Pre-monsoon bio-optical properties in estuarine, coastal and Lakshadweep waters

Abstract

Spatial patterns of bio-optical properties were studied in coastal, estuarine and Lakshadweep waters during the pre-monsoon period. This was carried out by analysing the profiles of downward irradiance, upward radiance and water samples collected from 26 stations. The apparent optical properties such as subsurface reflectance (R) and diffuse attenuation coefficient (k) derived from these profiles were processed to generate inherent optical properties, a (total absorption coefficient) and bb (total backscattering coefficient). Highly variable surface-coloured dissolved organic matter (CDOM) absorption (1.8–21.6 m⁻¹ at 440 nm) was observed in the coastal waters. The distribution of optical properties a and bb reflected the distribution of optically active constituents with CDOM contributing to most of the variation in coastal waters, sediment in estuarine waters, and chlorophyll a and water molecules in Lakshadweep waters. The spectral remote sensing reflectance (R_rs(λ)) is a linear function of a and b^b. k was lowest and R was at maximum in the green part of the electromagnetic spectrum (EMR) in estuarine and coastal waters while this is true in the blue region of EMR in Lakshadweep waters. A linear relationship was seen between downwelling diffuse attenuation coefficient (k_d) at 490 nm and attenuation of photosynthetically available radiation (k_PAR). Examination of the light field revealed maximum chlorophyll at greater depth in Lakshadweep waters and at shallow depth in coastal waters, with no well-defined maxima in the estuarine waters. An exponential inverse relation is evident between k_d (490) and depth of chlorophyll maxima. Through a radiative transfer computation, a ratio between water-leaving radiance at 400 and 670 nm is found to be an index of CDOM and hence could be used to retrieve it through an optical sensor.