A comparison of ship and satellite chlorophyll from California and Peru

Abstract

Ship surface chlorophyll measurements from coastal California and Peru have been processed for comparison with surface chlorophyll estimated during the lifetime of the coastal zone color scanner (CZCS). The ship data shows that chlorophyll concentrations are higher off Peru than California, consistent with the Peru Current system's being more productive. Off California the ship and satellite measurements yield similar estimates of chlorophyll concentration (the average ratio of ship to satellite is 1.1), but off Peru the satellite yields substantially lower concentrations (average ratio of 3.8). The lower satellite values off Peru are likely due to sparse satellite coverage and/or problems with atmospheric corrections. For California, the inshore (0–100 km) ship and satellite chlorophyll estimates are similar, but offshore (200–250 km) the satellite yields higher values. Both estimates, however, confirm the presence of a winter chlorophyll maximum in offshore California waters (likely due to increased nutrient concentrations), increases in chlorophyll per cell, and mixing of subsurface chlorophyll maximum to the surface. Conversely, off Peru, offshore ship and satellite values are in closer agreement than inshore values. As a result onshore‐offshore gradients are weaker in the satellite data for both California and Peru. The gradient problem is likely the result of the satellite algorithm. Different algorithms may be needed for coastal waters, dominated by relatively large and colonial phytoplankton, and open ocean waters, dominated by small solitary phytoplankton.