Red and Black Tides: Quantitative Analysis of Water-Leaving Radiance and Perceived Color for Phytoplankton, Colored Dissolved Organic Matter, and Suspended Sediments

Using field measurements and quantitative modeling, we demonstrate that red coloration of the sea surface is not associated with any particular group of phytoplankton and is strongly dependent on the physiology of the human visual system. Red or brown surface waters can be produced by high concentrations of most types of algae, colored dissolved organic matter, or suspended sediment. Even though light reflected by red tides commonly peaks in the yellow spectral region (570–580 nm), human color perception requires consideration of the entire spectrum of light relative to receptors within the human eye. The color shift from green to red is not due to any special optical properties of the algae but results from an overlap in spectral response of the eye’s red and green cones (centered at 564 and 534 nm, respectively). The spectral peak in light reflected from dense algal blooms coincides with a critical hinge point in color vision (570–580 nm), where fine-scale shifts in the spectral shape of water-leaving radiance due to algal absorption and backscattering properties lead to pronounced variations in the observed color. Of the taxa considered, only Chlorophytes and Prochlorophytes lacked sufficient accessory pigments to produce a red tide. Chlorophyll fluorescence and enhanced near-infrared reflectance (the ‘‘red edge’’) contribute negligibly to the perceived color. Black water events are produced when water is highly absorbing but lacks backscattering constituents. The first of the 10 plagues of Egypt may be one of the earliest recorded instances of a red tide: ‘‘and all the waters that were in the river turned to blood. And the fish that were in the river died, and the water stank’’ (The Bible, Exodus 7:20–21). Red tides have been known to occur throughout human history; however, human activities and population increases have contributed to a greater abundance of toxic and noxious algal blooms in coastal regions worldwide (Hallegraeff 2003; Glibert et al. 2005). Dense algal blooms are often called red tides because the sea surface becomes discolored red or ruddy brown. In addition, red tide is often used synonymously with harmful algal blooms (HAB), the term used by the scientific community to characterize all plankton events that have deleterious impacts. However, not all algal blooms that produce red or brown colored water are toxic. Conversely, not all harmful algal blooms are associated with red-colored waters (Anderson 1994), nor are they tidally driven. Despite these discrepancies, the term red tide has been widely adopted by the popular media and is commonly used to refer to intense algal blooms worldwide. Water color has long been used to define water masses since the introduction of the Forel-Ule color scale in the late 1800s (Hutchinson 1975; Arnone et al. 2004). Color No. 21 on this scale has a reddish-brown hue that could be associated with a red tide event. However, the reason for red coloration of intense algal blooms is often misunderstood. The Encyclopedia Britannica (2004) describes red tides as ‘‘a discoloration of sea water caused by dinoflagellates (phylum Protozoa) during periodic blooms (or population increases).’’ The underlying assumption is that red tide–forming phytoplankton contain a unique suite of 1 Corresponding author ([email protected]). Acknowledgments We thank Dariusz Stramski for generously providing phytoplankton absorption and scattering spectra and Grace Chang for permission to use her spectrum and photograph of a natural red tide. Collin Roesler and the anonymous reviewers provided numerous helpful comments that greatly improved this manuscript. Funding was provided by the David and Lucile Packard Foundation, NASA Ocean Biology and Biogeochemistry, Office of Naval Research Environmental Optics, and the NOAA Coastal Technology System (COTS, http://www.csc.noaa.gov/cots) under a partnership with the Center for Integrated Coastal Observation, Research and Education (CICORE) and Center for Integrated Marine Technology (CIMT). Limnol. Oceanogr., 51(6), 2006, 2646–2659 E 2006, by the American Society of Limnology and Oceanography, Inc.