Bio-optical characteristics of Gerlache and Bransfield Strait waters during an Antarctic summer cruise

Abstract

Abstract Spatial and temporal variations in bio-optical characteristics of the Gerlache and Bransfield Strait waters were determined during an Antarctic summer cruise (FRUELA) in December 1995 and January 1996. Chlorophyll concentration was estimated using a bio-optical model from natural fluorescence flux over the emission of chlorophyll a, incident irradiance, chlorophyll specific absorption coefficient, and quantum yield of fluorescence. The penetration of both photosynthetic active radiation (PAR) and ultraviolet radiation (UV) depended on the concentration of particulated material. Beam attenuation coefficient at 660 nm (c), a good estimator of the particulate material concentration, ranged from 0.85–2.5 m−1 in the upper mixed layer. The values of particulate beam attenuation and the spectral absorption coefficients suggest that the detritical contribution is relatively low and the package effect is relatively high. Both beam attenuation and vertical attenuation of downward radiation (Kd) were linearly correlated with chlorophyll concentration in the water column. Chlorophyll concentration in the study area varied two orders of magnitude (0.2–23 μg Chl a l−1). During the cruise, the maximal UV-B incident radiation at 305 nm was variable ranging from 0.8 to 9.7 μW cm−2. Great regional differences in penetration of UV-B were observed with Kd(305) values in the upper mixed layer, ranging from 0.01 to 0.56 m−1. On the other hand, the attenuation coefficient of downward PAR radiation (Kd(PAR)) in the upper mixed layer ranged from 0.08 to 0.5 m−1. Kd values of both PAR and UV were higher in the stations of Gerlache Strait, coastal waters of Trinity Peninsula, and in a front system north of Bransfield. The lowest Kd values were found in Bransfield, south of King George and Livingstone islands and were associated to low cell densities and chlorophyll concentrations. The spectral light attenuation between 300 and 800 nm also was analyzed. Blue and red regions of the spectra were drastically attenuated in the surface waters of Gerlache Strait.