Linking foraging strategies of marine calanoid copepods to patterns of nitrogen stable isotope signatures in a mesocosm study

The foraging modes of calanoid copepods differ in that stationary suspension-feeding is more easily detected by prey with strong escape responses (ciliates) than is 'cruising' or 'ambushing' feeding. Thus, the ability of a copepod to include heterotrophic prey in its diet may be associated with its foraging mode and, further, with its nitrogen stable isotope signature (delta(15)N).

This is because a more carnivorous diet may be expected to result in a higher delta(15)N. We tested this hypothesis in a mesocosm study using a density gradient (0 to 80 ind. 1(-1)) of calanoid copepods. We expected copepod delta(15)N to generally increase with decreasing copepod density because of increased food availability, and predicted stronger increases for cruising than for stationary suspension-fee ding species.

As an assemblage, copepods had a pronounced impact on the food web: diatoms and ciliates decreased, whereas nanoflagellates increased with increasing copepod density. As expected, Centropages hamatus, a cruising species, showed the strongest isotopic increase and also highest population growth at low copepod density, suggesting that it was the most efficient species in capturing ciliates.

Temora longicornis, a stationary suspension-feeder, showed a uniform isotopic increase in all mesocosms, which we believe resulted from nutritional stress arising from poor feeding on both ciliates (too fast for ingestion by T. longicornis) and nanoflagellates (too small). However, Pseudocalanus elongatus, a species equally categorised as a stationary suspension-feeder, showed increases in its delta(15)N similar to those for C. hamatus.

While this may indicate potential switching in its foraging mode, alternative explanations cannot be ruled out, partly because qualitative and quantitative aspects of trophic enrichment in our experiment could not be clearly separated. This study shows that consumer delta(15)N are difficult to interpret, even if potential food sources and aspects of the species' biology are known, and thus emphasises the necessity for further laboratory studies to help better interpret zooplankton delta(15)N in the field.