Impact of Cu, Ni and Fe on structure and function of a synthetic microcosm, comprising of four algae ( Chlorella vulgaris, Chlamydomonas sp., Anabaena doliolum and Oscillatoria formosa), two arthropods (Cyclops and Cypris) and a protozoan (Paramecium) was tested with respect to nutrient depletion, changes in algal population, chlorophyll a content and carbon fixation. The control (untreated) microcosm registered complete depletion of NO3-and PO43-on day 20, when the algal population had reached its maxima in terms of cell number, chlorophyll a content and carbon fixation. Metal treated microcosm, however, showed a delayed nutrient depletion, reduced algal maxima, and lower chlorophyll a content and carbon fixation rate than the control. Of the four algae tested Chlamydomonas sp. was found to be the most sensitive and Oscillatoria formosa, the most tolerant to all the metals used. The hierarchy of metal toxicity was Cu > Ni > Fe. Cu and Ni combination interacted synergistically in contrast to the antagonism of Cu + Fe and Ni + Fe. Carbon fixation was inhibited most as compared to nutrient depletion, algal number and chlorophyll a content. A comparison of the microcosm results with batch cultures using the same metals and algae revealed a similar pattern, but a reduced degree of inhibition in the former. This study therefore, demonstrates that results of laboratory toxicity bioassays can be extrapolated to the field level though with a reduced precision.