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INTERANNUAL VARIATIONS in ZOOPLANKTON BIOMASS in the GULF of ALASKA, and COVARIATION with CALIFORNIA CURRENT ZOOPLANKTON BIOMASS Iiichaiw I)

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INTERANNUAL VARIATIONS in ZOOPLANKTON BIOMASS in the GULF of ALASKA, and COVARIATION with CALIFORNIA CURRENT ZOOPLANKTON BIOMASS Iiichaiw I) BRODEUR ET AL.: VARIABILITY IN ZOOPLANKTON BIOMASS CalCOFl Rep., Vol. 37, 1996 INTERANNUAL VARIATIONS IN ZOOPLANKTON BIOMASS IN THE GULF OF ALASKA, AND COVARIATION WITH CALIFORNIA CURRENT ZOOPLANKTON BIOMASS IiICHAIW I). Ul1OIlEUK UliUCE W. FllOST STEVEN R.HAKE Alaska Fihrirs Science Criicrr, NOAA Sc.hool of Ocranograpliy Intcriintioii~l1'~ciiic tfhbiit <:omtiiii\ion 7600 Sand Point Wq NE 1301 357WJ V.0. Box 95009 Sr'lttle, W.lshlllgton '181 15 Uiii\ cr\ii). of W.i\hingtmi ScJttlr, W'lstlltl~t~~n981 45 Se'lttlr. Washln~on981 95 ABSTRACT tinguish their relative contributions (Wickett 1967; Large-scale atmospheric and oceanographic condi- Chelton et al. 1982; Roessler aiid Chelton 1987). tions affect the productivity of oceanic ecosystems both It has become increasingly apparent that atmospheric locally and at some distance froin the forcing mecha- and oceanic conditions are likely to change due to a nism. Recent studies have suggested that both the buildup of greenhouse gases in the atmosphere (Graham Subarctic Domain of the North Pacific Ocean and the 1995). Although there has been much interest in pre- California Current have undergone dramatic changes in dicting the effects of climate change, especially on fish- zooplankton biomass that appear to be inversely related eries resources (e.g., see papers in Beaniish 1995), dif- to each other. Using time series and correlation analy- ferent scenarios exist for future trends in basic physical ses, we characterized the historical nature of zooplank- processes such as upwelling (Bakun 1990; Hsieh and ton biomass at Ocean Station P (50"N,145"W) and froiii Boer 1992). Biological processes are niore laborious to offshore stations in the CalCOFI region. We found a monitor and difficult to predict because of their inher- statistically significant but weak negative relationship be- ent complexity. tween the domains. We investigated whether such a re- There are iiuiiierous examples showing that large- lationship arises froiii different forcing iiiechanisriis or as scale physical and biological changes have occurred an opposite response to the same niechanisni. We found throughout much of the Northeast Pacific Ocean over that the seasonal peak of both data sets occurred in the the last few decades (Francis and Hare 1994; Miller et suniiiier but that the CalCOFI data lagged the Ocean al. 1994). Indices which showed these changes include Station P data. A surface-drifi simulation model showed atmospheric (Ti-e-enberth 1990; Trenberth and Hurrell that winter trajectories started at Ocean Station P and 1994), oceanographic (Royer 1989; Hsieh and Boer along 145"W drifted niore into the California Current 1992; Miller et al. 1994; Lagerloef, 1995; Polovina et al. before the 1976-77 regime shift, and iiiore into the 1995), productivity (Venrick et al. 1987; Polovina et al. Alaska Current after the 1976-77 shift. We exaniiiied 1994), and biomass of various trophic levels (Brodeur physical and biological conditions which niay lead to and Ware 1992, 1995; McFarlane and Beaniish 1992; this inverse relationship between the two ecosystems, Beaniish 1994; Hare aiid Francis 3 995;Roeiniiiich aiid and we discuss the implications of these results for higher McChvan 1995a, b). A nuiiiber of studies have sug- trophic levels. gested that biological changes occurred rather suddenly sometinie arouiid 1976-77, coi~c~rreiitwith a draniatic shift in physical regimes (Francis and Hare 1994; Miller INTRODUCTION et al. 1994). Substantial temporal and spatial heterogeneity oc- Documenting the efixts of climate change on nia- curs in the production of oceanic ecosystems. Much of rine ecosystems requires long time series of saiiipling to this heterogeneity results froiii seasonal and geographic examine low-frequency periodicity. 111 the Northeast variations in nutrient availability, mixed-layer depths, or Pacific Ocean, two series are notable not only for their solar radiation. Processes that enhance productivity (e.g., length, but also for the broad suite of biological and phys- upwelling, wind and tidal mixing) teiid to be localized ical ~iieasureiiieiitsmade at each location. The first of and transient. Physical forcing in the form of the large- these, Ocean Station P (50"N,14.5'W; hereafter called scale circulation pattern redistributes the elevated pro- Station P) , iiomiiially represents a subarctic oceanic duction to areas less favorable for in situ production. ecosystem that was cain pled almost continuously froiii Thus production at any location can be affected by both 1936 to 1980 but only sporadically since then. The sec- local and remote processes, and it is often difficult to dis- ond, the CalCOFI grid, is an eastern boundary current 80 BRODEUR ET AL.: VARIABILITY IN ZOOPLANKTON BIOMASS CalCOFl Rep., Vol. 37, 1996 ecosystem that has been sampled since the