NUTRIENT ADDITIONS TO WETLANDS IN THE INTERLAKE REGION OF MANITOBA, CANADA - EFFECTS OF A SINGLE-PULSE ADDITION IN SPRING

This study examined the responses of algae and invertebrates to a sing le application of N and P in a series of experimental wetland enclosur es in the Interlake region of Manitoba during 1989 and 1990. N and P l evels in the water, sediment and vegetation were also monitored. The 3 fertilization treatments were: dissolved inorganic (6200 mug l-1 N, 4 20 mug l-1 P), dissolved inorganic (3200 mug l-1 N, 210 mug l-1 P) and organic (ground alfalfa meal: 6200 mug l-1 N, 420 mug l-1 P). Dissolv ed nutrients in the inorganic treatments were quickly depleted from th e water column, but dissolved N increased in the water column of the a lfalfa treatment as the alfalfa decomposed. No changes in N or P conce ntrations in the sediments or vegetation were detected. Phytoplankton biomass increased in all fertilized enclosures while epiphytic periphy ton exhibited only minor responses. Epipelon biomass increased in the alfalfa treatment and metaphyton standing crops were highest in the hi gh inorganic treatments. In the alfalfa treatment, high microbial resp iration rapidly reduced dissolved oxygen concentrations which negative ly affected invertebrates. This trend reversed as oxygen levels increa sed later in the experiment. Dominant nektonic and benthic invertebrat es increased in the high inorganic and alfalfa treatments. Orthocladii nae emergence increased in the high inorganic and alfalfa treatments, while Chironominae and Tanypodinae increased in the alfalfa treatment. Second year responses by algae and invertebrate communities to the fe rtilization treatments were minimal. Annual single pulse fertilization has the potential to increase the productivity of Interlake wetlands when nutrients are applied in the spring, however it should be noted t hat at the levels used in this study the effects did not extend to the second year.