Optical Properties of Otago Shelf Waters: South Island New Zealand

The optical properties and concentrations of optically active water components were measured at 31 stations on the Otago coast. For the broad-band (PAR), diffuse light attenuation decreased 15-fold (Kd=0·76 to 0·05 m 1) and reflectance 7-fold (R=11·95 to 1·45%) from the most turbid harbour water to the clearest oceanic sites 30 km offshore. The Kd values therefore suggested, that Otago shelf waters were appreciably clearer than at first thought. The absorption and scattering coefficients (a and b, respectively) for PAR were calculated using published nomograms. Multiple linear regression showed the dependence of a and b on changes in the concentration of gilvin, g440, chlorophyll a, [C], and inorganic particles, [I]: a(PAR)=0·019+0·221g440+0·037[C]+0·018[I] b(PAR)=0·239[C]+0·342[I] Substituting the mean concentrations of gilvin (0·078 m 1), chlorophyll a (1·58 mg m 3) and inorganic particles (3·23 g m 3) into the above equations suggested that gilvin absorbed 11%, phytoplankton 39%, inorganic particles 38% and water itself 12% of PAR and on average. Phytoplankton cells caused 25% and inorganic sediments 75% of the scattering. Spectral irradiance measurements showed, that absorption by gilvin and suspended sediments could dominate absorption in the blue-green waveband for the most turbid harbour waters. Gilvin and phytoplankton levels were lowest in winter and inorganic sediment concentration were generally low offshore (<1 g m 3). Thus, oceanic waters were clearer and brighter in winter when compared to spring (July: Kd=0·05 m 1; R=3·1%; November: Kd=0·11 m 1; R=2·1% on average). However, spectral irradiance measurements showed anomalous features in R( ) and Kd( ) spectra above 590 nm which suggested light emission resulting from Raman scattering and chlorophyll fluorescence.