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

In situ measurement of the growth rates of planktonic populationscan 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 growthof planktonic populations, there fore natural mixing conditionsshould be simulated as closely as possible during the incubation.A new device is described here which combines the advantagesof a dialysis chamber with a programmable vertical mixing regime.Realistic phytoplankton growth rates can thus be measured insitu under con ditions of vertical mixing and small-scale turbulence.The chamber made of transparent, UV-transmitting acrylic glasswas fitted at both ends with permeable polycarbonate membranes.It was moved vertically through the water column by a pocket-sizedlift and rotated simultaneously on its central axis. The methodwas applied to two typos of experiments on growth and lossesof phytoplankton in the River Severn, UK. The first one comparedchanges in biovolume of phytoplankton in a water parcel flowingdownstream (6% h^–1 decline) with those in a simultaneouslyincubated dialysis chamber moved between water surface and riverbottom (7% h^–1 increase). The difference equates to algallosses 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 mainstream and lateral dead zone was compared under different mixingconditions. Algae from the main stream grew faster than fromthe dead zone. Only cryptophytes preferred calm conditions,all the other algal groups grew faster in chambers moved throughthe water column than in stationary ones. Further possible applicationsin both standing and flowing waters are discussed.