Nutrient fluxes into the seasonal thermocline of the Celtic Sea

Estimates of vertical fluxes of nitrate (JN) into the subsurface chlorophyll<br/>maximum from the bottom mixed layer were made in a variety of hydrological<br/>regimes over the Celtic Sea in 2003 and 2005. Over a topographically flat shelf<br/>JN varied with the spring-neap tidal cycle (1.1 (neap) – 2.8 (spring) mmol m-2<br/>day-1), driven by changes in barotropic shear generated vertical diffusivities (Kz)<br/>at the base of the thermocline. Further increases in nitrate fluxes were possible<br/>through small shear perturbations. JN is further enhanced over topographic<br/>features, such as banks on the shelf or the shelf break, by the generation and<br/>dissipation of lee waves. The strength of mixing driven by the lee waves also<br/>varies with the spring-neap cycle, with higher Kz at the base of the thermocline<br/>occurring around spring tide, compared to neap tide, associated with the greater<br/>occurrence of short period internal waves. Over banks vertical nitrate fluxes<br/>varied between 2.9 (neap) – 15.7 (spring) mmol N m-2 day-1 and over the shelf<br/>break estimated vertical nitrate fluxes were 4 (neap) –15 (spring) mmol m-2<br/>day-1.<br/>These fluxes are capable of supporting new production of 207 mg C m-2 day-1<br/>over the Celtic Sea shelf, which over the summer stratified period is potentially<br/>greater than the new production taking place in the spring bloom. Enhanced<br/>production of 1200 mg C m-2 day-1 is supportable over regions of the shelf<br/>affected by the generation of lee waves over banks. This equated to a 4%<br/>increase in new production within the SCM over the Celtic Sea shelf. 31% of<br/>new production in the Celtic Sea was associated with the shelf break, where 660<br/>mg C m-2 day-1 could be supported in the shelf break region.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2007

Author dc:creator

• Tweddle, J.F.