El Niٌo related variations in nutrient and chlorophyll distributions off Oregon

As part of the NOAA-NSF supported Northeast Pacific GLOBEC program we have been studying the distribution of nutrients and chlorophyll off the coast of Oregon. Data from 1997–1999 allow the comparison of seasonal variations during the strong 1997–1998 El Niño with more normal seasonal distributions during 1998–1999. Maximum nutrient supplies and chlorophyll standing stocks were observed during the seasonal upwelling period (August/September 1998, July 1999). In comparison, both nutrients and chlorophyll standing stocks were severely depressed during the 1997 summer sampling, and nitrate availability appeared to limit primary production. Usually winter conditions result in intermediate nutrient levels with primary production being limited by light availability. During the 1997–1998 winter, the enhanced poleward flow of coastal water resulted in the movement of warm, high salinity water to the Oregon coast. This poleward moving southern water had a lower N:P ratio than typical coastal water. Thus, El Niño conditions had two direct effects on nutrient supply: (1) reduced upwelling resulted in severe nitrogen limitation during the normal upwelling season; and (2) enhanced poleward movement of warm, high salinity water resulted in anomalously low N:P ratios in the halocline. The latter effect contributed to nitrate limitation during the El Niño spring.