Nutrient Mapping and Recurrence of Coastal Upwelling Centers by Satellite Remote Sensing: Its Implication to Primary Production and the Sediment Record

Abstract

Satellite infrared (IR) images of the eastern North Pacific show surface thermal features which appear to be mesoscale eddies and fronts within the California Current and coastal upwelling zone. Coordinated satellite and shipboard measurements show that some of these are produced by interaction between the California Current and wind-induced upwelling near points and capes along the coastal boundary. The process of coastal upwelling as viewed from space is unlike the simple structure expected from classical concepts and may develop a cyclonic circulation, an anticyclonic circulation, or surface flows which extend far offshore as giant cold water plumes embedded in the California Current. Observations of a cyclonic upwelling system off Point Sur, California, show that these recurrent injections of nutrient-rich, cold waters produce the same distinctive structure in the nutrient field. When present, microplankton concentrate in the sharp thermal and nutrient fronts that define these systems. The combined effect of the distinctive advective process and redistribution of nutrients is a significant factor in determining the distribution of primary production and biological patchiness along the coast and in the California Current system. Since ships are too slow to synoptically study these large and rapidly changing upwelling systems, phytoplankton distribution and the nutrient structure of the sea surface is inferred from satellites. By calibrating satellite infrared data against in situ nutrient measurements, sea surface nutrient maps can be produced so that estimates of the true nutrient distribution and potential sites of biological activity can be made. Satellite ocean color imagery shows this relationship between chlorophyll-biomass blooms and sharp nutrient gradients. This upwelling center appears to operate like a “natural chemostat.” Such recurrent upwelling at favored locations along the coastal boundary may be reflected in the sediment record.