Phytoplankton population dynamics of a small reservoir: use of sedimentation traps to quantify the loss of diatoms and recruitment of summer bloom-forming blue-green algae

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Abstract
A vertical series of sedimentation traps was placed in a small reservoir (Guelph Lake, Southern Ontario, Canada) in the spring through summer of 1981 to intercept floating and sinking phytoplankton cells. Information from the sedimentation traps was used to determine the sinking loss rates of diatoms and to estimate the relative contribution of sedimentation to the loss of these species. Sinking loss rates varied with time and with depth. Sinking loss rates for Stephenodiscus astrea, Melosira granulata, and Asterionella formosa increased with the onset of thermal stratification. The abundance of Cyclotella meneghiniana declined as the surface pH increased. The results showed good agreement between the proposed sedimentary fluxes of diatoms and the corresponding maximum standing crop. Information from the trap catches was also used to examine the possibility of recruitment of summer blue-green species from the sediments. Specific migration rates of floating colonies of Microcystis aeruginosa, Gompliosphaeria lacustris and of Lyngbya Birgei filsments at 10 m were high (>1) for periods of time in August. For these blue-green algae, estimates of the population gain due to recruitment from the sediments ranged from 2 to 4% of the maximum standing crop. The high rates of accumulation of total phosphorus (TP) in the downward facing trap at 10 m provided further evidence that resuspension of material from the sediments occurred at this time. The appearance of these blue-green algae cointided with high surface temperatures and the development of anoxic conditions at 10 m. The growth or recruitment of Aphanizomenon flos-aquae was initiated under different environmental conditions than those for Mierocystis, Gomphosphaeria and Lyngbya. Evidence suggests that the Aphanizomenon filaments present at Station 1 originated from the Station 3 end of the lake and were advected towards Station 1.

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