Uncovering regional variations in the balance of physical and biological controls on phytoplankton ecology with underway flow cytometry

The balance of physical and biological controls on phytoplankton abundance and community structure remains an open question in biological oceanography. In the simplest case, where physical dynamics are the dominant control on biological distributions, the scales of variation of biological properties should match physical scales. However, we might also expect alternative scenarios where biological processes are the dominant control on biomass distributions and community structure, in which case physical and biological scales of variation would diverge. Here we test this hypothesis with an extensive, global dataset of continuously collected flow cytometry and underway data. We examine the relationship between the decorrelation length scales of physical and biological quantities. Although we often find that biological and physical scales coincide (i.e. physical processes dominate), there are large areas where physical and biological scales are significantly different. Our dataset spans a wide range of physical regimes, from the relatively stable subtropical gyres to the highly dynamic Kuroshio Extension region. We use eddy kinetic energy as a proxy for this range of physical activity, and explore how it relates to regions where physical and biological scales of variation either coincide or diverge.