THE POSSIBLE ROLE OF THE CYCLOPOID COPEPOD OITHONA IN RETARDING VERTICAL FLUX OF ZOOPLANKTON FECAL MATERIAL

Cyclopoid copepods of the cosmopolitan genus Oithona are often the mos t abundant metazooplankton in both coastal and oceanic waters. Reasons for their ubiquity have been discussed but their feeding habits are n ot well known. Field observations and results from experiments present ed here suggest that Oithona is also coprophagous. Data collected alon g the eastern shelf of Svalbard (Spitsbergen, Norway) during ice retre at showed that Oithona was the most abundant metazooplankter and a neg ative correlation between calanoid faecal material (volume) in the wat er column and Oithona abundance was found, but there was no significan t relationship between calanoids and their faeces. In experiments in w hich calanoid copepods were confined together with Oithona, the latter always removed and presumably ingested a significant amount of calano id faecal material at rates that appeared independent of ambient food concentrations. About 20 to 30% of the calculated daily carbon require ments of Oithona could have been met from faecal matter. We speculate that Oithona uses chemical cues to find and intercept sinking faecal m atter in the water column and that its ability to use this ubiquitous food source efficiently is one of the reasons underlying its extraordi narily widespread occurrence. Although it has been shown that generall y only a small percentage of calanoid faeces produced in the surface l ayer contributes to vertical flux, little is known about the fate of t he 'missing' faeces. We argue that metazoans are mainly responsible fo r retention of faecal material in the surface layer and that cyclopoid copepods are an important part of this 'coprophagous filter' that ret ards vertical flux in the water column.