Bio-optical characteristics of a red tide induced by Mesodinium rubrum in the Cariaco Basin, Venezuela

• Bio-optical characterization of a Mesodinium rubrum red tide in the Cariaco Basin
• Spectral absorption was related to specific pigments present in this organism.
• Based on reflection spectrum, a new band ratio is proposed to identify cryptophytes.

The bio-optical changes of the water induced by red tides depend on the type of organism present, and the spectral characterization of such changes can provide useful information on the organism, abundance and distribution. Here we present results from the bio-optical characterization of a non-toxic red tide induced by the autotrophic ciliate Mesodinium rubrum. Particle absorption was high [ap(440) = 1.78 m− 1], as compared to measurements done in the same region [ap(440) = 0.09 ± 0.06 m− 1], with detrital components contributing roughly 11% [ad(440) = 0.19 m− 1]. The remainder was attributed to absorption by phytoplankton pigments [aph(440) = 1.60 m− 1]. These aph values were ~ 15 times higher than typical values for these waters. High chlorophyll a concentrations were also measured (52.73 μg L− 1), together with alloxanthin (9.52 μg L− 1) and chlorophyll c (6.25 μg L− 1). This suite of pigment is typical of the algal class Cryptophyceae, from which Mesodinium obtains its chloroplasts. Remote sensing reflectance showed relatively low values [Rrs(440) = 0.0007 sr− 1], as compared to other Rrs values for the region under high bloom conditions [Rrs(440) = 0.0028 sr− 1], with maxima at 388, 484, 520, 596 and 688 nm. Based on the low reflection in the green-yellow, as compared to other red tides, we propose a new band ratio [Rrs(688)/Rrs(564)] to identify blooms of this particular group of organisms.