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 applications of these methods include long-term monitoring of coastal waters to indicate the degree to which present trends of HABs and human activities are linked, early warning systems to protect aquaculture sites and to signal the need for further investigations, and systems to characterize synoptically the distributions and physiological state of phytoplankton in an oceanographic context. Because phytoplankton strongly influence 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 properties can be related to the constituents of natural waters, including phytoplankton. The sensors measure radiometric quantities: consequently, rigorous calibration is possible and measurements can be compared between sites and over long periods of time. One passive optical system-a radiometer buoy-is shown to be useful for characterizing biological variability in surface waters over scales from minutes to months. A red tide was easily observed in measures of ocean color from the buoy; sensors for downwelling irradiance detected a subsurface bloom. Some optical instruments use artificial illumination to determine optical properties such as the coefficients of absorption and scattering. These measures can be made continuously in situ and effectively related to phytoplankton. 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 HABs, but biological interpretations of many optical measurements need further testing. Environmental and economic impacts of harmful algal blooms (HABs) have increased in recent decades (Smayda 1990; Hallegraeff 1993), concurrent with escalating direct influences of human activities on estuarine and coastal ecosystems. During these same years, some marine ecosystems have undergone fundamental changes that are apparently associated with natural variability of climate (Roemmich and McGowan 1995; Sugimoto et al. 1995). The possible influence of anthropogenic changes in greenhouse gases is also a concern (Fraga and Bakun 1993; Tester et al. 1993). In order to ensure sustainable development of critical coastal resources, it is imperative to know the degree to which present trends of HABs and human activities are linked (e.g. Kahru et al. 1994), if those trends will lead to unacceptable consequences, and if the means can be developed to mitigate harmful effects. Essential to this effort will be reliable, quantitative, and practical means to detect and characterize I Also with Satlantic, Inc., Halifax, Nova Scotia. Acknowledgments The comments of anonymous reviewers are appreciated. We thank the Office of Naval Research, NASA, NOAA, and NSERC for support. Some of this work was supported by the Natural Sciences and Engineering Research Council of Canada-Satlantic Industrial Research Chair in Environmental Observation Tcchnology, awarded to J.J.C. This arms-length research partnership is open to participation by other manufacturers of instruments. A.M.C. was supported by CNPq, Brazil. CEOTR Publication 1. HABs. Detection is required not only to describe the frequency of blooms and long-term trends, but also for the protection of aquaculture sites and for implementation of measures to protect public health. Historically, algal blooms have been noticed because the water turns red, brown, or yellow. Now we have relatively simple optical instruments to tell us quantitatively what a trained observer can easily see: that the water has changed color, and it is probably due to algae (Gower et al. 1984; Carder and Steward 1985; Cullen et al. 1994; Gitelson et al. 1994). In addition, phytoplankton pigment can be detected effectively with in situ fluorometers (see Falkowski and Kolber 1995). If such optical observations can be made autonomously and interpreted reliably, they would be ideal for the detection of HABs in coastal waters, even in remote locations. Species composition of blooms cannot yet be determined reliably from optical measurements (discussed below), and some harmful species can profoundly affect coastal resources without dominating the phytoplankton (Anderson 1983) or the optical characteristics of the water, so there are limitations to the usefulness of optical instruments for detecting HAB phenomena. Nonetheless, continuous optical measurements in coastal waters would be extremely useful for describing bloom dynamics and long-term trends as well as for detecting threats to public health or aquaculture. Although critical factors for bloom formation have been described for some species (Seliger et al. 1970; Anderson and Morel 1979; Tyler and Seliger 198 1; Franks and Anderson 1992b; Honjo 1993; Figueiras et al. 1994; Watanabe