Plankton populations at the bifurcation of the North Pacific Current

ABSTRACTAs the eastward-ﬂowing North Paciﬁc Currentapproaches the North American continent it bifur-cates into the southward-ﬂowing California Currentand the northward-ﬂowing Alaska Current. Thisbifurcation occurs in the south-eastern Gulf ofAlaska and can vary in position. Dynamic heightdata from Project Argo ﬂoats have recently enabledthe creation of surface circulation maps which showthe likely position of the bifurcation; during 2002 itwas relatively far north at 53 N then, during early2003, it moved southwards to a more normal posi-tion at 45 N. Two ship-of-opportunity transectscollecting plankton samples with a ContinuousPlankton Recorder across the Gulf of Alaska weresampled seasonally during 2002 and 2003. Theirposition was dependent on the commercial ship’soperations; however, most transects sampled acrossthe bifurcation. We show that the oceanic planktondiffered in community composition according to thecurrent system they occurred in during spring andfall of 2002 and 2003, although winter populationswere more mixed. Displacement of the planktoncommunities could have impacts on the plankton’sreproduction and development if they use cues suchas day length, and also on foraging of highertrophic-level organisms that use particular regions ofthe ocean if the nutritional value of the communi-ties is different. Although we identify some indicatortaxa for the Alaska and California currents, func-tional differences in the plankton communities oneither side of the bifurcation need to be betterestablished to determine the impacts of bifurcationmovement on the ecosystems of the north-eastPaciﬁc. Key words: Alaska Current, California Current,Continuous Plankton Recorder, Gulf of Alaska,North Paciﬁc Current, north-east Paciﬁc, plankton,Project ArgoINTRODUCTIONAs the eastward-ﬂowing North Paciﬁc Current(NPC) approaches the North American continent itbifurcates into the southward-ﬂowing CaliforniaCurrent (CC) and the northward-ﬂowing AlaskaCurrent (AC) which then forms the Alaska Gyre.This bifurcation occurs in the south-eastern Gulf ofAlaska (GoA) and can vary in position, although itsusual latitude is at about 45 N. Dynamic height datafrom Argo ﬂoats have recently enabled the creationof surface circulation maps of the Gulf of Alaskawhich show the likely position of the bifurcation andits interannual variation (Freeland and Cummins,2005). The lower trophic level characteristics of the CCand AC along the continental slope and on theshelf have been relatively well described (Morganet al., 2003; Mackas and Coyle, 2005; Zamon andWelch, 2005 and references therein); however, thedistributions of organisms related to the offshoreorigins of these currents is less well known andforms the focus of this paper. Two ship-of-oppor-tunity transects collecting plankton samples with aContinuous Plankton Recorder (CPR) across theGoA were sampled seasonally during 2002 and 2003.The position of these transects was dependent onthe commercial ship’s operations; however mosttransects sampled both offshore CC and AC waters,according to the Argo-derived circulation plots. Weexamined the plankton data to determine whetherpopulations were different in the two currents andassessed the effects of the bifurcation position on thedistribution of planktonic organisms in the oceanicGoA.