Physical controls on phytoplankton physiology and production at a shelf sea front: a fast repetition-rate fluorometer based field study

Observations of phytoplankton physiology collected using a fast repetition-rate fluorometer (FRRF) in the vicinity of a shelf-sea tidal-mixing front are presented. These data are combined with more traditional C-14-based measurements and observations of environmental parameters, including estimates of turbulent dissipation rates, in order to investigate the influence of physical forcing on the productivity of the system. Low nutrient concentrations on the stratified side of the front result in a reduction of photosynthetic efficiency. Conversely, the high degree of vertical mixing on the mixed side of the front constrains the ability of phytoplankton to adjust their photosynthetic apparatus to the ambient irradiance field. Redistribution of phytoplankton biomass and variations in physiological parameters also result from the spring-neap tidal cycle. FRRF- and C-14-derived physiological measurements are compared in the context of environmental gradients in the region. A strong correlation was found between independently measured functional absorption cross-sections (sigma(PSII)) and maximal photosynthetic rates (P-max(.)). Such a relationship was unlikely to have been causative and may have resulted from shifts in the balance between light-harvesting and carbon fixation across the front. The association of changes in P-max(.) with variations in sigma(PSII) provided the basis for the development of an empirical model, specific to the system and time of study, which utilised FRRF data to extrapolate between primary productivity rates measured at fixed sites. When applied to high-resolution cross-frontal data, the model suggested small-scale variations in productivity related to both spatial and temporal physical forcing including the spring-neap cycle.