Estimation of phytoplankton decomposition rates using two-stage continuous flow studies

Abstract

A two-stage continuous flow (TSCF) system was used to quantify phytoplankton decomposition rates and to characterize the decay process as a function of phytoplankton species, phytoplankton physiological state, and the presence and character of a decomposer community. A TSCF system allowed a steady-state phytoplankton culture, grown in the first stage, to be the feed to a darkened second stage decomposition culture. A mass balance on the phytoplankton biomass in the second stage quantitatively assessed the magnitude of the decomposition rate. The results of 12 experimental runs revealed that the relative order of decay rates among three phytoplankton species tested was Microcystis (blue-green) > Asterionella (diatom) > Scenedesmus (green). The results also suggest a strong dependence of decay rates on the presence and origin of the decomposer community in the second stage. A decomposer community originating from a hypereutrophic lake accounted for higher decay rates than did an inoculum from mesotrophic waters of Lake Ontario. Phytoplankton cultures not receiving a poser inoculum exhibited low decay rates indicative of endogenous respiration. Finally, two concurrently run TSCF experiments, where the only difference was the dilution rate in the lit first stage, illustrated that sub-optimal growth rates of phytoplankton result in a stress condition that renders the population more susceptable to death and decomposition.