THE IMPACT OF PHYTOPLANKTON ON SPECTRAL WATER TRANSPARENCY IN THE SOUTHERN-OCEAN - IMPLICATIONS FOR PRIMARY PRODUCTIVITY

Spectral water transparency in the Northern Weddell Sea was studied du ring Austral spring. The depth of the 1-% surface irradiance level ('' euphotic depth'') varied between 35 and 109 m and was strongly influen ced by phytoplankton biomass. Secchi depths were non-linearly related to euphotic depth. In phytoplankton-poor water, the most penetrating s pectral region was restricted to a relatively narrow waveband in the b lue (approximately 488 nm), but the range was broader, between 488 and 525 nm when phytoplankton were abundant. Water transparency in the re d spectral range was always low and only to a small extent affected by phytoplankton. Two independent procedures were used to quantify the i mpact of phytoplankton on spectral water transparency: (1) Regression analysis of spectral in situ vertical light attenuation coefficients i n the sea, against coincident chlorophyll concentrations. This method gave chlorophyll-specific light attenuation coefficients; the y-interc ept could be interpreted as a measure of light attenuation by pure wat er plus non-algal material. (2) Spectra of in vivo light absorption de rived by spectroscopy, using phytoplankton enriched to varying degrees onto filters. Thus chlorophyll-specific absorption cross-sections wer e determined. Estimates obtained by both procedures were in close agre ement, By integrating over the spectrum of underwater irradiance, in s itu chlorophyll-specific absorption cross sections of phytoplankton su spensions, related to all photosynthetically active radiation, were ca lculated. Light absorption by phytoplankton for photosynthesis is acco mplished mainly in the blue spectral range. Also dissolved and particu late organic matter contributed to the attenuation of blue light. Beca use in water poor in phytoplankton, underwater irradiance was progress ively restricted to blue light, chlorophyll-specific absorption cross- sections of phytoplankton, averaged over the spectrum of photosyntheti cally active irradiance, increased with water depth. In water with ele vated phytoplankton biomass, overall light attenuation was generally e nhanced. However, because the spectral composition of underwater light changed relatively little with depth, except immediately below the wa ter surface, light absorption cross-sections of phytoplankton changed little below 10 m depth. Vertical differences in the proportions of un derwater light absorbed by the phytoplankton community here were mainl y dependent on biomass variations. Because of the comparatively small attenuation of blue light by non-algal matter, the efficiency of light harvesting by phytoplankton at any given concentration of chlorophyll in Antractic waters is greater than in other marine regions. At the h ighest phytoplankton biomass observed by us, as much as 70% of underwa ter light was available for phytoplankton photosynthesis. When phytopl ankton were scarce, < 10% of underwater light was harvested by phytopl ankton.