Coccolithophore dynamics in non‐bloom conditions during late summer in the central Iceland Basin (July‐August 2007)

Abstract

Measurements of primary production (PP), calcification (CF), and coccolithophore abundance were made during late summer (July‐August 2007) in the Iceland Basin. Low numbers of coccolithophore cells and detached coccoliths (< 1 × 103 cells mL−1 and 1‐15 × 103 coccoliths mL−1, respectively) indicated a non‐bloom community, with Emiliania huxleyi as the dominant coccolithophore in terms of abundance, coccolithophore organic biomass, and cell calcite. PP ranged from 0.1 to 2 [mol C m−3 d−1, while CF ranged from 10 to 250 [mol C m−3 d−1, with both typically decreasing with depth. Coccolithophores were estimated to contribute 10‐20% toward total chlorophyll a, phytoplankton carbon, and PP within the euphotic zone. In these non‐bloom conditions, ~ 30‐60% of the total calcite in the water column was present as detached coccoliths rather than whole cells. Both cell numbers and variability in cell‐normalized CF controlled the magnitude of total CF, and hence both physiological limits to cell CF and growth, as well as mortality factors, need to be taken into account when examining oceanic coccolithophore communities. Combining cell‐normalized CF with an estimate of coccolith calcite gave coccolith production rates (0.4‐1.8 h−1) similar to those reported in the literature for laboratory cultures of E. huxleyi. None of the factors currently associated with coccolithophore blooms (irradiance, mixed‐layer depth, nitrate, phosphate, or calcite saturation) showed a clear correlation with community or cellular CF. Hence, although mortality is likely to control cell numbers, other factors such as trace metal (iron) availability may influence coccolithophore physiology in the central Iceland Basin during late summer.