The ratio of the xanthophyll pigments diadinoxanthin and diatoxanthin to chlorophyll a [(DD + DT): Chl a] was used as a photoadaptive index during the development of a large Antarctic diatom bloom. This index was found to track fluctuations in the incident solar irradiance and the in situ light field over a 3-order-magnitude change in the water column biomass. Depth profiles of the (DD + DT): Chl a ratio showed that the upper mixed layer, assessed by physical data, was in fact stable over the course of the month. Diel experiments conducted over the same period showed a delayed (5-8 h) response of the DD + DT pool to the instantaneous Qpar (400-700 nm) irradiance. These time-series results illustrate the potential use of xanthophyll pigments in assessing phytoplankton light histories and the degree of water column stability.
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