Effects of herbicide mixtures on microbial communities from a prairie wetland ecosystem

The Prairie Pothole wetlands of Saskatchewan and Manitoba serve an important ecological role in providing wildlife habitat, water storage, and water filtration and display a wide range of water quality parameters such as salinity and nutrients. These wetlands are regularly interspersed among agricultural operations where multiple pesticides are commonly used. This dissertation investigated the effects of glyphosate, an auxin-type herbicide mixture (2,4-D, MCPA, clopyralid, dicamba, dichlorprop, mecoprop), and a mixture of eight herbicides (including auxin-type herbicides, bromoxynil, and glyphosate) on pelagic and biofilm microbial communities. Three different experimental approaches were used including mesocosms, enclosures, and curtained whole wetlands. Effect assessment indices included: primary productivity, chlorophyll a content, bacterial productivity and numbers, protein and carbohydrate concentrations, bacterial carbon source utilization patterns, and algal pigment profiles. In the mesocosm experiment, effects of glyphosate as well as two herbicide mixtures (as noted above) were investigated. The glyphosate concentration utilized was 1000 times the environmentally relevant concentration (ERC). One herbicide mixture consisted of six auxin-type herbicides (listed above), each at 1000 times ERC. For the second mixture (eight herbicides: listed above), a dose-response approach was used with the ERCs of each herbicide as the base concentration. Results indicated that the eight herbicides mixture, even at low concentrations, produced effects on microbial communities. Glyphosate treatment suppressed algal productivity in both pelagic and biofilm communities. Auxin-type herbicide mixture, in general, had stimulatory effect on algae. This study indicated that glyphosate is more toxic to pelagic and biofilm wetland algal communities than the auxin-type herbicide mixture. To further investigate effects of the eight herbicides mixture (same as above) at maximum-exposure concentrations on microbial communities in ponds varying in salinity and nutrients, four wetlands (1 freshwater and 3 saline) were selected. Six enclosures (3 controls and 3 treatments) were installed in each pond. Results demonstrated that the herbicide mixture had a stimulatory effect on primary productivity in the nutrient-sufficient freshwater pond while no stimulatory effect was observed in the nutrient-deficient saline ponds. For the curtained wetland experiment, effects of eight herbicides mixture (same as previous) on microbial communities were investigated in an ephemeral and a semi-permanent wetland. Herbicide treatment at maximum-exposure concentration stimulated primary productivity in the ephemeral wetland likely due to the hormonal effect of auxin-type herbicides present in the mixture. In contrast, suppression of primary productivity (herbicidal effects) during the first week post-treatment was noted in the semi-permanent wetland, possibly a result of a concentration addition effect of the auxin-type herbicides. Biofilm bacterial carbon source utilization patterns and pigment profiles suggested a change in the community structure in both wetlands. This dissertation demonstrated the effects of herbicide mixtures on microbial communities using three different experimental approaches as well as in different types of wetlands