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

A vertical series of sedimentation traps was placed in a smallreservoir (Guelph Lake, Southern Ontario, Canada) in the springthrough summer of 1981 to intercept floating and sinking phytoplanktoncells. Information from the sedimentation traps was used todetermine the sinking loss rates of diatoms and to estimatethe relative contribution of sedimentation to the loss of thesespecies. 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 thermalstratification. The abundance of Cyclotella meneghiniana declinedas the surface pH increased. The results showed good agreementbetween the proposed sedimentary fluxes of diatoms and the correspondingmaximum standing crop. Information from the trap catches wasalso used to examine the possibility of recruitment of summerblue-green species from the sediments. Specific migration ratesof floating colonies of Microcystis aeruginosa, Gompliosphaerialacustris and of Lyngbya Birgei filsments at 10 m were high(>1) for periods of time in August. For these blue-greenalgae, estimates of the population gain due to recruitment fromthe sediments ranged from 2 to 4% of the maximum standing crop.The high rates of accumulation of total phosphorus (TP) in thedownward facing trap at 10 m provided further evidence thatresuspension of material from the sediments occurred at thistime. The appearance of these blue-green algae cointided withhigh surface temperatures and the development of anoxic conditionsat 10 m. The growth or recruitment of Aphanizomenon flos-aquaewas initiated under different environmental conditions thanthose for Mierocystis, Gomphosphaeria and Lyngbya. Evidencesuggests that the Aphanizomenon filaments present at Station1 originated from the Station 3 end of the lake and were advectedtowards Station 1. ¹Present address: Zoology Department, University of Alberta,Edmonton, Alberta T6G 2E1, Canada. ²Present address: CSIRO Division of Fisheries Research, GPOBox 1538, Hobart, Tasmania, Australia 7001.