The underwater light field in the Bellingshausen and Amundsen Seas (Antarctica)

Abstract

The underwater light field in the Bellingshausen andAdmundsen Seas was characterised using data collectedduring the R/V Polarstern cruise ANT XI/3, from12.1.94 to 27.3.94. The euphotic zone varied from 24to 100 m depth. Spectral diffuse vertical attenuationcoefficients (K _d (λ))were determined for 12narrow wavebands as well as for photosyntheticallyavailable radiation (PAR, 400–700 nm): K _d (490)ranged from 0.03 to 0.26 m^™1; K _d (550) from0.04 to 0.17 m^™1; K _d (683) from 0.04 to0.17 m^™1; and K _d (PAR) varied from 0.02 to0.25 m^™1. K _d (λ) for wavelengths centred at412 nm, 443 nm, 465 nm, 490 nm, 510 nm, 520 nm and550 nm were significantly correlated with chlorophyllconcentration (ranging from 0.1 to 6 mg m^™3). Thevertical attenuation coefficients for 340 nm and380 nm ranged from 0.10 to 0.69 m^™1 and from 0.05to 0.34 m^™1, respectively, and were also highlycorrelated with chlorophyll concentrations. These K _d values indicate that the 1% penetration depthmay reach maxima of 46 m and 92 m for 340 nm and380 nm, respectively. The spectral radiancereflectances (Rr(λ)) for 443 nm, 510 nm and 550 nmwere less than 0.01 sr^™1. Rr(λ) for 665 nm and683 nm increased with depth up to 0.2 sr^™1 because ofchlorophyll fluorescence. Using a model that predicts downwardirradiances by taking into account the attenuation bywater and absorption by chlorophyll, we show thatchlorophyll fluorescence has a significant influenceon the red downward irradiance (E _d (633, 665, 683))in deeper layers. The ability of the phytoplanktonpopulation to influence the light environment byautofluorescence and absorption processes depends onthe light conditions and on the photoacclimation ofthe cells, represented by the in vivo crosssection absorption coefficient of chlorophyll (a*). Theobtained mean chlorophyll-specific light attenuationcoefficients of phytoplankton in situ (k _d ) are higherthan the in vivo absorption coefficient of chlorophyll,more than to be excepted from the scattering. a*(λ), m² mg chl^™1, decreased due topackaging effect with increasing chlorophyllconcentrations.