Project Summary

Ann Bucklin and Dennis J. McGillicuddy

U.S. GLOBEC: Maintenance of Pseudocalanus spp. populations on Georges Bank

The planktonic copepod sibling species, Pseudocalanus moultoni and P. newmani, occur sympatrically during the spring and summer on Georges Bank. Despite their marked morphological similarity, the species differ in many aspects of their ecology, including: patterns of distribution and abundance; preferred habitat; and seasonal timing of reproduction. In comparison to P. newmani, P. moultoni is more abundant on the well- mixed crest of Georges Bank within the 60 m isobath (Bucklin et al., 1998, Mar. Biol., in press). We hypothesize that populations of P. moultoni are maintained on Georges Bank by local reproduction and recruitment, and that the species is differentially retained within the tidal- mixing front located along the 60 m isobath by behavior (vertical distribution) in relation to physical processes (dynamics of the tidal- mixing front). This hypothesis will be tested by characterization of Bank- wide, stage-specific distribution and abundance of P. moultoni and P. newmani in relation to circulation patterns (based on Broad-scale Survey samples collected from January to June, 1999) and three-dimensional small-scale distributions in relation to advection and mixing in the tidal- mixing front (during field experiments proposed for May and June, 1999). The goal of this work is to understand how P. moultoni may persist and proliferate in the complex and variable flow field over Georges Bank. The technical approach will be to identify all life stages of P. moultoni using species-specific competitive PCR, and to perform numerical modeling experiments using observed distributions in a known physical field. A subsidiary goal is to evaluate molecular genetic (mtDNA sequences) and biochemical (differential gene expression) variation within P. moultoni, for possible use as indicators of rates of exchange between Georges Bank and adjacent populations. The significance of this project is to understand the interplay between population dynamics and ocean processes in determining the distribution and abundance of important members of the Georges Bank zooplankton assemblage.