OPTICAL-DETECTION AND ASSESSMENT OF ALGAL BLOOMS

Concerns about harmful algal blooms (HABs) have grown in recent years. There is a pressing need for robust, quantitative, and cost-effective methods to detect and characterize algal blooms. Critical application s of these methods include long-term monitoring of coastal waters to i ndicate the degree to which present trends of HABs and human activitie s are linked, early warning systems to protect aquaculture sites and t o signal the need for further investigations, and systems to character ize synoptically the distributions and physiological state of phytopla nkton in an oceanographic context. Because phytoplankton strongly infl uence the fate of light in the ocean, optical methods are well suited for HAB applications. Passive optical systems can measure ocean color or the penetration of solar irradiance in surface waters; both propert ies can be related to the constituents of natural waters, including ph ytoplankton. The sensors measure radiometric quantities: consequently, rigorous calibration is possible and measurements can be compared bet ween sites and over long periods of time. One passive optical system-a radiometer buoy-is shown to be useful for characterizing biological v ariability in surface waters over scales from minutes to months. A red tide was easily observed in measures of ocean color from the buoy; se nsors for downwelling irradiance detected a subsurface bloom. Some opt ical instruments use artificial illumination to determine optical prop erties such as the coefficients of absorption and scattering. These me asures can be made continuously in situ and effectively related to phy toplankton. Several types of fluorometers can be used to characterize the abundance, pigmentation, and physiological state of phytoplankton. All of these optical technologies should be useful for the study of H ABs, but biological interpretations of many optical measurements need further testing.