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.