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

A J Poulton,E P Achterberg,C Dumousseaud,S Richier,H E Lawson,M C Stinchcombe,G A Lee,J R Young,D J Suggett,D C E Bakker

doi:10.5194/bg-11-3919-2014

Abstract

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.

Highlights

High cellular levels of calcite production in coccolithophores, maintained through the rapid production of individual cellular plates of calcite, facilitate this group with a strong influence on the marine carbon cyclePublished by Copernicus Publications on behalf of the European Geosciences Union.A
A number of distinct hydrographic environments were sampled around the NW European Shelf, including the open ocean (BB, PEACE site (PEA)), shelf-break (MRf, Atlantic coast (Atl), Shetland Islands (Sh), south of the Faroe Islands (sFI)), seasonally stratified (Cel, E1, North Sea (NS)) and fully mixed (Figs. 1 and 2) environments
Open-ocean sites generally had the deepest mixed layers (> 45 m), while mixed layer depths were similar for shelf-break and stratified sites (< 30 m) and fully mixed sites were mixed to the seafloor (∼ 40 m) (Table 1; Fig. 2)

Summary

Introduction

High cellular levels of calcite production in coccolithophores, maintained through the rapid production of individual cellular plates of calcite (coccoliths), facilitate this group with a strong influence on the marine carbon cycleA. High cellular levels of calcite production in coccolithophores, maintained through the rapid production of individual cellular plates of calcite (coccoliths), facilitate this group with a strong influence on the marine carbon cycle. Poulton et al.: Coccolithophores on the north-west European shelf through the production and export of calcite, as well as modification of air–sea carbon dioxide (CO2) fluxes (Holligan et al, 1993a). Coccolithophores are globally distributed, from the subpolar Arctic to the Antarctic and from the open ocean to shelf seas. Many coccolithophore species (e.g. Emiliania huxleyi, Gephyrocapsa muellerae) have cell diameters of 5–10 μm, making them a potentially important component of the nanoflagellate ( < 10 μm) community

Objectives

Methods

Results

Discussion

Conclusion