Articles | Volume 11, issue 14
Biogeosciences, 11, 3919–3940, 2014

Special issue: Field investigation of ocean acidification effects in northwest...

Biogeosciences, 11, 3919–3940, 2014

Research article 25 Jul 2014

Research article | 25 Jul 2014

Coccolithophores on the north-west European shelf: calcification rates and environmental controls

A. J. Poulton1, M. C. Stinchcombe1, E. P. Achterberg2,3, D. C. E. Bakker4, C. Dumousseaud2, H. E. Lawson1,2, G. A. Lee4, S. Richier2, D. J. Suggett5,6, and J. R. Young7 A. J. Poulton et al.
  • 1National Oceanography Centre, University of Southampton Waterfront Campus, European Way, Southampton, UK
  • 2Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, Southampton, UK
  • 3GEOMAR Helmholtz Centre for Ocean Research, 24148 Kiel, Germany
  • 4Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University of East Anglia, Norwich Research Park, Norwich, UK
  • 5University of Essex, Colchester, Essex, UK
  • 6Plant Functional Biology and Climate Change Cluster, University of Sydney, Sydney, Australia
  • 7University College London, London, UK

Abstract. Coccolithophores are a key functional group in terms of the pelagic production of calcium carbonate (calcite), although their contribution to shelf sea biogeochemistry, and how this relates to environmental conditions, is poorly constrained. Measurements of calcite production (CP) and coccolithophore abundance were made on the north-west European shelf to examine trends in coccolithophore calcification along natural gradients of carbonate chemistry, macronutrient availability and plankton composition. Similar measurements were also made in three bioassay experiments where nutrient (nitrate, phosphate) and pCO2 levels were manipulated. Nanoflagellates (< 10 μm) dominated chlorophyll biomass and primary production (PP) at all but one sampling site, with CP ranging from 0.6 to 9.6 mmol C m−2 d−1. High CP and coccolithophore abundance occurred in a diatom bloom in fully mixed waters off Heligoland, but not in two distinct coccolithophore blooms in the central North Sea and Western English Channel. Coccolithophore abundance and CP showed no correlation with nutrient concentrations or ratios, while significant (p < 0.01) correlations between CP, cell-specific calcification (cell-CF) and irradiance in the water column highlighted how light availability exerts a strong control on pelagic CP. In the experimental bioassays, Emiliania-huxleyi-dominated coccolithophore communities in shelf waters (northern North Sea, Norwegian Trench) showed a strong response in terms of CP to combined nitrate and phosphate addition, mediated by changes in cell-CF and growth rates. In contrast, an offshore diverse coccolithophore community (Bay of Biscay) showed no response to nutrient addition, while light availability or mortality may have been more important in controlling this community. Sharp decreases in pH and a rough halving of calcite saturation states in the bioassay experiments led to decreased CP in the Bay of Biscay and northern North Sea, but not the Norwegian Trench. These decreases in CP were related to slowed growth rates in the bioassays at elevated pCO2 (750 μatm) relative to those in the ambient treatments. The combined results from our study highlight the variable coccolithophore responses to irradiance, nutrients and carbonate chemistry in north-west European shelf waters, which are mediated by changes in growth rates, cell-CF and species composition.

Final-revised paper