Nutrient fluxes into the seasonal thermocline of the Celtic Sea

Abstract

Estimates of vertical fluxes of nitrate (JN) into the subsurface chlorophyll
maximum from the bottom mixed layer were made in a variety of hydrological
regimes over the Celtic Sea in 2003 and 2005. Over a topographically flat shelf
JN varied with the spring-neap tidal cycle (1.1 (neap) – 2.8 (spring) mmol m-2
day-1), driven by changes in barotropic shear generated vertical diffusivities (Kz)
at the base of the thermocline. Further increases in nitrate fluxes were possible
through small shear perturbations. JN is further enhanced over topographic
features, such as banks on the shelf or the shelf break, by the generation and
dissipation of lee waves. The strength of mixing driven by the lee waves also
varies with the spring-neap cycle, with higher Kz at the base of the thermocline
occurring around spring tide, compared to neap tide, associated with the greater
occurrence of short period internal waves. Over banks vertical nitrate fluxes
varied between 2.9 (neap) – 15.7 (spring) mmol N m-2 day-1 and over the shelf
break estimated vertical nitrate fluxes were 4 (neap) –15 (spring) mmol m-2
day-1.
These fluxes are capable of supporting new production of 207 mg C m-2 day-1
over the Celtic Sea shelf, which over the summer stratified period is potentially
greater than the new production taking place in the spring bloom. Enhanced
production of 1200 mg C m-2 day-1 is supportable over regions of the shelf
affected 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% of
new production in the Celtic Sea was associated with the shelf break, where 660
mg C m-2 day-1 could be supported in the shelf break region.

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