Photoadaptive response during the development of a coastal Antarctic diatom bloom and relationship to water column stability

The ratio of the xanthophyll pigments diadinoxanthin and diatoxanthin to chlorophyll a [(DD + DT): Chi 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): Chi a ratio show that the upper mixed layer, assessed by phys­ ical 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 Qpac (400-700 nm) irradiance. These time-series results illus­ trate the potential use of xanthophyll pigments in assessing phytoplankton light histories and the degree of water column stability. The Southern Ocean supports a rich biotic ecosystem and has been recognized as playing an important role in many global processes, such as biogeochemical cycling (Nelson and Smith 1991) and the sequestration of atmospheric CO2 (Siegenthaler and Sarmiento 1993). These processes are ul­ timately dependent on autotrophic production by phyto­ plankton. Although a continued subject of debate (Chisholm and Morel 1991), the primary mechanism controlling phy­ toplankton production over much of the Southern Ocean is thought to be light limitation as a result of the deep vertical mixing, characteristic for the region (Mitchell et al. 1991; Nelson and Smith 1991; Sakshaug et al. 1991). This is es­ pecially true in neritic and shelf water where potentially lim­