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 recorded the ratio of downwelling irradiances at 440 and 524 nm, which is strongly influenced by phytoplankton abundance. The fluorometers recorded the red fluorescent response of phytoplankton pigments to a blue flash. Profiles of water properties were made almost daily near the mooring, in order to assess the reliability of automatic bio-optical measurements of phytoplankton abundance. Each type of instrument had limitations, but all were able to estimate chlorophyll concentration with a precision of 10-14% over the observed ranges of 1 to 21 or 33 mg m ³, giving an r² (between the bio-optical signal and extracted chlorophyll) of about 0.8. There were large day-to-day, and, sometimes, hour-to-hour changes in amount of phytoplankton. These were largely the result of (non-tidal) water movements: biological changes were slow. Implications for the monitoring of phytoplankton blooms in coastal waters are discussed.