Climatic Forcing of Zooplankton Dynamics Is Stronger during Low Densities of Planktivorous Fish

We analyze interannual variation in zooplankton biomass in the southwestern Barents Sea in spring (May) and summer (June–July) 1959–1993. Using a threshold modeling approach, we quantify spatial, climatic, and acrossseason autoregressive effects under contrasting regimes of low and high densities of planktivorous fish (capelin and herring). Main findings: (1) zooplankton biomass is mainly influenced by capelin feeding, first in the offshore parts of the western Barents Sea in spring and subsequently farther east and north in summer, whereas direct effects of herring are quantitatively less important; (2) effects of climate are stronger during the zooplankton increase phase in spring than in summer and are better predicted using the North Atlantic Oscillation index than using reconstructed sea surface temperature and Atlantic water influx; (3) regional anomalies in zooplankton biomass persist from spring to summer but not from summer to the subsequent spring, indicating that the observed fluctuations have little effect on next year's dynamics; and (4) effects of climatic variation and planktivorous fish interact: climatic (and autoregressive) effects are mainly evident when and where the effect of feeding is weak. Zooplankton dynamics thus appear to be under shifting top-down and climatic control, the relative importance of the two processes varying spatially, seasonally, and interannually.