Effects of acidification and primary production on coccolith weight: Implications for carbonate transfer from the surface to the deep ocean

Abstract

The coccolithophores are not only an important group of oceanic primary producers; they are also one of the main carbonate producers in the ocean. Because the calcium carbonate plates (coccoliths) secreted by these unicellular algae are so small and light (a few picograms each), they cannot be directly weighed, with the result that very little is known about the effect of primary production and dissolution on the coccoliths. Using a new method that allows a rapid estimate of the weight of discrete coccoliths, we analyzed the effect of dissolution. The results of two acidification experiments, using cultured and fossil coccoliths, indicate that acidification does not change significantly the weight and size of coccoliths. On the basis of these experiments, we identify four parameters for identifying the effects of dissolution on a coccolith assemblage. These parameters were applied in samples from two sediment trap time series taken at the same tropical Atlantic site (EUMELI), but separated by 2000 m water depth. There was no evidence of increased dissolution with water depth between 250 and 2500 m. This is indicative of the absence of dissolution of coccoliths between the photic zone and the lysocline. Other studies have identified supra‐lysoclinal dissolution, and we propose that most of this takes place in the photic zone and possibly in the guts of grazers. Most of the coccolith weight changes observed over the duration of trap deployment were related to the occurrence of two coccolithophore blooms which resulted in significant increases in average coccolith weight of the dominant coccolithophore species during the blooms. Changes in growth rate and cell density appear to have a significant effect on the weight of coccoliths. To test this observation, we cultured E. huxleyi at different P and N concentrations. This test confirmed that coccoliths weigh more when coccolithophore production is highest. An important consequence for the carbon pump is that during higher primary production events the coccoliths will increase the ballasting of the aggregates in which they are carried to the seafloor, increasing the rate and amount of organic matter sequestered into the deep sea.