Coccolithophore ecology at the HOT station ALOHA, Hawaii

Cell densities of total coccolithophores and dominant taxa were determined in 183 samples from the upper 200 m of the water column at about monthly in tervals between January 1994 and August 1996 at the HOT station ALOHA, Hawa ii. High cell densities were observed twice a year, in March (up to 41 x 10 (3) cells 1(-1)) and in September/October (up to 52 x 10(3) cells 1(-1)). I n the intervening months, cell densities were extremely low (0-20 x 10(3) c ells 1(-1)), reflecting a strong seasonality. The main production of coccol ithophores took place in the middle photic zone between 50 and 100 m water depth. In total 125 coccolithophore species were identified but only five c onstituted on average more than 30% of the community: Emiliania huxleyi, Um bellosphaera irregularis, U. tenuis, Florisphaera profunda and Gephyrocapsa ericsonii. The generally low, but seasonally dynamic coccolithophore cell density variability is compared with in situ measurements of environmental parameters. Correlation analyses between cell density variability of the do minant taxa and potentially controlling environmental parameters show signi ficant correlation coefficients when the data set was separated into upper and lower photic zone. Cell densities of all dominant taxa are most highly correlated with temperature variability. U. irregularis is positively corre lated in the upper photic zone, whereas E. huxleyi and G. ericsonii are neg atively correlated. In the lower photic zone, F. profunda cell densities ar e positively correlated with light, which corresponds to the maximum bottom -up control (i.e. by physical forcing) of any species encountered. The surp risingly low correlations of cell densities with nitrate and phosphate may be caused by insufficient sampling resolution, nutrient levels close to det ection limits, or both. (C) 2001 Elsevier Science Ltd. All rights reserved.