Spatial and temporal variations in variable fluoresence in the Ross Sea (Antarctica): Oceanographic correlates and bloom dynamics

During two cruises to the Ross Sea, Antarctica in austral spring and summer, fast repetition rate fluorometry was used to investigate the relationship between phytoplankton photophysiology and water mass characteristics, micronutrient availability, and composition. Particulate organic matter proxies for phytoplankton biomass (chlorophyll a, particulate organic carbon and nitrogen, and biogenic silica) were all elevated in the photic zone during spring and summer. Biogenic silica concentrations were an order of magnitude higher in summer relative to spring, reflecting a shift in composition from Phaoecystis antarctica to diatoms. Quantum yields of PS II (Fv/Fm) were generally higher in spring relative to summer, coincident with weaker vertical and horizontal gradients in hydrographic properties. Reduced Fv/Fm values (<0.4) were observed in the upper 30m in both seasons, with maximum values (ca. 0.55) observed near base and below the euphotic zone. No significant relationship between Fv/Fm values and dissolved Fe could be identified in the merged spring/summer data set. Functional absorption cross sections were significantly higher in spring than summer, presumably reflecting adaptations to lower irradiance in spring; little variation with depth was observed. Phytoplankton composition did not appear to be a major determinant of bulk quantum yield, although diatom-dominated waters exhibited significantly higher functional absorption cross sections when compared to waters dominated by P. antarctica. Dominance of P. antarctica appears to be related to greater photophysiological resilience and faster photoacclimation to changing light conditions, whereas diatoms were prevalent in shallow summer mixed layers, which likely reflects their enhanced photosynthetic capacity at high irradiance levels.