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

[摘要] Coccolithophores are a key functional group in terms of the pelagicproduction of calcium carbonate (calcite), although their contribution toshelf sea biogeochemistry, and how this relates to environmental conditions,is poorly constrained. Measurements of calcite production (CP) andcoccolithophore abundance were made on the north-west European shelf toexamine trends in coccolithophore calcification along natural gradients ofcarbonate chemistry, macronutrient availability and plankton composition.Similar measurements were also made in three bioassay experiments wherenutrient (nitrate, phosphate) and pCO₂ levels were manipulated.Nanoflagellates (< 10 μm) dominated chlorophyll biomass andprimary production (PP) at all but one sampling site, with CP ranging from0.6 to 9.6 mmol C m⁻² d⁻¹. High CP and coccolithophore abundanceoccurred in a diatom bloom in fully mixed waters off Heligoland, but not in two distinct coccolithophore blooms in the central North Sea and WesternEnglish Channel. Coccolithophore abundance and CP showed no correlation withnutrient concentrations or ratios, while significant (p < 0.01)correlations between CP, cell-specific calcification (cell-CF) and irradiancein the water column highlighted how light availability exerts a strongcontrol on pelagic CP. In the experimental bioassays, Emiliania-huxleyi-dominated coccolithophore communities in shelf waters (northernNorth Sea, Norwegian Trench) showed a strong response in terms of CP tocombined nitrate and phosphate addition, mediated by changes in cell-CF andgrowth rates. In contrast, an offshore diverse coccolithophore community (Bayof Biscay) showed no response to nutrient addition, while light availability or mortality may have been more important in controlling thiscommunity. Sharp decreases in pH and a rough halving of calcite saturationstates in the bioassay experiments led to decreased CP in the Bay of Biscayand northern North Sea, but not the Norwegian Trench. These decreases in CPwere related to slowed growth rates in the bioassays at elevated pCO₂(750 μatm) relative to those in the ambient treatments. Thecombined results from our study highlight the variable coccolithophoreresponses to irradiance, nutrients and carbonate chemistry in north-westEuropean shelf waters, which are mediated by changes in growth rates, cell-CFand species composition.

[学科分类] 地球化学与岩石