgC/l day. In the bottom sediments, CO2-assimilation rate s ranged from 0.8 to 28.0 mkgC/kg day, increasing near the shrimp aggr egations to 56 mgV/kg day. In the most active plume zone of the long-l iving TAG field, bacterial production of organic matter from carbon di oxide was as high as 170 mgC/m(2) day, the share of autotrophic proces s, bacterial chemosynthesis, reaching 90% (156 mgC/m(2) day). Thus, th e value of the organic matter chemosynthetic production in September-O ctober is almost equal to the photosynthetic production in this oceani c region. Bacterial production of organic matte above the Broken Spur hydrothermal field is one order lower and reaches only 20 mgC/m(2) day .