The possible role of the cyclopoid copepod Oithona in retarding vertical flux of zooplankton faecal material

Abstract

Cyclopoid copepods of the cosmopolitan genus Oifhona are often the most abundant metazooplankton in both coastal and oceanic waters. Reasons for their ubiquity have been discussed but their feeding habits are not well known. Field observations and results from experiments presented here suggest that Oithona is also coprophagous. Data collected along the eastern shelf of Svalbard (Spltsbergen, Norway) during ice retreat showed that O~thona was the most abundant metazooplankter and a negative correlation between calanoid faecal material (volume) in the water column and Oithona abundance was found, but there was no significant relationship between calano~ds and their faeces. In experiments in which calanoid copepods were confined together with Oithona, the latter always removed and presumably ingested a significant amount of calanoid faecal material at rates that appeared independent of ambient food concentrations. About 20 to 30% of the calculated daily carbon requirements of Oithona could have been met from faecal matter. We speculate that Oithona uses chemical cues to find and intercept sinking faecal matter in the water column and that its ability to use this ubiquitous food source efficiently is one of the reasons underlying its extraordinarily widespread occurrence. Although it has been shown that generally only a small percentage of calanoid faeces produced in the surface layer contributes to vertical flux, llttle is known about the fate of the 'missing' faeces We argue that metazoans are mainly responsible for retention of faecal material in the surface layer and that cyclopoid copepods are an important part of this 'coprophagous fllter' that retards vertical flux in the water column.