Copepod omnivory in the North Water Polynya (Baffin Bay) during autumn: spatial patterns in lipid composition

To deduce spatial patterns in copepod lipid composition and feeding strategy (i.e., degree of omnivory) in the North Water Polynya (Baffin Bay), three dominant species were sampled extensively over a broad geographical area (∼75–78°N; 77–69°W). Calanus hyperboreus CV, C. glacialis CV and Metridia longa females were collected in shallow and deep strata at 16 stations during autumn 1999 (August–October). Principal components analysis (PCA) revealed that all species fed omnivorously in the southeastern (SE) region of the polynya. Here, copepods generally had elevated levels of carnivorous (e.g., 18 : 1 ( n - 9 ) ), dinoflagellate (e.g., 18 : 4 ( n - 3 ) ; 22 : 6 ( n - 3 ) ) and bacterial fatty acid markers (e.g., odd-numbered and/or branched; 18:1( n - 7 )). Copepods in the SE contained low proportions of diatom (e.g., 16 : 4 ( n - 1 ) ; 20 : 5 ( n - 3 ) ) and phytoplankton (e.g., polyunsaturated fatty acids) markers, relative to animals from northwest stations. Values of the omnivory index ‘UC’ (i.e., unsaturation coefficient) were also low in SE copepods, which implied reduced phytoplankton ingestion. Spatial patterns in seston fatty acid composition resembled the dietary signatures in that dinoflagellate and bacterial indices were highest in SE waters. Estimates of primary production, particulate organic carbon, carbon to chlorophyll ratios, and abundances of diatoms, dinoflagellates and bacteria, provided further evidence of the importance of the microbial loop at SE stations. Comparable spatial patterns in feeding strategy were observed in both sampling layers, indicating that copepods from the entire water column were feeding on a similar food source. Several interesting species-specific trends also emerged from the PCA. In general, C. hyperboreus fed the most herbivorously, followed by C. glacialis and M. longa. C. glacialis showed a stronger connection to the microbial food web than the other two species, and M. longa fed herbivorously throughout much of the polynya. These latter two findings contradict the conventional wisdom and highlight the need for future work. In particular, the microbial community should be emphasized and characterized in subsequent studies on the feeding ecology of C. glacialis. Although the three species investigated responded quite differently to available prey by adopting specialized feeding strategies, copepod omnivory in the polynya was generally inversely related to the availability of diatoms.