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This thesis investigates the physical controls of variability in phytoplankton abundance in the subpolar North Atlantic. A multi-decadal set of monthly SST data (HadISST1) is used to identify the dominant variability in the SST annual cycle during the period 1870-2009. Long-term variability in phytoplankton abundance is examined using data from the in situ Continuous Plankton Recorder (CPR) observations for 1946-2007. Physical factors that determine the strength of the annual phytoplankton bloom are identified using a state-of-the-art coupled physical and biogeochemical model. Mixed layer depth (MLD) in particular is examined because of its importance in controlling the growth of phytoplankton through determining the distribution of light and nutrients in the water column. Multi-decadal variability is observed in the amplitude of the SST annual cycle, though the variability is inconsistent across the subpolar basin. The leading EOF modes of seasonal SSTs reveal that about 70% of the observed variability is explained by the seasonal variation of the strength and action centers of the dominant climatic indices (e.g. NAO). Phytoplankton abundance in the eastern shelf region shows variability similar to SST on decadal scales. However, phytoplankton abundance in the subpolar basin is less influenced by SST and is more sensitive to changes in stratification and MLD. The northeastern subpolar basin is examined in more detail because it is a region where the model simulates observations reasonably well. The hypothesis tested is that the timing and characteristics of the MLD shoaling determines the timing, duration and strength of the following spring phytoplankton bloom. The results show that there are two contrasting scenarios in MLD development and bloom strength. Years with early and lengthy MLD shoaling are characterised by repeated short episodes of vertical mixing when stratification is disturbed, and have a weak and continuous spring bloom as a result. Years with late and rapid MLD shoaling have intense but short spring phytoplankton blooms. MLD influences the growth of zooplankton indirectly, which also modulates the spring phytoplankton bloom. The occurrence of early MLD shoaling in the late winter coincides with a prevalence of atmospheric blocking events (high pressure features) in the northeastern subpolar. |
