Abstract
The Ross Sea is the region of greatest primary production in the Southern Ocean due to massive blooms of two phytoplankton groups, the diatoms and Prymnesiophyceae (Phaeocystis antarctica). Dynamics of such blooms (e.g., timing, magnitude, and composition) are mainly controlled by seasonal variability of the micronutrient iron and irradiance, which are predicted to change under a +2°C scenario. Phytoplankton play a key role at the base of the food web, elemental cycling and carbon uptake, and so alteration in their biomass, community composition and structure may propagate to higher trophic levels affecting the Ross Sea ecosystem. To investigate the relationship between phytoplankton physiology, community composition and environmental conditions in the Ross Sea region, this thesis combined field and experimental work across scales from cell to community and ecosystem. Special attention was given to the spatial and temporal distribution of phytoplankton at different taxonomic resolution (Class, Species, and genotype) to characterize phytoplankton diversity in relation to physicochemical variability and evaluate the physiological response of key taxa to future climate scenarios. DNA-metabarcoding analysis of the 18S rRNA gene of phytoplankton collected during two summer-autumn voyages showed that seasonal variability in community composition in the oligotrophic oceanic (off-shelf) region of the Ross Sea is lower than on the more productive continental shelf, which has historically received more attention. Additionally, DNA-metabarcoding revealed distinct latitudinal patterns among P. antarctica genotypes that confirmed biogeographic trends previously reported. Moreover, mixed layer depth, traditionally viewed as a primary determinant of community composition and relative dominance of diatoms and P. antarctica in the Ross Sea, was less significant than macronutrient concentration and salinity. Continuous culture experiments conducted in the laboratory investigated the response of P. antarctica and Fragilariopsis sp. to a future scenario of warmer temperature (+2°C) and higher irradiance and revealed that colony-forming P. antarctica have greater resilience to projected future conditions than anticipated. In both species, pigments from the xanthophyll cycle increased at higher irradiance and temperature, with higher pigment concentration in Fragilariopsis sp. but a higher rate of epoxidation in P. antarctica. Indicating different photoacclimation strategies to cope with excess light exposure. Fragilariopsis sp. cell size decreased without change in cellular Si content, indicating the presence of smaller cells with denser frustules in the future, whereas future conditions did not affect P. antarctica cell size but increased colony size. Further experiments exposing natural communities from three different regions of the Ross Sea to a higher temperature and iron availability, as projected for the future by Earth System Models, showed a lack of effect of temperature alone; conversely, changes in community structure and composition were observed in the treatments with iron addition with Fragilariopsis and Pseudo-nitzschia as the main beneficiaries in the continental shelf region. Observed response to iron addition reflected the initial seed community and trophic conditions, indicating that responses to future change will be regionally variable with implications for trophic processes and carbon export in the Ross Sea. Overall, this thesis advances knowledge of Ross Sea phytoplankton community composition, by incorporating high resolution molecular approaches with a mechanistic understanding of key phytoplankton species and communities’ response to future conditions, to improve and validate biogeochemical models.