Utilising Chlorophyll Fluorescence to Assess the Variability of Phytoplankton Biomass and Productivity in the North-West European Shelf Seas

Marine primary productivity by phytoplankton drives the conversion of CO2 into organic carbon, thus supporting key marine ecosystem services such as biogeochemical nutrient cycles and fisheries. Therefore, the ability to accurately quantify and predict the environmental influences that govern this process is of paramount importance. Through surveying and experimental approaches, I investigated the variability of phytoplankton physiology and biomass across a range of light and nutrient environments. I found that growth and physiological responses differ in geographically distinct regimes of nutrient limitation across the North Sea. Variable nutrient stoichiometry across the region could therefore lead to alternative growth and productivity rates that reflect the populations suited to the different conditions. Fast repetition rate fluorometry (FRRf) measurements of photosystem II (PSII) characteristics during a spring bloom were shown to change with hydrography and community composition. Thus, providing further support that the physiological state of PSII can be used as an indicator of bloom status and community composition. FRRf-derived productivity parameters were also measured and shown to change in response to changes in taxonomy. Through geostatistical approaches the spatial distribution of phytoplankton was characterised during a spring bloom. I show both in situ and satellite sampling approaches possess the ability to capture mesoscale variability in phytoplankton distribution, but ocean colour estimates lose accuracy in highly heterogenous conditions. This thesis provides a step towards capturing the extent of spatial and temporal variability in phytoplankton stocks and rates in temperate shelf seas, in part, by providing a better understanding of the strengths and limitations of the use of fluorescence-based measurements.