ALBERT

Notes: Abstract Copepod faecal pellets have often been considered as rapid transporters of material out of the euphotic zone. Laboratory experiments on their degradation and sinking rates support this view, but field data on the distribution and flux of pellets through the water colomn present contradictory evidence. We suggest that due to the exclusion of metazoans from previously published degradation experiments, such studies may have little relevance to the natural environments. In 1987/1988 we carried out experiments using adult copepods of mixed species but dominated byCentropages hamatus collected in Kiel Bight (FRG). We have demonstrated that copepods can be highly adept at breaking up their own pellets while ingesting only a small proportion, a behaviour we define as “coprorhexy”. The microbiota is probably unable to cause significant modification to faecal pellets before they are fragmented within a few hours of their production. Thereafter, microbial remineralisation will become important. Many of the “difficult” field data can be readily explained if the process of coprorhexy is taken into account and, indeed, breakage of large particles by crustacean zooplankton may be an important process in modifying material transport in the ocean. Copepods appear to perform coprorhexy by removing the peritrophic membrane with its attached bacterial flora and this may then be ingested. We speculate on the nutritional value of such a behaviour and the possible significance of “ghost” pellets, consisting of a membrane with little or no apparent solid content.

Notes: Abstract During a 25 d Lagrangian study in May and June 1990 in the Northeast Atlantic Ocean, marine snow aggregates were collected using a novel water bottle, and the composition was determined microscopically. The aggregates 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 concentration of bacteria and cyanobacteria was much greater on the aggregates than when free-living by factors of 100 to 6000 and 3000 to 2 500 000, respectively, depending on depth. Various species of crustacean plankton and micronekton were collected, and the faecal pellets produced after capture were examined. These often contained the marine snow signature, indicating that these organisms had been consuming marine snow. In some cases, marine snow material appeared to dominate the diet. This implies a food-chain short cut wherby material, normally too small to be consumed by the mesozooplankton, and considered to constitute the diet of the microplankton can become part of the diet of organisms higher in the food-chain. The micronekton was dominated by the amphipod Themisto compressa, whose pellets also contained the marine snow signature. Shipboard incubation experiments with this species indicated 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 be defecated at its day-time depth of several hundred meters. Plankton and micronekton were collected with nets to examine their vertical distribution 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 measured simultaneously by drifting sediment traps and 〈5% when all migrants are considered. The in situ abundance and distribution of marine snow aggregates (〉0.6 mm) was examined photographically. A sharp concentration peak was usually encountered in the depth range 40 to 80 m which was not associated with peaks of in situ fluorescence or attenuation but was just below or at the base of the upper mixed layer. The feeding behaviour of zooplankton and nekton may influence these concentration gradients to a considerable extent, and hence affect the flux due to passive settling of marine snow aggregates.