Optical monitoring of phytoplankton blooms in Loch Striven, a eutrophic fjord

During May 1991, a mooring equipped with a blue-green colour sensor and two fluorometers was deployed in Loch Striven, Scotland. The colour sensor rec orded the ratio of downwelling irradiances at 440 and 524 nm, which is stro ngly influenced by phytoplankton abundance. The fluorometers recorded the r ed fluorescent response of phytoplankton pigments to a blue flash. Profiles of water properties were made almost daily near the mooring, in order to a ssess the reliability of automatic bio-optical measurements of phytoplankto n abundance. Each type of instrument had limitations, but all were able to estimate chlorophyll concentration with a precision of 10-14% over the obse rved ranges of 1 to 21 or 33 mg m(-3), giving an r(2) (between the bio-opti cal signal and extracted chlorophyll) of about 0.8. There were large day-to -day, and, sometimes, hour-to-hour changes in amount of phytoplankton. Thes e were largely the result of (non-tidal) water movements: biological change s were slow. Implications for the monitoring of phytoplankton blooms in coa stal waters are discussed.