late 1940s, lasting 6 to 12 years each. A cold era (Type A) is asso- although not always with the same geographic and tem- ciated with a weak Aleutian Low, relatively weak circu- poral intensity, Although Ekman pumping of deep, iiu- lation in the Alaska Gyre, strong upwelling inshore of trient-rich water is a feature common to both these the California Current, negative sea-surface temperature ecosystems, the mechanisms behind biological produc- (SST) anomalies throughout the coastal Northeast Pacific tion in the two systems chffer (Ware and McFarlane 1989). Ocean, and positive SST anomalies in the central North Over the past few decades, there has been much spec- Pacific Ocean (centered at 40”N).A warm era (Type B) ulation on the nature of the eastern bifurcation of the is associated with a strong Aleutian Low, strong gyral cir- Subarctic Current into the Alaska Current and California culation, and reduced upwelling and high temperatures Current and its possible effects on biological production to the south (figure 2). in these two large ecosystems (figure 1).Wickett (1967), Francis (1993), Francis and Hare (1994), and Hare Chelton and Davis (1982), and Chelton (1984) have and Francis (1995) find similar although longer (20-30 speculated that the intensities of the flows in the Alaska year) periods of oscillating “wariii” and “cool” regimes, and California Currents fluctuate in opposition to one which they relate to the production dynamics of Alaska another. They hypothesized that north-south shifts in salmon. In addition, Francis (1993) speculated that the the bifurcation of the Subarctic Current (West Wind interdecadal variations in salmon production in these Drift) could be forced by physical factors occurring in two oceanic domains are inversely correlated. the western or central Pacific Ocean. This paper examines factors related to long-term Using environmental indices arid fish-recruitment changes in production in the subarctic Pacific Ocean, data, Hollowed and Wooster (1992) and Francis (1993) using examples maiiily from Station P. We examine, in have characterized two alternating interdecadal states of particular, trends in zooplankton bioniass as an indica- atmospheric and oceanic circulation in the Northeast tor of changes in productivity in this oceanic ecosystem. Pacific Ocean which result in very different components Using tinie-series analysis, we then compare these trends of fisheries production (e.g., groundfish, salmon) in the to those evident in the offshore region of the California Alaska Current and Cahfornia Current domains. Hollowed Current to examine the hypothesis of Wickett (1 967) and Wooster (1992) have characterized these states as that production in both systems may be affected by an 64 60 56 52 48 44 40 Subarctic Boundary 36 32 1 Central Subarctic Domain 2 Coastal Downwelling Domain 28 3 Coastal Upwelling Domain 24 I I I I I I I I I 1 20 130 140 150 160 170 180 170 160 150 1 40 130 1 I Figure 1. Large-scale near-surface circulation in the North Pacific Ocean, and Ocean Station P (star) and the CalCOFl sampling region (shaded area). Also shown are the different ocean production domains of Ware and McFarlane (1989) and Pearcy (1991). 81 BRODEUR ET AL.: VARIABILITY IN ZOOPLANKTON BIOMASS CalCOFl Rep., Vol. 37, 1996 WINTER 1972 65N 70N 60 60 55 50 50 40 45 40 30 35 30 20 25 140E 160 180 160 140 120W N iio ria 150 iio iio iio I WINTER 1977 65N 60 msn COLUMBIA 55 50 45 40 35 30 20 1 25 140E 160 180 160 140 120W w ti0 Ih 150 li0 ,io 120 I Figure 2. Winter mean sea-level pressure from Emery and Hamilton (1985) for two contrasting years, and the alternate states of atmospheric and oceanic circu- lation patterns in the eastern North Pacific Ocean proposed by Hollowed and Wooster (1992). influx of water from the same source-the Subarctic Sampling gear was changed from a 0.42-m-diameter Current. We examine physical data relevant to regime- NORPAC net to a 0.57-m SCOR net in August 1966, shift changes in flow between the Alaska Current and although the mesh size remained the same (0.351 mm). California Current. Finally, we discuss implications of Fulton (1983) estimated that the catching efficiency of regime shifts to higher trophic levels and suggest new the SCOR net was 1.5 times greater than the NOR- hypotheses and further studies that could be undertaken PAC net, based on a series of intercalibration tows. But to address these hypotheses. a new estimation based on the original data presented by Fulton (1983) suggests that the correction factor should DATA SOURCES AND METHODS be higher, somewhere in the range of 1.6-2.1~, with 1.77~being the most likely value (Waddell and McKmnel Zooplankton Data Sets 1995; Frost, Ware, and Brodeur, unpubl. data), which Zooplankton biomass data from 24 years (1956-80) is what we used in this analysis. of vertical net sampling at Station P were used in our Zooplankton displacement volumes from the central analyses (figure 1). Before 1969, sampling was conducted part of the CalCOFI grid (lines 77-93) over a longer over alternate 6-week periods, but after this time, sam- time frame (1951-94) were provided by Paul Smith pling was continuous (Fulton 1983).
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