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. Copyright 2006 by the American Geophysical Union.