The importance of algae and different algal species to the chironomid diet was investigated through examination of the guts of chironomids taken from 5 locations in Big Bay (Bay of Quinte, Lake Ontario) at 6 times during the ice-free period . A comparison of the density and condition of algal cells at the two ends of the gut provided information on the digestibility of each algal species and the algal biomass assimilated by the chironomids . Diatoms and two species of green algae were assimilated over much of the summer, but the blue green algae were digested only late in the season when their populations were aged and/ or dying .
The shortcomings of many established toxicity criteria for metals have resulted from a simplification of bioassays performed with single metals . A more comprehensive approach was needed to diagnose the effects of metal mixtures on aquatic organisms . Using Chlorella, Ankistrodesmus and Scenedesmus as test organisms, we experimented on a number of factors which affected metal toxicity bioassays. These included metal interactions, algal competitions, species sensitivities, the ratio of an excess metal to other metals and nutrient levels . An alternative technique was finally established which involved an evaluation of individual assays of Chlorella and Ankistrodesmus in separate media and a tissue-metal analysis on Chlorella. Chlorella fusca, commonly found in lakes with high metal concentrations, showed high tolerance to mixed-metal solutions, while Ankistrodesmus proved to be very sensitive . By determining the maximum yield ratio between Ankistrodesmus and Chlorella (i .e . the A/Ch ratio) it was possible to compare the toxic strength of harmful metals according to an established standard curve of A/Ch ratio versus mixed-metal concentrations . The levels of tissue-metal analysed in Chlorella also gave some indication as to which metals were responsible for the toxicity .
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