We examined stressors that have led to profound ecological changes in the Lake Ontario ecosystem and its fish community since 1970. The most notable changes have been reductions in phosphorus loading, invasion by Dreissena spp., fisheries management through stocking of exotic salmonids and control of sea lamprey (Petromyzon marinus), and fish harvest by anglers and double-crested cormorants (Phalacrocorax auritus). The response to these stressors has led to (i) declines in both algal photosynthesis and epilimnetic zooplankton production, (ii) decreases in alewife (Alosa pseudoharengus) abundance, (iii) declines in native Diporeia and lake whitefish (Coregonus clupeaformis), (iv) behavioral shifts in alewife spatial distribution benefitting native lake trout (Salvelinus namaycush), threespine stickleback (Gasterosteus aculeatus), and emerald shiner (Notropis atherinoides) populations, (v) dramatic increases in water clarity, (vi) predation impacts by cormorants on select fish species, and (vii) lake trout recruitment bottlenecks associated with alewife-induced thiamine deficiency. We expect stressor responses associated with anthropogenic forces like exotic species invasions and global climate warming to continue to impact the Lake Ontario ecosystem in the future and recommend continuous long-term ecological studies to enhance scientific understanding and management of this important resource. 490Résumé : On trouvera ici un examen des facteurs de stress qui ont modifié profondément l'écosystème du lac Ontario et sa communauté de poissons depuis 1970. Les changements les plus importants ont été la réduction de l'apport de phosphore, l'invasion des Dreissena spp., la gestion de la pêche, notamment l'empoissonnement de salmonidés exotiques et le contrôle de la grande lamproie marine (Petromyzon marinus), ainsi que la récolte des poissons par les pêcheurs sportifs et les cormorans à aigrette (Phalacrocorax auritus). La réaction à ces facteurs a eu pour conséquen-ces: (i) le déclin de la photosynthèse des algues et de la production du zooplancton épilimnétique, (ii) la diminution de l'abondance du gaspareau (Alosa pseudoharengus), (iii) la réduction des Diporeia indigènes et des grands corégonesCan.

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The paper is based on the study of a bloom of Eudorina elegans and Trinema lineare which occurred in a pond in April, 1963. The organisms showed some interesting pattern of vertical distribution .The pond is more or less 0 .25 million sq. m in surface area and is about 3 m deep . It receives large quantities of sewage through a canal all the year round from the city of Hyderabad which is more or less at 10 km distance. Eichhornia crassipes SOLMS covers almost the entire surface of the pond .On 20th April, 1963, when the bloom was noticed a series of samples were collected from surface, middle and bottom layers at the interval of 2 hours . The samples from lower layers were collected with the help of a flask fitted with an outlet and an inlet tube . As the depth is very small, the opening and closing of the apparatus did not involve much difficulty and could be manipulated with hand . The algal material was preserved in 4 % formalin .The temperatures of the different strata of water were recorded with the help of a thermometer inserted into a thermos flask with an inlet and an outlet tube . The biological samples were allowed to sediment for a period of one month in the glass columns of the pattern described by WELCH (1948) . The frequencies of the genera were calculated from the counts of their individuals occurring in 0 .25 ml of the sedimented sample . The results are reported as the frequency of the species occurring in one litre of the original sample .

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