Phytoplankton growth rates estimated from depth profiles of cell concentration and turbulent diffusion

Abstract

We develop a theory for estimating the net population growth rate (reproductive rate minus mortality rate) of a phytoplankton population in situ from biological observations of cell concentrations and physical measurements of turbulance. High resolution vertical profiles of cell concentration and turbulent diffusion rates from the central North Atlantic measured on successive days were used to study depth-dependent growth rates for categories of ultraphytoplankton defined by flow cytometry. The four categories and their equivalent spherical diameters were cyanobacteria (≈1 μm), the abundant newly discovered very small red fluorescing bodies (prochlorophytes <1 μm), small eukaryotes (<2 μm) large eukaryotes (mean 2–4 μm, mode 2–3 μm). For all groups, the highest net population growth rates occurred in areas of peak cell concentration near the top of the nitracline and the bottom of the euphotic zone around the 1% light level. Cyanobacteria grew most rapidly between 85 and 95 m, whereas eukaryotes grew most rapidly between 100 and 115 m; the maximum population growth rates were 0.48 ± 0.07 d −1 for cyanobacteria, and 1.43 ± 0.32 d −1 and 0.20 ± 0.03 d −1 for small and large eukaryotes, respectively. Oriented motility toward the region of peak cell concentration may have contributed to the apparently high maximum growth rate of the small eukaryotes. The most negative rates of population growth occurred adjacent to the regions of peak cell concentration, consistent with previous observations of densest concentrations of zooplankton grazers around and above the deep chlorophyll maximum. Below about 150 m, at light intensities <0.1% of surface values, cell concentrations of all ultraphytoplankton groups decreased exponentially with depth, corresponding to net population growth rates ranging from −0.5 to −2.0 d −1 in this region.