Phycoerythrin-containing picocyanobacteria in the Arabian Sea in February 1995: diel patterns, spatial variability, and growth rates

The abundance of phycoerythrin-containing picocyanobacteria in the surface mixed layer was measured both along-shore and offshore between 8 and 23 Feb ruary 1995 in the Northwestern Arabian Sea. Water samples from 3m depth wer e taken at 2-h intervals and picocyanobacterial abundance and frequency of dividing cells were determined by epifluorescence microscopy, Cell counts s howed an average diel change from a mid-day minimum of similar to 50 x 10(3 ) cells ml(-1) to an evening maximum of similar to 180 x 10(3) cells ml(-1) . The diel change was greater than the differences observed between physica lly and spatially discrete water masses. By counting the frequency of divid ing cells (FDC) and using a novel approach to estimating the length of time required to complete cell division, growth and loss rates were both estima ted to be similar to 2.9 d(-1) with daily turnover being 140% of the mean s tanding stock. If differences in the intrinsic population growth rate (mu) and the net rate of change in cell number (r) are assumed to be due to graz ing, then grazing occurred throughout the day at a relatively constant rate (reflecting phytoplankton loss rates of similar to 0.12h(-1)). Cell divisi on rates peaked in the late afternoon and early evening. FDC decreased thro ughout the night, suggesting that dark-inhibition of cell division is weak or nonexistent in the picocyanobacteria we studied. While all cell types in cluded in this study would be identified as Synechococcus by flow cytometry because they were small unicells with bright phycoerythrin fluorescence, m orphological variability suggests that the community was actually taxonomic ally diverse and included cells other than Synechococcus, including Synecho cystis. Despite this diversity, the strong diel patterns we observed persis ted throughout the study region, suggesting that great care should be taken when interpreting picocyanobacterial survey data and experimental results that do not account for the effects of time-of-day. (C) 2001 Elsevier Scien ce Ltd. All rights reserved.