Measurement of in situ growth rates of phytoplankton under conditions of simulated turbulence

Abstract

In situ measurement of the growth rates of planktonic populations can be improved by using dialysis chambers ('cage cultures') to avoid shifts in the chemical environment during incubation. Vertical mixing and small-scale turbulence affect the growth of planktonic populations, therefore natural mixing conditions should be simulated as closely as possible during the incubation. A new device is described here which combines the advantages of a dialysis chamber with a programmable vertical mixing regime. Realistic phytoplankton growth rates can thus be measured in situ under conditions of vertical mixing and small-scale turbulence. The chamber made of transparent, UV-transmitting acrylic glass was fitted at both ends with permeable polycarbonate membranes. It was moved vertically through the water column by a pocket-sized lift and rotated simulataneously on its central axis. The method was applied to two types of experiments on growth and losses of phytoplankton in the River Severn, UK. The first one compared changes in biovolume of phytoplankton in a water parcel flowing downstream (6% h-1 decline) with those in a simultaneously incubated dialysis chamber moved between water surface and river bottom (7% h-1 increase). The difference equates to algal losses prevented in the chamber but suffered along the river (mainly sedimentation and grazing of benthic filter feeders). Loss rate of diatoms was three times higher than those of chlorophytes. In another experiment growth of phytoplankton from the main stream and lateral dead zone was compared under different mixing conditions. Algae from the main stream grew faster than from the dead zone. Only cryptophytes preferred calm conditions, all the other algal groups grew faster in chambers moved through the water column than in stationary ones. Further possible applications in both standing and flowing waters are discussed.