Seston Distribution and Phytoplankton Production in a New, Eutrophic Reservoir subject to Artificial Mixing

Phytoplankton ecology of a new, artificially mixed Thames Valley Reservoir at Wraysbury, Buckinghamshire, was investigated during the period from late 1971 to early spring 1974. Biological, chemical, and physical aspects and their inter-relationships are discussed in reference to artificial mixing. Nitrates and phosphates were never limiting, and showed little fluctuation with phytoplankton growths. Although silicate concentrations decreased during vernal and autumnal diatom pulses of Stephanodiscus astraea (Ehrenb.) Grun. ≈ Stephanodiscus rotula (Kutz.) Hendey, dissolved silica seemed to be of secondary importance in limiting rates of growth or maximum yields. Particular emphasis was placed on the interactions of the seston, in particular the ratio of productive to non-productive particles as determined by particle frequency-size analyses using Coulter Counter Models A and B with 100 and 200. diameter aperture tubes. It was found by this method that non-productive particles strongly influenced the penetration of solar radiation. Although phytoplankton innocula from the River Thames were of little ecological importance, the turbid river water did alter the under water light ecology of the reservoir by changing the ratio of productive to non-productive particles. The suspension of non-productive particles was dependent on natural and artificial turbulence. Although the direct effects of mixing were the maintenance of vertically homogeneous temperature and oxygen distributions and at most times vertically homogeneous phytoplankton populations, it was the indirect effects of mixing, the suspension of non-productive particles from the seston of the River Thames, which limited growth rates, and interacted with temperature and solar radiation to limit production in the highly eutrophic reservoir.