US-GLOBEC: Cross-Frontal Processes and Recruitment Variabiliorges Bank -- Diet of Calanus Finmarchicus Copepodiduplii

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US-GLOBEC: Cross-Frontal Processes and Recruitment Variability on Georges Bank - Diet of Calanus finmarchicus Copepodids and Nauplii This program will examine the biological and physical processes controlling the distribution and abundance of marine animals in time and space, especially as related to cross-frontal processes. The specific focus of this proposal is to describe quantitatively the contribution of nano- and micro- plankton prey fields to recruitment of target copepod populations on the Bank. We hypothesize that copepods and nauplii located in frontal regions and on the well-mixed (on-Bank) side of fronts experience an enriched food environment relative to the stratified (off-Bank) side of fronts. We will first define the fronts by hydrographic surveys which include CTD/rosette sampling of the nano- and microplankton (i.e. plant and animal prey 2-200 um in size) prey field which our studies in Phases 1 and 2 of the GLOBEC Northwest Atlantic Program have demonstrated are heavily utilized by target copepods and larval fish. Exchange rates of these organisms will be evaluated and compared to exchange rates of conservative water column properties from the same hydrographic casts. We will evaluate prey fields and measure consumption of nano- and microplankton organisms by copepodid and feeding naupliar stages of the calanoid copepod Calanus finmarchicus in two cross-frontal areas which are characteristic of two different seasons: (1) the Northeast Peak Front, which exhibits its strongest surface thermal signal in February-March and (2) the Tidal Mixing Front which separates waters of the Bank crest from the southern flank during late spring and summer (late May-August). We focus on C finmarchicus because this species dominates the zooplankton biomass of Georges Bank during the January-June period. We emphasize fronts on the Northeast Peak and the Southem Flank because these are potential sites for cross-frontal transport of water, nutrients and plankton onto the Bank. We focus on the Northeast Peak because it is an area of enhanced production of both plankton and benthos relative to the rest of the Bank. And we compare zooplankton across the tidal mixing front between the Bank crest and the Southern Flank because U.S.-GLOBEC studies conducted during 1994-1996 indicate that, although C. finmarchicus nauplii are advected onto the Bank crest early in the season, strong cohorts do not subsequently develop, possibly as a result of intense predation pressure in this area. Ingestion rates of nauplii and copepodids will be measured directly in shipboard experiments using natural prey assemblages. The proximate influence of omnivorous feeding will be interpreted in the context of the in situ prey field and physical regime. Ultimately, the influence of plant versus animal prey in consumer diet will be addressed collaboratively in the context of measurements made by other investigators of consumer distribution, condition, life history parameters and production.