Diel cycles in surface waters of the equatorial Pacific

Diel variations in the abundance, cellular light scattering, and fluorescence of picophytoplankton (Prochlorococcus and Synechococcus) and ultraphytoplankton in the equatorial Pacific are well-documented. Cycles of abundance are driven primarily by cell division, which in these groups generally occurs in the late afternoon or at night. Cell division is well-phased within each group, but its timing varies to some extent among groups and between different depths and locations. Grazing mortality acts to reduce abundance over the course of the day and can itself vary over the diel cycle, although a consistent picture of this variation has not yet emerged. Diel cycles in cellular light scattering appear to be largely driven by cell size changes, which reflect the balance between cell growth and division. Decreases in light scattering generally coincide with increases in cell number, and (in the case of Prochlorococcus) with periods of active cell division as determined by cell cycle analysis. Diel variability in cellular fluorescence is most apparent in Prochlorococcus and Synechococcus, and appears to be driven by changes in both pigment content (mediated by pigment synthesis and cell division) and light-dependent non-photochemical fluorescence quenching. Diel variations in bulk fluorescence and beam attenuation are consistent (at least qualitatively) with the observed cycles of cellular abundance, light scattering, and fluorescence. Direct and quantitative assessments of the relative contributions of variations in cell abundance, pigment quota, and fluorescence yield among phytoplankton groups to diel cycles in bulk fluorescence are not yet available. In the case of beam attenuation a few such quantitative assessments have been made, and although the results are not all in agreement, diel changes in the size and abundance of Prochlorococcus and ultraphytoplankton appear to explain the observed diel variation in beam attenuation to a large degree. Estimates of primary production and phytoplankton growth rate can be derived from this diel variation, but owing to the assumptions they incorporate, these estimates should be interpreted with caution. (C) 2002 Elsevier Science Ltd. All rights reserved.