Predicting phytoplankton composition from space—Using the ratio of euphotic depth to mixed-layer depth: An evaluation

A technique to remotely characterize the taxonomic composition of phytoplankton would have application in several fields of environmental study. Satellite imagery available presently and in the near future will likely not be able to accomplish this, except in unique cases, using spectral methods. As an alternative approach, we empirically evaluated a technique that uses the ratio of euphotic depth (Zeu) to mixed-layer depth (Zm) as a parameter to predict the relative abundance of three major algal groups—diatoms, dinoflagellates, and coccolithophores— in the surface layer of the temperate North Atlantic and North Pacific Oceans. The ratio can be ascertained without in situ measurements; Zeu can be estimated from ocean color imagery, and Zm can be derived from hydrographic models. Diatoms were found to dominate the phytoplankton community, in terms of cell concentration, at stations possessing significantly greater values of the ration Zeu:Zm than those stations where dinoflagellates dominated. This is contrary to the generally accepted view that diatoms occupy less stratified water columns than dinoflagellates. The result, which may merely reflect the data set employed and as such requires further testing, could aid in classifying the phytoplankton on a regional basis. However, we conclude that the use of the ratio Zeu : Zm is not likely to provide a general, nonspectral technique to characterize the taxonomic composition of phytoplankton.