Species-specific differences in lipid composition and omnivory indices in Arctic copepods collected in deep water during autumn (North Water Polynya)

To discern species-specific patterns in omnivory indices in copepods from the North Water Polynya, lipid classes and fatty acids were determined in Calanus hyperboreus Krøyer copepodite stage 5 (CV), C. glacialis Jaschnov (CV) and Metridia longa Lubbock (females) sampled below the mixed layer during autumn 1999. Generally, M. longa contained higher proportions of triacylglycerols, polar lipids and 18:1(n-9) than the other species. M. longa also had lower relative amounts of wax esters (WE), polyunsaturated fatty acids (PUFA), 20:1(n-9), 22:1(n-11), and lower absolute (g lipid copepod^-1) and relative (% lipid) lipid levels. Unsaturation coefficients (UC; ratio of polyunsaturated to total WE) were usually lowest in M. longa. These differences probably relate to a lesser dependence on phytoplankton in M. longa, and hence more ingestion of PUFA-poor prey. Moreover, levels of the carnivory index 18:1(n-9)/18:1(n-7) were highest in M. longa. Our data support the widespread contention that M. longa is more omnivorous than C. hyperboreus and C. glacialis. Proportions of bacterial fatty acids [odd-numbered and/or branched (OBFA); 18:1(n-7)] and PUFA/SFA (SFA: saturated fatty acids) ratios were largely unrelated to feeding strategy. Relationships between relative and absolute amounts of 16:1(n-7) and 18:1(n-7) in copepods suggest that the latter fatty acid is formed in vivo by chain elongation of the former. However, elevated levels of 18:1(n-7) and OBFA in M. longa at stations dominated by the microbial loop imply that these indices can be used to track ingestion of PUFA-poor protozoans when diatoms are scarce.Communicated by J.P. Grassle, New Brunswick