Photosynthetic performance and productivity of phytoplankton in the Southern Ocean

Marine phytoplankton account for approximately half of global primary production, an amount equivalent to their terrestrial counterpart. These short-lived organisms, with turnover rates between one and three weeks, support nearly all life in the ocean and have a profound effect on global biogeochemical cycles and climate. The connection between marine phytoplankton and climate is intimate and changes to either will profoundly affect the other. Over the years, due to high operational costs and distance from major human settlements, the Southern Ocean has been the least studied ocean, despite its signicance in the distribution of nutrients to the world oceans, especially the lower latitudes, and controlling global climate. In order to capture the response of the phytoplankton to environmental change across the vast Southern Ocean, a method with high spatio-temporal resolution is desirable. By focusing on the Australian sector of the Southern Ocean, this dissertation examines the productivity and physiology of natural phytoplankton communities in situ using the fast repetition rate (FRR) uorometry technique. The FRR uorometry technique was used to derive direct estimation of in situ primary productivity in the Southern Ocean during the SAZ-Sense (Sub-Antarctic Zone Sensitivity to Environmental Change) voyage in Jan-Feb 2007. A statistically significant correlation between FRR- and 14C-derived primary production was observed (r2 = 0.85, slope = 1.230.05, p < 0.01, n = 85) but the relationship between the methods differed vertically and spatially, mainly due to the effect of non-photochemical quenching under high irradiance. This indicates the FRR uorometry technique can be used to determine in situ primary productivity in the Southern Ocean but care should be taken in the interpretation of the data. In addition to the primary production measurements, the photosynthetic performance of phytoplankton was investigated to provide a better understanding of how natural phytoplankton communities acclimate to different environmental variables, especially in the ironreplete Subantarctic Zone (SAZ) and iron-depleted Polar Frontal Zone (PFZ). High effective photochemical effciency of photosystem II (F0 q/F0 m > 0.4), maximum photosynthesis rate (PB max), light-saturation intensity (Ek), maximum rate of photosynthetic electron transport (1/PSII), and low photoprotective pigment concentrations observed in the SAZ correspond to high chlorophyll a and iron concentrations. In contrast, phytoplankton in the PFZ exhibits low F0 q/F0 m ( 0.2) and high concentrations of photoprotective pigments under low light environment. Strong negative relationships between iron, temperature, and photoprotective pigments demonstrate that cells were producing more photoproctive pigments under low temperature and iron conditions, and are responsible for the low biomass and low productivity measured in the PFZ. FRR uorometry data from 31 transects collected aboard MV I'Astrolabe between 2002 and 2009, were used to assess the photosynthetic performance of phytoplankton along a repeated transect from Hobart (42.8degrees S, 147.3 degrees E) to the French Antarctic station, Dumont d'Urville (66 degrees S, 140 degrees E). The maximum photochemical effciency of photosystem II (Fv/Fm) values were high in the Subtropical Zone and water close to the Antarctic continent, but low in the PFZ. Spring Fv/Fm were higher than other seasons, suggesting higher nutrient supply. High Fv/Fm observed in the Subtropical Zone and Antarctic Zone is consistent with moderate to high iron concentrations in these regions. Overall, phytoplankton photophysiology in the Southern Ocean is governed by nutrient distributions, especially iron, which are affected by atmospheric and oceanic physical processes.