Small copepods add zooplankton to their diet: consequences for pelagic functioning, trophic webs and carbon fluxes

Small copepods ( [ view full abstract ]

Small copepods (<2 mm) are the most abundant metazoans in the world’s oceans, a keystone in pelagic food webs. Small copepods are considered to be omnivorous, feeding on unicellular protists and phytoplankton, with carnivorous feeding –i.e. preying on zooplankton- restricted to larger copepods and cyclopoid copepods. We developed a PCR based technique that detected a diverse range of zooplankton species (eight families of decapods, bivalves, fish and also other copepods) within the digestive tract of copepods traditionally considered to be “herbivorous”. This finding provides evidences that a significant component of copepod diets has been overlooked owed to the methodologies traditionally been carried out –visual inspection and pigments analysis. Such predatory behaviour extends the trophic role of small copepods in upwelling food webs, affecting the mesozooplankton communities through intraguild predation. Furthermore, this study helps to explain the discrepancies found between quantified phyto/microzooplankton ingestion and metabolic demands of copepods.
In order to infer the consequences of this overlooked predatory behaviour to the flux of Carbon in the whole pelagic ecosystem, we screened the literature to obtain an estimate of weight specific ingestion rates (WSIR) based on their averaged weight for adult calanoid (n=121) and cyclopoid copepods (n=41), in the field (n=125) and under laboratory conditions (n=37). By multiplying the estimated WSIR by the standing stock of adult copepods in the upper 100 m of the ocean -assuming that i) copepods are approximately 80% of the total zooplankton biomass ii) only 50% are adults and iii) calanoids comprise between 30 to 40% of total copepods (the rest being cyclopoid copepods) - our carbon budget estimates show that on a global scale, copepods process, between 3.41 - 3.70 up to 20.75 - 21.87 gigaton (Gt) C yr-1 through zooplankton predation based on field and laboratory measured WSIR, respectively. This conservative field estimate reveals that copepods are channelling 31.28 - 33.95% more carbon towards upper trophic levels than previous estimates focused on phytoplankton and microzooplankton consumption (10.9 Gt C yr-1). Hence, copepod intraguild predation should definitely be considered in oceanic biogeochemical cycles and ecosystem models.