cyclic shifts in the opposite direction to those in the northeast Atlantic. As C.
helgolandicus moves north, C. finmarchicus and even arctic zooplankton species tend
to shift south and west. This is connected to greater advection out of the Labrador
Sea, caused by the same shifts in North Atlantic low- and high-pressure centers
recorded by the NAO. The changes are less dramatic, but definite, and are spelled out
by Greene and Pershing (2000).
(^) The North Sea and general North Atlantic (Hátun et al. 2009) plankton changes are
typical of the broad coastal current systems. They involve both changes in conditions
locally and changes in the plankton delivered by the prevailing currents. To learn
whether the biological changes will prove in the very long term to be predictors of
persistent climate changes, we will have to wait. Possibly, higher-latitude plankton
will adapt, shifting the mobile, oscillating patterns back to cycles similar to those of
available time-series. Possibly, high latitudes will be permanently occupied by low-
temperate and even tropical species.
CalCOFI: Shifts in the California Current
(^) Plankton samples from the CalCOFI series were rarely counted in much specific
detail, apart from the fish larvae. A few years were thoroughly studied, but that was
all that the available funding and human energy allowed. However, total zooplankton
displacement volume data (drain the sample on a fine mesh, resuspend in a measured
volume of water and measure again) were scrupulously kept. John McGowan and
colleagues (e.g. Roemmich & McGowan 1995a, b) have provided us with analyses,
and Lavaniegos and Ohman (2007) have offered some reinterpretations. The most
complete time-series have been sustained in the area offshore of the Southern
California Bight (Fig. 16.22a) Displacement volume of oblique plankton hauls fell off
progressively from approximately 1976 until 1998 (Fig. 16.22b), reaching ∼25% of
the long-term mean, then sharply rebounded to near the pre-1976 mean in 1999. Of
course, emphasizing that trend skips across the strong pulse in 1984–1986 following
the extreme 1983 El Niño. Nevertheless, the decreasing trend corresponded to a sharp
upward step in mean temperature (Fig. 16.22c) and a shift in the PDO (Fig. 16.22d).
Unlike the NAO, the PDO is derived directly from Pacific temperature fields north of
20°N as the first principal component of the spatial variation (Mantua et al. 1997).
Many biological variables across that region are correlated with it. Of course, the
correlations need not arise from direct effects of temperature. Atmospheric pressure
indices like the NAO exist for the Pacific and also correlate at interannual time-scales
with the PDO. McGowan et al. (2003) have suggested that the effect of the
temperature increase on California Current zooplankton was mediated by increased
stratification, reducing upward transfer of nutrients by upwelling and mixing. A
contingency comparison in color (Plate 16.4) makes their point by visual inspection.