Quantitative physiological techniques for seagrasses which are non-destructive have long been sought. This is the first report of spectral reflectance measurements and reflectance indices of physiological processes in seagrasses. Low salinity was the variable. Seedlings of Thalassia testudinum were tested over a range of salinities 32 ppt to 16 ppt, incubation time 24 hr, while spectral reflectance was measured from 308 nm to 1138 nm. The spectral reflectance differences between 32 ppt and 16 ppt were statistically significant (P¡Ü0.05). Many specimens¡¯ mature blades turned yellow and brown at 16 ppt. Reflectance changes at 525 and 650-680 nm suggested changes in xanthophylls and chlorophylls. Several different reflectance indices were employed. The normalized difference vegetation index (NDVI) was reduced at 16 ppt from that at 32 ppt for mature blades, but was slightly increased for new blades at 16 ppt. The structure-independent pigment index (SIPI) was higher in mature blades at 16 ppt than new blades, indicating a higher carotenoid: chlorophyll ratio in new blades. The photochemical reflectance index (PRI) suggested a lower photochemical efficiency on reduced photosynthetic potential at lower salinities. Salinity lethal limits were established at 20 ppt after 72 hours for Thalassia testudinum (Thorhaug and Marcus, 1981). Massive changes altering salinities have and are now occurring in estuaries which may alter seagrass ecosystem health. Detailed data is necessary. Additionally this reflectance data may be useful for remote sensing of seagrass in shallow waters to differentiate them from other benthic habitats. In comparison, blade reflectance of Thalassia was the same general shape as terrestrial species, but of lesser peak height in the near infrared 750-950 nm range (20% than that of terrestrial species). In comparison to other habitat plants, near infrared reflectance of Thalassia was intermediate (18%) between the green (12%) and the brown (27%) habitat algae.

Key words: NDVI, pigments, PRI, salinity, seagrass, SIPI, spectral reflectance, Thalassia testudinumI