Size-specific growth and grazing rates for picophytoplankton in coastal and oceanic regions of the eastern Pacific

Estimates of growth and grazing mortality rates for different size classes and taxa of natural picophytoplankton assemblages were measured in mixed-layer experiments conducted in 3 regions of the eastern Pacific: the California Current Ecosystem, Costa Rica Dome, and equatorial Pacific. Contrary to expectation, size-dependent rates for cells between 0.45 and 4.0 μm in diameter showed no systematic trends with cell size both in and among regions. For all size classes, mean ± SD growth rates ranged from –0.70 ± 0.17 to 0.83 ± 0.13 d−1 and grazing rates between –0.07 ± 0.13 and 1.17 ± 0.10 d−1. Taxon-specific growth rates for Prochlorococcus ranged from 0.17 ± 0.12 to 0.59 ± 0.01 d−1, for Synechococcus from 0.68 ± 0.03 to 0.97 ± 0.04 d−1, for picoeukaryotes from 0.46 ± 0.13 to 1.03 ± 0.06 d−1, and for all cells combined between 0.45 ± 0.03 and 0.65 ± 0.02 d−1. For grazing, Prochlorococcus rates ranged between 0.02 ± 0.12 and 0.66 ± 0.02 d−1, Synechococcus rates between 0.24 ± 0.08 and 0.92 ± 0.05 d−1, for picoeukaryotes between 0.19 ± 0.10 and 0.78 ± 0.09 d−1, and for all cells between 0.16 ± 0.05 and 0.75 ± 0.02 d−1. When comparing rates among taxa, only Prochlorococcus had consistently lower rates than Synechococcocus in all regions. No other trends were apparent. Temperature relationships based on the Metabolic Theory of Ecology were able to explain more of the variability among grazing rates than among growth rates for each taxon considered.