Growth- and Loss-Rate Responses of Phytolankton to Intermittent Artificial Mixing and their Potential Application to the Control of Planktonic Algal Biomass

Abstract

The dynamic responses of individual species and of the total biomass of phytoplankton in a large, limnetic enclosure (Lund Tube C, Blelham Tarn, England [UK]) to an artificially-imposed, 3- to 4-wk cycle of alternations in column mixing and restratification are described. Inorganic nutrients were added to maintain a high fertility and to avert nutrient limitation of algal growth. Diatoms, desmids and Oscillatoria achieved rapid rates of growth during mixing which then declined or were overtaken by loss rates during quiescent, restratifying episodes; Eudorina, Sphaerocystis grew well during quiescence but declined during mixing episodes. The growth rates of other summer species (of Anabaena, Ceratium, Volvox, Microcystis) were depressed during mixing, though they generally maintained their existing biomass. By thus controlling the biomass of fast-growing (r-selected) species and delaying the crop maxima of the slower-growing (K-selected) summer species, the total standing crop of phytoplankton was kept below 40 .mu.g chla [chlorophyll a] l-1 until late Aug. The mean April-Sept. biomass (equivalent to 15.5 .mu.g chla l-1) was well below the mean nutrient-carrying capacity (> 45 .mu.g l-1). Possible applications of intermittent mixing to reduce the biomass of phytoplankton in eutrophic storage reservoirs are discussed. At present, the potential water-quality improvements to be gained by intermittent operation of existing destratification systems seen insufficient to outweigh increased costs of monitoring and control. Continuous, low-intensity mixing is more likely to achieve cost-effective improvements in water quality. Further studies on the dynamic responses of reservoir phytoplankton to artificial mixing are suggested.