Phosphate addition to surface waters of the ultraoligotrophic, phosphorus-starved eastern Mediterranean in a Lagrangian experiment caused unexpected ecosystem responses. The system exhibited a decline in chlorophyll and an increase in bacterial production and copepod egg abundance. Although nitrogen and phosphorus colimitation hindered phytoplankton growth, phosphorous may have been transferred through the microbial food web to copepods via two, not mutually exclusive, pathways: (i) bypass of the phytoplankton compartment by phosphorus uptake in heterotrophic bacteria and (ii) tunnelling, whereby phosphate luxury consumption rapidly shifts the stoichiometric composition of copepod prey. Copepods may thus be coupled to lower trophic levels through interactions not usually considered.

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Because the downward flux of zooplankton faecal pellets (FP) measured in sediment traps is typically lower than FP production in overlaying waters, it is inferred that FP are recycled. One suggested explanation for low export fluxes of FP is retention through processes such as FP ingestion (coprophagy) and FP fragmentation (coprorhexy). It has become accepted that the globally important cyclopoid copepod Oithona spp. are one of the main organisms causing high retention of copepod FP, however, the experimental evidence supporting this hypothesis is limited. We conducted 3 experiments to test whether the raptorial feeding Oithona spp. removed sinking FP material produced by Calanus finmarchicus either by grazing or fragmentation. No such evidence was found. We therefore suggest that alternative explanations for pellet recycling should be sought. We believe that Oithona spp. may be indicator species for a 'retention ecosystem', but they are probably not the single factor explaining FP retention in the world oceans.

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