BIOGEOCHEMICAL SIGNIFICANCE OF ATTACHED AND FREE-LIVING BACTERIA AND THE FLUX OF PARTICLES IN THE NE ATLANTIC-OCEAN

In the NE Atlantic during May 1990, a period of high aggregate abundan ce, amorphous aggregates contained substantially higher concentrations of bacteria, cyanobacteria and flagellates than the surrounding seawa ter. Those from 45 to 55 m, at the aggregate maximum just below the se asonal thermocline, contained 2.1 to 25.4 x 10(8) bacteria, 1.0 to 4.7 x 10(7) cyanobacteria and 1.3 to 33.0 x 10(6) flagellates ml-l aggreg ate. Leucine incorporation rates by bacteria attached to aggregates ra nged from 12 to 206 x 10(-21) mol cell(-1) d(-1). The concentration of bacteria associated with faecal pellets was generally higher than the concentration on the amorphous aggregates. Bacteria attached to aggre gates were equivalent to 10 and 14% of free-living bacterial carbon in tegrated through the water column to 100 and 300 m, respectively. This could rise to 25 and 34% if the different carbon content of attached and free-living bacteria was taken into account. However, this study c oincided with the maximum marine snow concentration measured 150 km so uthwest, so at other times when marine snow concentrations are lower, the proportion of attached bacteria will be less. The contribution of attached bacteria to total bacterial production in the top 100 and 300 m ranges between 1.8 and 3.4%. The contribution of free-living bacter ial carbon to suspended POC (particulate organic carbon) was between 2 5 and 33%, and after correcting for their retention on the glass fibre filters, this contribution could be 28 to 40%. Flux studies during 19 89 and 1990 indicated that a smaller proportion of POC flux (9%) and b acterial carbon flux (10%) reached 3100 m than mass flux (25 and 35%), indicating there are processes involved which preferentially utilise or reduce the POC and bacterial components. Bacterial detachment from sinking particles could contribute 2.4% of the integrated bacterial bi omass per annum. The fraction of sinking POC lost between 150 and 3100 m may be an important carbon source to the mid/deep-water bacterial p opulation, capable of supplying around 90% of the bacterial carbon dem and during April to July 1989 at 47 degrees N, 20 degrees W. In the de eper waters, between 600 and 3100 m, there was sufficient depth-dissip ated sinking POC to potentially supply the carbon demand of the bacter ia. However, above 600 m an additional source of organic carbon is req uired to support their growth.