Background
Over the past few years, we have reviewed a number of papers that broach the question of a possible regime shift in the Pacific Ocean somewhere near the turn of the last century (Gedalof and Smith, 2001; Freeland et al., 2002; Barth, 2003; Chavez et al., 2003), which may have returned the region to pre-1976 cooler conditions and associated ecosystem characteristics.  The paper by Mullin et al. further addresses this possibility.

What was done
The authors review what is known about the history of macrozooplankton found within the California Current in relation to the regime-shift hypothesis, after which they present some new data on the subject based on biovolume measurements made with the optical plankton counter on preserved samples of macrozooplankton taken in the non-El Niņo years of 1955, 1956, 1966, 1981, 1984, 1991, 1995, 1996 and 1999.

What was learned
With respect to the work of others, Mullin et al. report that Roemmich and McGowan (1995a,b) demonstrated "a decline of approximately 70% in the biomass of zooplankton, and a warming of the surface layer of up to 1.5°C, in the southern California sector of the California Current System (CCS) over the 42 years ending in 1993."  They also note that "the decrease in macrozooplankton has continued at least to 1998, exacerbated by the 1997-1998 El Niņo (Hayward et al., 1999; Hayward, 2000)," but that macrozooplankton biomass has subsequently "increased dramatically."

Although Roemmich and McGowan (1995a) originally presented their data as showing a smooth trend, Mullin et al. remark that they and others have subsequently emphasized "the idea of a 'regime shift' - a relatively rapid change in the mid-1070s, from one quasi-steady state of relatively high biomass in the CCS to another of relatively low biomass."  This interpretation, they say, "is supported by North Pacific climatology (e.g. Miller et al., 1994), concurrent (but oppositely directed) ecological changes in the Alaska Gyre, and atmosphere/ocean models linking the two (e.g. Francis and Hare, 1994; Brodeur et al., 1996; Mantua et al., 1997)."  In addition, they note that "recent climate and ocean data suggest the 1999 El Viejo may also be the start of a new decadal-scale regime (Stephens et al., 2001; Mackas et al., 2001; Greene, 2002)."

As for their own work, Mullin et al. report that their results from 1955 to 1996 "could be interpreted either as a linearly decreasing trend (total biovolume decreased by 45%) or as a regime shift (decrease of 38% from pre- to post-1975 regimes)."  Thereafter, however, in a single year, they say that "total biovolume increased in 1999 to the pre-1975 level, consistent with a possible shift to a new regime."

What it means
Mullin et al. conclude that their 1999 data "are consistent with a shift from the low biovolume regime of 1976-1996 to one more like the high biovolume regime of 1955-1975."  However, they say it is still to early to decide "whether the increased biovolume in 1999 is brief (e.g. ENSO-related) or lasting (e.g. a regime)."  More time and more data will be required to resolve the issue.

References
Barth, J.A.  2003.  Anomalous southward advection during 2002 in the northern California current: Evidence from Lagrangian surface drifters.  Geophysical Research Letters 30: 10.1029/2003GL017511.

Brodeur, R.D., Frost, B.W., Hare, S.R., Francis, R.C. and Graham Jr., W.J.  1996.  Interannual variations in zooplankton biomass in the Gulf of Alaska, and covariation with California Current zooplankton biomass.  CalCOFI Report 37: 80-99.

Chavez, F.P., Ryan, J., Lluch-Cota, S.E. and Niquen C., M.  2003.  From anchovies to sardines and back: multidecadal change in the Pacific Ocean.  Science 299: 217-221.

Francis, R.C. and Hare, S.R.  1994.  Decadal-scale regime shifts in the large marine ecosystems of the Northeast Pacific: A case for historical science.  Fisheries Oceanography 3: 279-291.

Freeland, H.J., Gatien, G., Huyer, A. and Smith, R.L.  2002.  Cold halocline in the northern California Current: An invasion of subarctic water.  Geophysical Research Letters 30: 10.1029/2002GL016663.

Gedalof, Z. and Smith, D.J.  2001.  Interdecadal climate variability and regime-scale shifts in Pacific North America.  Geophysical Research Letters 28: 1515-1518.

Greene, K.  2002.  Coastal cool-down.  Science 295: 1823.

Mackas, D.L., Thomson, R.E. and Galbraith, M.  2001.  Changes in the zooplankton community of the British Columbia continental margin, 1985-1999, and their covariation with oceanographic conditions.  Canadian Journal of Fisheries and Aquatic Sciences 58: 685-702.

Mantua, M.J., Hare, S.R. Zhang, Y.K., Wallace, J.M. and Francis, R.C.  1997.  A Pacific interdecadal climate oscillation with impacts on salmon production.  Bulletin of the American Meteorological Society 78: 1069-1079.

Miller, A.J., Cayan, D.R., Barnett, T.P., Graham, N.E. and Oberhuber, J.M.  1994.  The 1976-7 climate shift in the Pacific Ocean.  Oceanography 7: 21-26.

Roemmich, D. and McGowan, J.A.  1995a.  Climatic warming and the decline of zooplankton in the California Current.  Science 267: 1324-1326.

Roemmich, D. and McGowan, J.A.  1995b.  Sampling zooplankton: correction.  Science 268: 352-353.

Stephens, C., Levitus, S., Antonov, J. and Boyer, T.P.  2001.  On the Pacific Ocean regime shift.  Geophysical Research Letters 28: 3721-3724.