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 light-absorbing pigments that make them reddish in color. However, absorption properties of red tide–forming phytoplankton are not generally unique from other phytoplankton, and hence reddish pigments cannot solely be responsible for the color of red tides (Millie et al. 1997; Schofield et al. 1999; Roesler et al. 2003). In addition to pigmentation, algal scattering, particularly backscattering, is influenced by the cell size and potential growth phase of the phytoplankton (McLeroy-Etheridge and Roesler unpubl. data) and is an important optical property determining the magnitude and spectral shape of the emergent light field.
A comprehensive analysis of the optics of red tides has been hampered by the use of radiometers incapable of fully resolving the visible spectrum (i.e., from 400 to 700 nm). Most in-water optical sensors launched over the last few decades measure light in only six or seven visible channels that match those measured from most space-borne ocean color sensors (Yoder 2000). These channels are not evenly spaced throughout the visible spectrum and are concentrated in blue and green wavelengths between 412 and 555 nm. Consequently, a large spectral gap occurs between the green channel at 555 nm and the red channel around 665 nm. Light reflected from red tides commonly peaks from 570–580 nm, a part of the visible spectrum that is not generally quantified by these multispectral sensors. Moreover, the region between 555 and 665 nm is critical for accurately modeling the color perceived by the human eye. The development of a new class of hyperspectral sensors that provide continuous spectral coverage over the entire visible spectrum (Chang et al. 2004) has allowed us to quantify the spectrum of light incident upon the human eye.
Following on past research (Morel and Prieur 1977; Carder and Steward 1985; Roesler et al. 2003), we use radiative transfer modeling to estimate the water-leaving radiance from sea surface expressions of different concentrations and types of phytoplankton, colored dissolved organic matter (CDOM), and minerals. The color of the water perceived by a human eye is estimated for each spectrum using the color matching functions defined by the Commission Internationale de l’Eˆ clairage (CIE 1991). The perceived color assumes that the individual is looking directly down at a relatively calm sea surface with no contrasting skylight or sea surface glint. Color shifts from blue to green to brown and red can be quantitatively modeled from typical absorption and backscattering properties of phytoplankton, CDOM, and/or minerals.