Optical Measurements on the Louisiana Shelf off the Mississippi River

Optical measurements (UV-B downward irradiance (Ed(310)), photosynthetically available radiation (PAR), light scattering b(655), beam attenuation c(631), ratios of nadir upward radiances (i.e. colour-index), Secchi disc depth and chlorophyll a fluorescence) and CTD measurements were gathered along a cross shelf transect at approximately 92 Won the Louisiana Shelf during one cruise in April 1987 and two cruises in February and July 1988. Large cross shelf variability in the optical properties was found over a relatively small spatial scale of about 150 km. In terms of optical water mass classification, Jerlov coastal water type 7 was found near the coast while Jerlov oceanic water type IB was predominant at the shelf edge. The UV-B irradiance measurements showed a negative correlation with salinity which points to the Mississippi and Atchafalaya rivers as a major source of coloured dissolved organic matter (CDOM) in the region. Very clear water was found on the shelf edge in July 1988 and extremely high UV-B irradiance transmission was measured. The 1%-depth for UV-B downward irradiance was 65·2 m, corresponding to a Kd(310)-value of 0·0706 m 1. This value is approximately 50% lower than the pure water value, Kw(310) of 0·116 m 1 as given by Smith and Baker (Applied Optics, 20, 177–184) and often used in inverse methods. This result suggests that absorption in the UV-B region by pure water should be re-examined.