Variability of phytoplankton production rates in the Atlantic Ocean as observed using the Fast Repetition Rate Fluorometer

This thesis examines some aspects of in situ phytoplankton physiology and subsequent production rates within the Atlantic Ocean, as observed using a novel instrument, the Fast Repetition Rate Fluorometer (FRRF). The underlying theory and use of this instrument is described in detail. High resolution FRRF data collection was performed during three oceanographic cruises: RV Pelagia, March 1998, RRS James Clark Ross, May-June 1998 and RRS Challenger, August 1999. These data observe characteristics of phytoplankton physiology and, therefore, production, over daily (diel), small (turbulent) and broad (seasonal) scales. The sampling sites for all cruises were chosen within a variety of hydrographic regimes to further assess the light-nutrient dependencies of this variability. Phytoplankton physiology is described by the functional absorption cross section (σPSII) and the quantum yield of photochemistry (Fv/Fm which relate to the rate at which photosystem II (PSII) saturates with light and the proportion of functional PSII reaction centres, respectively. Changes in both σPSII and Fv/Fm are most evident at the diel scale. σPSII correlates with corresponding changes in PSII pigments indicating non-photochemical quenching of excess solar energy as part of a diel rhythm in cellular constituents. A novel calculation for the number of in situ PSII reaction centres (nPSII), based on FRRF measurements, is described and tested and shows similar diel variability. Smaller-scale variations in σPSII are also observed continually throughout the diel period apparently as an attempt to balance the distribution of energy between PSII and PSI and, therefore, maintain high rates of photosynthesis. Such smaller-scale processes are most obvious in low nutrient (oligotrophic) waters where hydrographic variability and consequently new nutrient input, remains relatively low. FRRF estimates of production were most related to nutrient conditions in these oligotrophic waters. Conversely, production correlated with light in waters where nutrients were in abundance. FRRF production estimates compared well with corresponding in situ gross O2 measurements but were typically a factor of 3-4 higher than 14C production estimates. This difference can be accounted as the stochiometry between O2 evolution and carbon uptake for photosynthesis but may also represent the limitations associated with the calculation of production from one or both techniques. These limitations are discussed as a premise for further work