Seaglider observations of blooms and subsurface chlorophyll maxima off the Washington coast

From September 2003 to December 2007, autonomous, underwater Seaglider continuously ran a V-shaped transect off Washington State from about 200-m water depth (i.e., at the break between the shelf and slope) to offshore waters with depths. 2700 m. Seaglider visited the offshore vertex at 47 degrees N, 128 degrees W, where our observations concentrated, approximately monthly. Seaglider measured temperature, conductivity, and dissolved oxygen to 1000 m and also recorded chlorophyll a (Chl a) fluorescence and particulate optical backscatter to 150 m. Distinct interannual variation was documented in timing and depths of winter mixing, transition to a shallow summer pycnocline, and onset of mixed-layer erosion in autumn. Chl a concentrations estimated from fluorescence were directly comparable among the seven laboratory-calibrated sensors used, but their estimates exceeded concurrent, satellite-derived concentrations by a factor of three. Seaglider optical profiles enabled interpretation of satellite imagery by revealing that the apparent autumn bloom after destratification was instead a vertical redistribution of phytoplankton from the subsurface maximum to a depth where they could be observed by satellites. Results of 4 yr of sampling within 25 km of the vertex demonstrate the value of gliders in ocean observing and their capability to carry out multiyear, fully autonomous operations under any sea state. The true power of glider programs will be realized in combination with other measurement platforms, including larger spatial coverage by satellites and more comprehensive biogeochemical measurements from moorings and occasional ship-based sampling.