Spectral variations of UV-A and PAR solar radiation in estuarine waters of Goa

Abstract

The spectral solar radiation measurements in the range 350-800 nm were carried out in the estuarine waters of Goa using hyperspectral radiometer. The results of the analysis of solar light in the spectral range of photosynthetically available radiation (400 to 700 nm), PAR and ultraviolet radiation in the range 350-400 nm (UV-A) are presented here. The mean PAR values at the surface were 327 W/m sup(2) and reduced to 84 W/m sup(2) at first optical depth, Z sub(90) (m) in water. The first optical depths of the water column varied from 2 - 7m. The mean solar radiation in the UVA range measured from 7.8 to 35.74 W/m sup(2) with a mean value of 25.58 W/m sup(2). The PAR value at Z sub(90) depth is about 36% of the PAR value at surface and at this depth of Z sub(90), the UV-A values were about 11.5% of its value at the surface. In the PAR region, the highest penetration of light were observed in the spectral range of 560 to 600 nm, while in the UV-A region the wavelength of light that had highest penetration in water were within the spectral range of 396-400 nm. The spectral range of highest penetration of light decreased with increase in penetration depth Z sub(90), indicating that with decrease in transparency of water, the light at longer wavelengths penetrated the most in water. A linear correlation was observed of the diffuse attenuation coefficient, K sub(d) (m sup(-1)) at UV-A and at PAR, which would help in developing an algorithm to derive K sub(d) in the UV-A region. Similar patterns of variations in solar radiation within the euphotic depth of water were also observed. The light in the PAR region at euphotic depth was about 1% of the light at surface, however at the same depth the percentage of light in the UV-A region was above 1% of the UV-A light at surface, indicating that UV-A light energy at euphotic depth may reduce productivity, affect reproduction and development. Hence in coastal waters UV-A light could be available for biogeochemical processes at deeper depths