Nutrient fluxes into the seasonal thermocline of the Celtic Sea

Estimates of vertical fluxes of nitrate (JN) into the subsurface chlorophyllmaximum from the bottom mixed layer were made in a variety of hydrologicalregimes over the Celtic Sea in 2003 and 2005. Over a topographically flat shelfJN varied with the spring-neap tidal cycle (1.1 (neap) – 2.8 (spring) mmol m-2day-1), driven by changes in barotropic shear generated vertical diffusivities (Kz)at the base of the thermocline. Further increases in nitrate fluxes were possiblethrough small shear perturbations. JN is further enhanced over topographicfeatures, such as banks on the shelf or the shelf break, by the generation anddissipation of lee waves. The strength of mixing driven by the lee waves alsovaries with the spring-neap cycle, with higher Kz at the base of the thermoclineoccurring around spring tide, compared to neap tide, associated with the greateroccurrence of short period internal waves. Over banks vertical nitrate fluxesvaried between 2.9 (neap) – 15.7 (spring) mmol N m-2 day-1 and over the shelfbreak estimated vertical nitrate fluxes were 4 (neap) –15 (spring) mmol m-2day-1.These fluxes are capable of supporting new production of 207 mg C m-2 day-1over the Celtic Sea shelf, which over the summer stratified period is potentiallygreater than the new production taking place in the spring bloom. Enhancedproduction of 1200 mg C m-2 day-1 is supportable over regions of the shelfaffected by the generation of lee waves over banks. This equated to a 4%increase in new production within the SCM over the Celtic Sea shelf. 31% ofnew production in the Celtic Sea was associated with the shelf break, where 660mg C m-2 day-1 could be supported in the shelf break region

Phytoplankton photoacclimation and photoadaptation in response to environmental gradients in a shelf sea

Variability in the photosynthetic performance of natural phytoplankton communities, due to both taxonomic composition and the physiological acclimation of these taxa to environmental conditions, was assessed at contrasting sites within a temperate shelf sea region. Physiological parameters relating to the structure of the photosystem II (PSII) antenna and processes downstream from PSII were evaluated using a combination of fast repetition rate fluorescence, oxygen flash yields, spectral fluorescence, and 14C photosynthesis versus irradiance measurements. Parameters relating to PSII antenna structure, specifically the functional absorption cross-section (sPSII) and the chlorophyll to PSII reaction center ratio, varied principally as a result of spatial (horizontal) taxonomic differences. Phenotypic plasticity in the size of the PSII light-harvesting antenna appeared to be limited. In contrast, parameters related to electron transport rates (ETRs) downstream of PSII, including the maximum ETR (1/tPSII), the chlorophyll-specific maximum rate of carbon fixation (P*max), and the light-saturation intensity (Ek), all decreased from the surface to the subsurface chlorophyll maximum (SCM) in stratified waters. The primary photoacclimation response to the vertical light gradient thus resulted in decreasing light-saturated carbon fixation per reaction center with increasing depth. Increases in the ratio of PSII reaction centers to carbon fixation capacity thus dominated the phenotypic response to decreased irradiance within the SCM. Perhaps counterintuitively, phytoplankton populations within fully mixed water columns, characterized by low mean irradiance, were acclimated or adapted to relatively high irradiance. <br/