TRANSFORMATIONS OF BIOGENIC PARTICLES DURING SEDIMENTATION IN THE NORTHEASTERN ATLANTIC

The vertical flux and transformation of biogenic particles are importa nt processes in the oceanic carbon cycle. Changes in the magnitude of the biological pump can occur in the north eastern Atlantic on both a seasonal and interannual basis. For example, seasonal Variations in ve rtical flux at 47 degrees N 20 degrees W are linked to seasonal ocean productivity variations such as the spring bloom. The size and organic and inorganic content of phytoplankton species, their development and succession also play a role in the scale and composition of the biolo gical pump. The majority of flux is in the form of fast sinking aggreg ates. Bacteria and transparent exopolymer particle production by phyto plankton have been implicated in aggregate production and mass flux ev ents. Zooplankton grazing and faecal pellet production, their size and composition and extent of their vertical migration also influence the magnitude of vertical flux. Aggregates are formed in the upper ocean, often reaching a maximum concentration just below the seasonal thermo cline and can be a food resource to mesozooplankton as well as to the high concentrations of attached bacteria and protozoa. Attached bacter ia remineralize and solubilize the aggregate particulate organic carbo n. The degree of particle solubilization is likely to be affected by f actors controlling enzyme activity and production, for example tempera ture, pressure or concentration of specific organic molecules, all of which may change during sinking. Attached bacterial growth is greatest on particulate organic matter collected at 500 m which is the depth w here studies of(210)po reveal that there is greatest break-up of rapid ly sinking particles. Break-up of particles by feeding zooplankton can also occur. The fraction of sinking poc lost between 150-3100 m at on e station in the north eastern Atlantic could supply about 90 % of the bacterial carbon demand. Some larger, faster sinking aggregates escap e solubilization and disaggregation in the upper 1000 m and arrive in the deep ocean and on the deep-sea bed. Seasonally varying rates of se dimentation are reflected at the deep-sea floor by deposition of phyto detrital material in summer. Approximately 2-4 % of surface water prim ary production reaches the sea floor in 4500 m depth at 47 degrees N 2 0 degrees W after a sedimentation time of about 4-6 weeks. In this reg ion, concentrations of chloroplastic pigments increased in summer by a n order of magnitude, whereas seasonal changes in activity or biomass parameters were smaller. Breakdown of the generally strongly degraded organic matter deposited on deep-sea sediments is mainly accomplished by bacteria. Rates of degradation and efficiency of biomass production depend largely on the proportion of biologically labile material whic h decreases with advancing decay. It is likely that different levels o f organic matter deposition influence the bioturbation rates of larger benthos, which has an effect on transport processes within the sedime nt and presumably also on microbial degradation rates.