INTERANNUAL VARIATIONS IN ZOOPLANKTON BIOMASS IN THE GULF OF ALASKA , AND COVARIATION WITH CALIFORNIA CURRENT ZOOPLANKTON BIOMASS

Large-scale atmospheric and oceanographic conditions affect the productivity of oceanic ecosystems both locally and at some distance froin the forcing mechanism. Recent studies have suggested that both the Subarctic Domain of the North Pacific Ocean and the California Current have undergone dramatic changes in zooplankton biomass that appear to be inversely related to each other. Using time series and correlation analyses, we characterized the historical nature of zooplankton biomass at Ocean Station P (50"N, 145"W) and froiii offshore stations in the CalCOFI region. We found a statistically significant but weak negative relationship between the domains. We investigated whether such a relationship arises froiii different forcing iiiechanisriis or as an opposite response to the same niechanisni. We found that the seasonal peak of both data sets occurred in the suniiiier but that the CalCOFI data lagged the Ocean Station P data. A surface-drifi simulation model showed that winter trajectories started at Ocean Station P and along 145"W drifted niore into the California Current before the 1976-77 regime shift, and iiiore into the Alaska Current after the 1976-77 shift. We exaniiiied physical and biological conditions which niay lead to this inverse relationship between the two ecosystems, and we discuss the implications of these results for higher trophic levels. INTRODUCTION Substantial temporal and spatial heterogeneity occurs in the production of oceanic ecosystems. Much of this heterogeneity results froiii seasonal and geographic variations in nutrient availability, mixed-layer depths, or solar radiation. Processes that enhance productivity (e.g., upwelling, wind and tidal mixing) teiid to be localized and transient. Physical forcing in the form of the largescale circulation pattern redistributes the elevated production to areas less favorable for in situ production. Thus production at any location can be affected by both local and remote processes, and it is often difficult to distinguish their relative contributions (Wickett 1967; Chelton et al. 1982; Roessler aiid Chelton 1987). I t has become increasingly apparent that atmospheric and oceanic conditions are likely to change due to a buildup of greenhouse gases in the atmosphere (Graham 1995). Although there has been much interest in predicting the effects of climate change, especially on fisheries resources (e.g., see papers in Beaniish 1995), different scenarios exist for future trends in basic physical processes such as upwelling (Bakun 1990; Hsieh and Boer 1992). Biological processes are niore laborious to monitor and difficult to predict because of their inherent complexity. There are iiuiiierous examples showing that largescale physical and biological changes have occurred throughout much of the Northeast Pacific Ocean over the last few decades (Francis and Hare 1994; Miller et al. 1994). Indices which showed these changes include atmospheric (Ti-e-enberth 1990; Trenberth and Hurrell 1994), oceanographic (Royer 1989; Hsieh and Boer 1992; Miller et al. 1994; Lagerloef, 1995; Polovina et al. 1995), productivity (Venrick et al. 1987; Polovina et al. 1994), and biomass of various trophic levels (Brodeur and Ware 1992, 1995; McFarlane and Beaniish 1992; Beaniish 1994; Hare aiid Francis 3 995; Roeiniiiich aiid McChvan 1995a, b). A nuiiiber of studies have suggested that biological changes occurred rather suddenly sometinie arouiid 1976-77, coi~c~rreii t with a draniatic shift in physical regimes (Francis and Hare 1994; Miller Documenting the efixts of climate change on niarine ecosystems requires long time series of saiiipling to examine low-frequency periodicity. 111 the Northeast Pacific Ocean, two series are notable not only for their length, but also for the broad suite of biological and physical ~iieasureiiieiits made at each location. The first of these, Ocean Station P (50"N, 14.5'W; hereafter called Station P) , iiomiiially represents a subarctic oceanic ecosystem that was cain pled almost continuously froiii 1936 to 1980 but only sporadically since then. The second, the CalCOFI grid, is an eastern boundary current et al. 1994).