ARE THE INHERENT OPTICAL PROPERTIES OF PHYTOPLANKTON RESPONSIBLE FOR THE DISTINCT OCEAN COLORS OBSERVED DURING HARMFUL ALGAL BLOOMS? Stacey L. McLeroy-Etheridge and Collin S. Roesler Department of Marine Sciences, University of Connecticut, Groton, CT 06340 USA 1. ABSTRACT Harmful algal blooms are becoming more frequent phenomena in the coastal environment. As this escalating trend continues, an early warning system based upon non- invasive rapid detection of harmful algal blooms is desired. A discoloration of water is often associated with these blooms, suggesting the feasibility of detection via ocean color measurements. Previous research has focused on absorption characteristics and pigment compositions of the algae as being responsible for the unique optical signatures observed. Harmful algal species do not contain unique pigments, and thus absorption alone can not explain the distinct changes in water color. However, taxon-specific optical properties, in particular scattering and backscattering, in combination with high concentrations of a monodispersed population may be responsible for the significant changes in ocean color during bloom events. We measured the inherent optical properties (IOPs) of several harmful algal species to determine the source for the frequently observed changes in ocean color during blooms. Four common harmful algal species (Prorocentrum minimum, Gymnodinium splendens, Heterosigma akashiwo, and Aureococcus anophagefferens) were investigated under controlled growth conditions to determine IOPs and particle size distributions (PSD) for exponential and stationary phase cells in order to understand how ocean color might change over the course of a bloom. Only slight distinctions in the shape of the absorption spectra were observed between species or growth phase, indicating that pigments are not responsible for the distinct ocean colors associated with blooms. The exception was G. splendens which was in a heterotrophic mode, and therefore contained few pigments, resulting in low absorption. The data demonstrate that absorption is not the source of distinct ocean color during harmful algal blooms, but rather the algal scattering and backscattering properties. The scattering and backscattering spectra are affected by cell size and growth phase, providing a major contribution to the changes in water color associated with blooms as the PSD changes from a polydispersed to a monodispersed population and as the phytoplankton physiology changes. Therefore our ability to detect these blooms optically will depend upon our ability to determine and interpret accurately scattering and backscattering spectra. 2. INTRODUCTION There are approximately 60-80 phytoplankton species that comprise the category harmful algae. Their modes of destruction include the production of potent biotoxins, physical damage imparted to other organisms (i.e. grazers), the creation of eutrophic and anoxic conditions in their surrounding environment, as well as numerous other problems (Smayda, 1997; Taylor, 1990). The occurrence of these toxic outbreaks is increasing