Seasonal and inter-annual variability of SeaWiFS-derived chlorophyll a concentrations in waters off the Washington and Vancouver Island coasts, 1998–2002

Abstract

Seasonal and inter-annual variability in satellite-derived estimates of near-surface chlorophyll a concentration and primary production were examined in three regimes (shelf, slope and deep oceanic waters) off Washington and Vancouver Island from 1998 to 2002. Wind-induced upwelling predominates in late spring and summer, coinciding with the maximum in solar radiation, leading to increased accumulations of phytoplankton biomass. Shelf chlorophyll concentrations varied from 2 to 10 mg m−3 and were generally lower in April and May and maximal in September. Chlorophyll concentrations on the slope followed a similar seasonal pattern (ranging from 0.5 to 4 mg m−3); however, concentrations were always greater on the Vancouver Island slope compared to the Washington slope for any 30-day period. Oceanic chlorophyll concentrations were generally less than 1 mg m−3 and averaged approximately 0.3 mg m−3. The Pacific Northwest is often cloud covered; data density maps provide an index of confidence in the ‘regional’ applicability of the summary statistics. One consequence of the 1997–1998 El Niño appears to be a reduction in chlorophyll concentration, and inferred rates of primary productivity, in slope and oceanic waters off Vancouver Island in 1998. An increased frequency of El Niño events may lead to a reduction in offshore carbon transport and sequestration, and these results help develop an understanding of the variability in carbon cycling in this dynamic upwelling region. Six-month median chlorophyll concentrations were maximal in 2002 for all subregions off Vancouver Island and for shelf and slope regions off Washington. The dramatic increases in chlorophyll concentrations seen in 2002 are consistent with hypothesized increased advection of nutrient-rich Subarctic waters into the California Current.