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 47uN, 128uW, 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.