Primary production and nitrate uptake within the seasonal thermocline of a stratified shelf sea

Photosynthesis versus irradiance relationships were used to estimate primary production (PP) at a number of locations across the seasonally stratified Celtic Sea during summer in 2003 and 2005. A subsurface chlorophyll maximum (SCM) was ubiquitously located at the base of the seasonal thermocline on the nitracline. Average PP for the stratified waters was 170 to 390 mg C m−2d−1, of which 40 to 50% occurred within the thermocline, the range reflecting cloudy or sunny conditions. Phytoplankton photosynthesis was light-limited at the SCM, resulting in PP in the thermocline being more sensitive to surface irradiance conditions than that in the surface mixed layer. During the stratified period, production fuelled by nitrate from below the thermocline was estimated to be ~10 to 19 g C m−2, around half that determined for the spring bloom. Comparison of carbon (14C) fixation and nitrate (15NO₃⁻) uptake versus irradiance experiments revealed that the proportion of15NO₃⁻ uptake compared to14C fixed was higher at the SCM than in the upper part of the thermocline where nitrate was depleted. The proportion also decreased with increasing irradiance. Estimates of15NO₃⁻ and14C uptake, dissolved oxygen production and the potential vertical diffusive nitrate flux from below suggested that ‘excess’ nitrate is assimilated relative to photosynthetic carbon fixation in the thermocline, particularly under cloudy conditions.