• Lipid accumulation in three species of Neocalanus copepod in the northern Gulf of Alaska

      Coleman, Delaney M.; Hopcroft, Russell; Danielson, Seth; Hennon, Gwenn (2022-05)
      The Northern Gulf of Alaska experiences pronounced seasonality and inter-annual variability characterized by a significant bloom of phytoplankton in the spring. Neocalanus copepods in the NGA have evolved to match their lifecycle to the seasonality of the Gulf of Alaska and feed upon the spring phytoplankton bloom. All three of these Neocalanus species utilize diapause as an over-wintering strategy; acquiring large stores of lipid to sustain them through winter hibernation and subsequent reproduction. Zooplankton were sampled with 150 and 505 µm mesh nets from 0 to 1200 m along the Seward Line and within Prince William Sound in the Northern Gulf of Alaska during 2018-2020 to track the physiological process of Neocalanus copepods preparing for diapause. We measured lipid sac area, lipid volume and percent lipid to quantify lipid content. Neocalanus showed significant interannual variability in final lipid accumulation both at depth and in the surface during the study period. For all three species, lipid content increased with increasing stage and prosome length. Lipid content increased from spring to summer for N. flemingeri, remaining steady into fall as animals molted into adults and descended to depth for diapause. Neocalanus plumcrhus stored lipid from spring to summer before descending slightly after N. flemingeri. Neocalanus cristatus exhibited dissimilar behavior to the other two species, storing consistently low amounts of lipid, alluding to a different lifecycle. Each Neocalanus species displayed similar lipid accumulation behavior with offset timing from one another. Neocalanus exhibits an earlier developmental timing as compared to other lipid accumulating copepods giving them a competitive advantage to reach maturity in time to feed on the early phase of the spring phytoplankton bloom faster than other species. Our data provided some evidence for both the lipid accumulation hypothesis and the developmental program hypothesis being utilized in Neocalanus populations in the Northern Gulf of Alaska. This work serves as the first detailed study of body condition and lipid sac condition in Neocalanus populations throughout the water column within the Northern Gulf of Alaska.