A biomarker perspective on coccolithophorid growth and export in a stratified sea

Summer cruises to the Gulf of California and adjacent Eastern Tropical North Pacific between 2004 and 2008 provided data on non-bloom coccolithophorid abundance and production in nitrate-depleted surface waters. Using lipid biomarkers specific to the dominant coccolithophorids in this region, it was found that these organisms routinely exhibited maximum production rates at depths associated with subsurface chlorophyll features and the nitracline, as opposed to the N-depleted surface. Estimates of integrated coccolithophorid production, relative to bulk primary production, showed no systematic relationship to availability of the limiting macronutrient (nitrate) in the euphotic zone. Our observations suggest that, in sufficiently well-stratified settings, the nutrient-depleted and nutrient-replete portions of the euphotic zone may become so sharply partitioned and temporally stable that a coccolithophorid-favorable ‘mid-to-low nutrient’ niche is absent. Consequently, the relative abundance and production rate of coccolithophorids are low, and the relative contribution of alkenones to carbon export is driven by variability in the significantly larger bulk carbon component. Study of this region provides important insights into the ecology of these calcifying organisms in a warm, quiescent ocean. Our findings suggest that, if water columns akin to those surveyed become more common, coccolithophorid-derived inorganic carbon export may serve as a weaker positive feedback on atmospheric CO2 than previously suggested.