Contrasting the vertical differences in the phytoplankton biology of a dipole pair of eddies in the south-eastern Indian Ocean

A dipole pair consisting of one warm-core (WC) and one cold-core (CC) eddy formed off Western Australia in the winter of 2003. Within these eddies the vertical distributions of phytoplankton, both in terms of community and physiology, were characterized in an effort to understand the impact of eddies on the phytoplankton ecology of the region. The WC eddy had a 275-m deep mixed layer in the centre and a vertically dispersed photoautotrophic community dominated by large diatoms, Pelagophyceae and coccolithophorids. The CC eddy had a shallower and more consistent mixed-layer depth with a deep chlorophyll a maximum (chlamax) well developed throughout the eddy. The euphotic zones and mixed layers of both eddies were very low in nitrate. Phytoplankton characteristics that are known to acclimate to variation in irradiance but require different amounts of time to reach steady state were measured at the surface and deep chlamax in both eddies. Differences between the surface and chlamax were found for characteristics that can diverge over a range of time scales from very short (seconds) to much longer (months) for physiological and ecological parameters such as: Ik and fluorescence quantum yield from PAM fluorometry, pigments (photoprotective and photosynthetic), pigment ratios, carbon to chlorophyll a ratios, PB and PBmax (measured over 24 and 4 h, respectively) community composition based on cell counts and pigments, and deep chlamax formation. Absolute time scales for change of these characteristics were obtained from the literature and used to quantify vertical mixing rates in the two eddies. Overall, vertical mixing is shown to be more pronounced, up to 20 times faster and deeper, in the WC eddy than in the CC eddy, especially near the WC eddy centre. This high rate of vertical mixing is hypothesized to allow the development and persistence of a vertically homogeneous community of large diatoms that spreads from the surface down to 275 m, well below the euphotic depth. As nitrate concentrations in the surface mixed layers of both eddies were very low there was no capacity for a significant increase in phytoplankton biomass within the eddies without additional nitrogen inputs. The longer-term biological trajectories of the eddies are discussed.