MEPS 463:39-57 (2012)  -  DOI: https://doi.org/10.3354/meps09836

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

Anna E. Hickman1,2,*, C. Mark Moore2, Jonathan Sharples3, Michael I. Lucas4, Gavin H. Tilstone5, Vladimir Krivtsov6, Patrick M. Holligan2

1Department of Biological Sciences, University of Essex, Wivenhoe Park, Colchester, CO4 3SQ, UK
2Ocean and Earth Sciences, University of Southampton, National Oceanography Centre, Southampton, European Way, Southampton, SO14 3ZH, UK
3School of Environmental Sciences, University of Liverpool, and National Oceanography Centre, 4 Brownlow Street, Liverpool, L69 3GP, UK
4Department of Zoology, University of Cape Town, Rondebosch 7701, South Africa
5Plymouth Marine Laboratory, Prospect Place, Plymouth, PL1 3DH, UK
6School of the Built Environment, Herriot-Watt University, Edinburgh, EH14 4AS, UK

ABSTRACT: 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−2 d−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 of 15NO3 uptake compared to 14C 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 of 15NO3 and 14C 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.


KEY WORDS: Phytoplankton · Photosynthesis · Primary production · Carbon fixation · Nitrate uptake · Irradiance · Continental shelf · Celtic Sea


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Cite this article as: Hickman AE, Moore CM, Sharples J, Lucas MI,Tilstone GH, Krivtsov V, Holligan PM (2012) Primary production and nitrate uptake within the seasonal thermocline of a stratified shelf sea. Mar Ecol Prog Ser 463:39-57. https://doi.org/10.3354/meps09836

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