Small scale physical processes and phytoplankton growth in self seas

Moore, Christopher Mark (2002) Small scale physical processes and phytoplankton growth in self seas. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Detailed physical and biological observations collected in shelf sea regions, dominated by tidal mixing fronts or internal tides, are analysed in terms of the effect of physical forcing on phytoplankton growth. Microstructure measurements are used to quantify differences in vertical mixing and hence nutrient and light supply across regimes. The Fast Repetition Rate Fluorometer (FRRF) is used to obtain high-resolution data on the physiological state of the associated phytoplankton communities, with the aim of testing hypotheses relating growth to changing environmental conditions. Observations were made during three research cruises, RRS Challenger, in the Western English Channel, 1999 and RV Kaharoa, off the North East New Zealand coast, 1998 and 2000. The work represents one of the first attempts to collect and interpret FRRF data in a defined physical context within natural ecosystems.

Results from the 1999 cruise indicate that physical forcing in the region of a tidal mixing front has a pronounced influence on the physiology of the associated phytoplankton communities. FRRF based observations were consistent with nutrient limitation towards the stratified side of tidal mixing fronts, while deep vertical mixing caused light limitation within the mixed region. The observed enhancement of chlorophyll in the region of tidal mixing fronts therefore results, at least partly, from in situ growth.

Results from the 1998 and 2000 cruises demonstrate that internal tide dissipation can be responsible for high vertical nitrate fluxes and pronounced variations in phytoplankton light climate. Interpretation of physiological data from the 2000 cruise was difficult due to the complexity of the physical processes observed. However, physiological variability due to vertical motions and upwelling in the region is demonstrated.

Changes in FRRF derived photophysiology, as described by the photosynthetic efficiency (F_v/F_m) and the functional absorption cross section (σ_PSH), are shown to be related to photosynthetic parameters measured by ¹⁴C derived carbon fixation. In particular a striking inverse correlation between FRRF derived σ_PSH and ¹⁴C P* vs. E derived maximal photosynthetic rates (P*_max) was observed. FRRF derived photosynthetic rates are shown to compare favourably with ¹⁴C derived rates, given the limitations of both techniques.