Particles in "Particle-free" Seawater: Growth of Ultraphytoplankton and Implications for Dilution Experiments

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Abstract
Ultraphytoplankton cells were detected by flow cytometry in seawater that passed through filters of the finest pore size commonly used in aquatic research (0.22-μm Millipore®, 0.45-μm Millipore®, 0.2-μm Nuclepore®, Whatman®GF/F). Although the relative number of cells in freshly prepared filtrate was very small (generally less than 1% of the intact assemblage), the number of such cells increased rapidly during incubation. The implication of this apparent growth was investigated in dilution experiments where diluent was mixed with unfiltered seawater in varying proportions. Plots of apparent rate of change versus dilution factor were generally hyperbolic, with high rates at high dilutions. Simple calculations indicated that fluorometric measurements of bulk chlorophyll a would not have been able to detect these changes.