Maintenance of Pseudocalanus spp. on Georges Bank

A. Bucklin and D. J. McGillicuddy

The planktonic copepod sibling species, Pseudocalanus moultoni and P. newmani (Crustacea, Copepoda), are abundant in waters over Georges Bank from late winter until mid-summer and are thought to reproduce throughout this period. The two species cannot be reliably distinguished using morphological characters, but are readily identified and distinguished by simple, rapid, and inexpensive molecular protocols based on sequence variation of mitochondrial DNA (mtDNA). DNA sequence variation of a portion of the mitochondrial cytochrome oxidase I (mtCOI) confirmed the presence of P. moultoni and P. newmani on Georges Bank; the mtCOI sequences were used to design species-specific oligonucleotide primers for use in a competitive multiplexed species-specific Polymerase Chain Reaction (PCR). Species-specific PCR was used to determine the relative abundances of the two species in sub-samples of zooplankton collections from U.S. GLOBEC Georges Bank Study Broad-scale Surveys from February to June, 1997 and January to June, 1999. Based on monthly visualizations, we inferred the spring-summer evolution of the two species' distributions and abundances on Georges Bank. During both years, both species' overall abundances increased during these months. The two species differed in that the peak in distribution of P. moultoni was most frequently on the crest of the Bank, while P. newmani was more abundant to the south and east of the Bank. The spring-summer increase of P. moultoni may result from the persistence of reproducing individuals over the Bank and/or from advective transport from adjacent regions. In contrast, P. newmani may be transported to Georges Bank from upstream populations on the Scotian Shelf and Browns Bank. The processes responsible for the observed patterns cannot be determined from this series of monthly snap-shots alone; ongoing studies use numerical models to examine the biological and physical dynamics causing these distributions.