Seasonal, interannual and event scale variation in North Pacific ecosystems

Abstract

We synthesize information on changes in ecosystems of the North Pacific at seasonal, interannual, and event time scales. Three approaches are used to cope with inadequate temporal, spatial and trophic resolution in generating this synthesis. First, we use highly spatially and temporally resolved data on physical forcing and chlorophyll (SeaWiFS data from 1998 to 2005) to describe basin-wide spatial patterns and seasonal to interannual time scales. The second approach is to compare time series of zooplankton at selected spatial sites at which sampling resolution is sufficient to describe seasonal biomass/abundance patterns, and where multiple years of data exist to examine interannual variability. The third approach is to infer trophic relationships, and broaden the first two approaches to higher trophic levels, by examining the impacts of several event scale phenomena on many trophic levels, but only over a rather limited geographic region. The 8 years of satellite chlorophyll data clearly show that interannually persistent seasonal patterns exist in most regions in the North Pacific, even in the tropical waters. From frequency analysis (Lomb periodograms), the annual cycle was the strongest in most regions, but in the tropics and eastern boundary current regions, periods greater than 1 year were significant. In mid- to high-latitude regions, periods of less than 1 year were also significant in addition to the annual period indicating double peaks with varying intervals. Seasonal progression of the timing of annual peak chlorophyll concentration in the North Pacific showed a different pattern compared with the Atlantic or Indian Ocean, largely due to the presence of the subarctic high nutrient–low chlorophyll (HNLC) and equatorial upwelling regions, which had later phytoplankton blooms than would have been predicted based on a simple equatorial to pole progression of bloom timing. Seasonal cycles in zooplankton were more or less synchronized (concomitant with or slightly lagged) with those of phytoplankton with a few exceptions. Exceptions occur in the Eastern Subarctic Gyre where annual peaks of chlorophyll occur in autumn, after the peak in zooplankton biomass. Interannual variation of annually averaged chlorophyll in 30 regions show three patterns, one positively related to El Niño, one negatively related to El Niño, and one with longer-scale variation, possibly related to climate regimes. Nine regions did not match any of the three patterns. Interannual variation in zooplankton abundance/biomass from selected regions indicates that El Niño may be the major source of interannual variability with its effects modulated by longer-scale variation, such as by the Pacific Decadal Oscillation. Two well-documented environmental events in the Northern California Current, in 2002 and 2005, exemplify how short-term disruption in atmospheric forcing causes changes in ocean hydrography and circulation that has significant impacts on primary production and ripple effects throughout multiple trophic levels of the ecosystem. We conclude that a closer look at the data often yields interesting results that might not necessarily be gained by considering the broad generalizations. Specifically, we observe that short-term disruptions of the ecosystem at the primary producer level may impact higher trophic levels in nonlinear ways that lead to unpredic impacts when one considers the entire food chain.