Optical properties of Gas-vacuolate cells and colonies of Microcystis in relation to light Attenuation in a Turbid, Stratified Reservoir (Mount Bold Reservoir, South Australia)

Abstract

Profiles of downwelling and upwelling irradiance were measured in a stratified, turbid reservoir, when Microcystis aeruginosa formed a significant proportion of the phytoplankton community. The attenuation coefficient (K_d) was c. 2.1 m^-1 and the reflectance (R) c. 0.03. Application of Kirk's simulation model relating apparent and inherent optical properties enabled calculation of coefficients of absorption (a; 1.3-1.6 m-') and scattering (6; 5-7 m^-1). The asymptotic diffuse backscattering coefficient (0.2 m^-1) was derived from the relationship b′_b = 3.6 R_aK_d, a slight modification of Kirk's original equation. Turbidity measurements supported the general rule that nephelometric turbidity was numerically equivalent to the scattering coefficient.

There was good agreement between the measured light profile and one reconstructed from inherent optical properties using the relationship between K_d, a and b.

The optical properties of cells and colonies of Microcystis were investigated before and after the collapse of gas vacuoles. The Chla-specific absorption coefficient for cells (0.0138 m² mg,^-1 Chla) was higher than for colonies (0.0106 m² mg,^-1 Chla) at a depth equivalent to 0.2 m. Both coefficients decreased with increasing depth as the spectral composition changed. The Chla-specific scattering coefficient for vacuolate cells (0.14 m² mg,^-1 Chla) was greater than for colonies (0.11 m² mg,^-1 Chla), and a similar correspondence occurred for non-vacuolate cells and colonies (0.029 and 0.020 m² mg,^-1 Chla respectively). These measurements illustrate the package effect and also that c. 80% of light scattering is due to gas vacuoles. The relationship between pressure-sensitive turbidity and gas-vacuole volume suggested that 1 gL mL^-1 was equivalent to a turbidity of 2 NTU. These optical characteristics, combined with the buoyant nature of Microcystis, suggest that it is a canopy species.