This study examined the responses of algae and invertebrates to a single application of nutrients in a series of experimental wetland enclosures in the Interlake Region of Manitoba during 1989 and 1990. Water, sediment and vegetation chemistry were also monitored. The 3 fertilization treatments were: dissolved inorganic high (6200 $ mu$g/l N, 420 $ mu$g/l P), dissolved inorganic low (3200 $ mu$g/l N, 210 $ mu$g/l P) and organic high (alfalfa: 6200 $ mu$g/l N, 420 $ mu$g/l P). / Dissolved nutrients in the high and low treatments were quickly depleted from the system but dissolved N increased in the alfalfa treatment as decomposition progressed. No changes in sediment or vegetation chemistry were detected. Phytoplankton biomass increased in all the fertilized enclosures while epiphytic periphyton exhibited only minor responses. Epipelon biomass increased in the alfalfa treatment and metaphyton standing crops were extensive in the high treatment enclosures. / In the alfalfa treatment, high microbial respiration rapidly decreased dissolved oxygen concentrations which negatively affected invertebrates. This trend reversed as oxygen levels increased. Dominant nektonic and benthic herbivores-detritivores increased in the high and alfalfa treatment enclosures. Orthocladiinae emergence increased in the high and alfalfa treatments while Chironominae and Tanypodinae increased in the alfalfa treatment. Responses by algae and invertebrate communities to the fertilization treatments were minimal during 1990. Annual single pulse fertilization has the potential to increase the productivity of Interlake wetlands.

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