Plankton are not passive tracers: Plankton in a turbulent environment

Abstract

Spectral analysis was performed on a series of oceanographic transects collected using an optical plankton counter and conductivity‐temperature‐depth probe. The “physical” time series (i.e., temperature) power spectra showed a single passive scaling relationship across the entire range of sampling scales (1–8192 s) that was expected from turbulence. However, the “biological” time series possessed more than one scaling region. The Chl a fluorescence had three scaling regions, a flattened (whitened) intermediate range bound by passive regions at scales approximately <30 and >300 s. Cross‐spectral analysis indicated that the chlorophyll‐temperature spectra were similar at these scales. The zooplankton biomass had a single break in the power spectrum and was passive only at scales >300 s, the zooplankton‐temperature spectra being similar only at these scales. The zooplankton‐chlorophyll cross‐spectra were often negatively coupled at the intermediate (300–30 s) scale giving a strong indication that zooplankton grazing was affecting the phytoplankton distributions.