MARINE SNOW STUDIES IN THE NORTHEAST ATLANTIC-OCEAN - DISTRIBUTION, COMPOSITION AND ROLE AS A FOOD SOURCE FOR MIGRATING PLANKTON

During a 25 d Lagrangian study in May and June 1990 in the Northeast A tlantic Ocean, marine snow aggregates were collected using a novel wat er bottle, and the composition was determined microscopically. The agg regates contained a characteristic signature of a matrix of bacteria, cyanobacteria and autotrophic picoplankton with inter alia inclusions of the tintiniid Dictyocysta elegans and large pennate diatoms. The co ncentration of bacteria and cyanobacteria was much greater on the aggr egates than when free-living by factors of 100 to 6000 and 3000 to 250 0000, respectively, depending on depth. Various species of crustacean plankton and micronekton were collected, and the faecal pellets produc ed after capture were examined. These often contained the marine snow signature, indicating that these organisms had been consuming marine s now. In some cases, marine snow material appeared to dominate the diet . This implies a food-chain short cut whereby material, normally too s mall to be consumed by the mesozooplankton, and considered to constitu te the diet of the microplankton can become part of the diet of organi sms higher in the food-chain. The micronekton was dominated by the amp hipod Themisto compressa, whose pellets also contained the marine snow signature. Shipboard incubation experiments with this species indicat ed that (1) it does consume marine snow, and (2) its gut-passage time is sufficiently long for material it has eaten in the upper water to b e defecated at its day-time depth of several hundred meters. Plankton and micronekton were collected with nets to examine their vertical dis tribution and diel migration and to put into context the significance of the flux of material in the guts of migrants. ''Gut flux'' for the T compressa population was calculated to be up to 2% of the flux measu red simultaneously by drifting sediment traps and < 5% when all migran ts are considered. The in situ abundance and distribution of marine sn ow aggregates (> 0.6 mm) was examined photographically. A sharp concen tration peak was usually encountered in the depth range 40 to 80 m whi ch was not associated with peaks of in situ fluorescence or attenuatio n but was just below or at the base of the upper mixed layer. The feed ing behaviour of zooplankton and nekton may influence these concentrat ion gradients to a considerable extent, and hence affect the flux due to passive settling of marine snow aggregates.