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theses

The biophysical processes regulating primary productivity and biomass of phytoplankton in Antarctic coastal seas are both highly variable in time and space. This dissertation integrates multi-platform observations to understand the physical drivers of phytoplankton in coastal waters of Antarctica, with a greater focus in the West Antarctic Peninsula (WAP). The heads of cross-shelf canyons in the WAP are considered biological “hotspots”, yet the physiology and composition of the phytoplankton blooms and the physical mechanisms driving them are not well understood. Incubation experiments were conducted at three of the WAP canyons to test the role of light availability and upwelling of mUCDW in the increased productivity observed at those locations. Results showed that light, and in particular photoadaptation mechanisms are responsible for increased phytoplankton. This work determined an ecologically relevant MLD for coastal Antarctica to further investigate the role of light in these canyon systems. The mixed layer depth (MLD) determined by the maximum of the buoyancy frequency criteria was found to correlate the best with the vertical distribution of phytoplankton estimated by chlorophyll fluorescence. This metric was then applied to a high-resolution glider dataset with the aim to characterize the dynamics of the spring phytoplankton bloom in submarine canyons in the WAP. Both stability, due to increased freshwater input, and mixed layer depth (MLD), and therefore light availability, have been linked to increased chlorophyll fluorescence. To evaluate how the photophysiology of phytoplankton respond to physical forcing, the glider was equipped with a PAR sensor and integrated with a Fluorescence Induction and Relaxation (FIRe) sensor, the first sensor of its kind to be integrated in a glider. The concurrent high-resolution, vertically-resolved and autonomous measurements of physiological variables together with physical oceanographic data allows investigations on how photosynthetic processes are affected by environmental factors, as it is highly sensitive to environmental stresses. Analyses comparing different MLD regimes have shown different photoadaptations resulting from differences in solar radiation exposure conditions (both time and intensity), reflected in the depth of the ML. Potentially different photoacclimation regimes can be evaluated by comparing light saturation parameters (Ek) determined based on the relationship between Photosynthetic Available Radiation (PAR) and photosystem II photosynthetic efficiency (Fv/Fm). With decreasing sea ice trends and increased winds reported for some Antarctic coastal regions undergoing rapid climatic changes, the increased phytoplankton exposure to highly dynamic irradiance levels, especially with deeper MLD, have significant ecological and biogeochemical implications, particularly in the carbon cycling.

ETD_7985

Ph.D.

Includes bibliographical references

by Ana Filipa Miguel Carvalho

doi:10.7282/T30G3P1S

ETD doctoral

The author owns the copyright to this work.