Bio-optical properties of the southwestern Ross Sea

Abstract

The bio-optical properties of the southwestern Ross Sea were measured as part of the Antarctic research program Research on Atmospheric Variability and Atmospheric Response in the Ross Sea (ROAVERRS). The study area contained three distinct phytoplankton blooms, distinguishable by species composition. The largest in area was located to the north of the Ross Ice Shelf and was dominated by the prymnesiophyte Phaeocystis antarctica; chlorophyll a (Chl a) ranged from 0.45 to 8.2 mg m-3. Beam attenuation and particle absorption at 435 nm were as high as 3.4 m-1 and 0.35 m-1, respectively. A bloom of diatoms was more spatially restricted, located to the north and west of the P. antarctica bloom, with Chl a generally below 4 mg m-3. Neither diatoms nor P. antarctica exhibited evidence of the level of pigment packaging measured in waters near the Antarctic Peninsula during the Research on Antarctic Coastal Ecosystem Rates (RACER) program, possibly because of their smaller sizes. A much smaller cryptophyte bloom, located south of the Drygalski Ice Tongue, displayed a lower pigment-specific absorption spectra than did P. antarctica or diatoms, a sign of greater pigment packaging. Pigment-specific diffuse attenuation coefficients were consistent with the pigment-specific particle absorption coefficients (aph∗), both being ∼3 times greater than similar measurements made during RACER. Spectral absorption by solutes determined through regression analysis of Kd against Chl a for the ROAVERRS data set was nearly identical to that measured during RACER. Total diffuse attenuation spectra at a given station could be reconstructed by summing the inherent optical properties of the major optical components (pure water, soluble material, detritus, phytoplankton) measured there. Differences in the absorption ratio of aph∗(λ) at 490 nm to aph∗(λ) at 555 nm among the three dominant phytoplankton taxa in the southwestern Ross Sea were responsible for most of the variability in the ratio of remote sensing reflectance (Rrs) at these same wavelengths. At a given concentration of Chl a, the ratio log [Rrs(490):Rrs(555)] was greatest in cryptophyte-dominated waters, which also possessed the lowest aph∗(490):aph∗(555) ratio, and lowest in P. antarctica-dominated waters. These bio-optical differences suggest that no simple empirical relationship between Chl a and log [Rrs(490):Rrs(555)] will apply to all three taxonomically distinct phytoplankton blooms in the southwestern Ross Sea.