Effects of a Subsurface Trichodesmium spp. Bloom on the Optical Reflectance Measured in the Upper 150 m of the Water Column in the North Pacific Subtropical Gyre

Ricardo Letelier and Mark Abbott

College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331-5503

Abstract

During July 1996, and as part of the first ALOHA-CLIMAX transect cruise, we measured natural fluorescence by phytoplankton chlorophyll and optical reflectance in the upper 150 m of the water column. At each of several transect stations, a Tethered Spectral Radiometer Buoy (TSRB, Satlantic Inc.) and a Profiling Reflectance Radiometer (PRR, Biospherical Instruments) were deployed close to local noon. The TSRB recorded downwelling irradiance at 490 mn (Ed490), subsurface temperature and upwelling radiance at 412, 443, 490, 510, 555, 670 and 683 mn. The PRR measured profiles (5-150 m) of downwelling and upwelling radiances at 412, 443, 490, 510, 555, 665 and 683 run, as well as temperature.

A large subsurface algal bloom (upper chlorophyll maximum layer), centered at the base of the mixed-layer (approximately 25-45 m depth) was observed north of Station ALOHA and south of the CLIMAX area with chlorophyll concentration equal to or greater than the ubiquitous deep chlorophyll maximum layer [DCML] located at a depth of 100-125 m. Based on visual inspection of net tow samples, this upper water column bloom was attributed to Trichodesmium spp. Although no significant differences were found when comparing the TSRB results for stations within the bloom area with those outside the area of subsurface Trichodesmium aggregations, the PRR depth profile of upwelling radiance at 683 mu and downwelling light attenuation presented distinct differences in the presence of these aggregations.

Based on previous results from Station ALOHA, the vertical position of this Trichodesmium bloom is close or within the water column region of maximum carbon assimilation efficiency per unit chlorophyll a. Hence, this subsurface aggregation of cells may contribute significantly to the total autotrophic carbon fixation in the water column. However, because the increase in chlorophyll concentration peaks below the first optical depth, this feature is not detected in upwelling radiances measured near the sea surface (TSRB). Nevertheless, without knowing the frequency and magnitude of these subsurface events it is difficult to speculate on their relevance with respect to our attempts to model oceanic primary production based on remote sensing of ocean color.