Dissolved oxygen dynamics in the Dunnville Marsh on the Grand River, Ontario, Canada

Dissolved oxygen (DO) is one of the most important environmental factors necessary to sustain aquatic life and the overall health of aquatic ecosystems. The Grand River is the largest watershed in southern Ontario. It has been adversely affected by agricultural activities and a growing urban population which has led to periods of the year when oxygen supplies in river waters are extremely low. The southern part of the river near its mouth is unusual compared to upstream reaches by having extensive marsh and open water wetlands along its banks. This study focuses on the Dunnville Marsh, a typical example of wetlands along the southern end of the Grand River. The spatial and temporal variation in dissolved oxygen was studied in the Dunnville Marsh and in a stretch of the Grand River immediately beside it over a one year cycle during 2007 to 2008. Dunnville Marsh exhibited little influence on the oxygen regime of the river. The Grand River; however, could influence the oxygen regime in the marsh during the spring when waters are high but exerts little influence during the rest of the year. There were no great differences in DO between the wetland and the river during the high water spring melt period; however notable differences occurred in the summer and fall.;Oxygen stable isotopes and diel O2 measurements showed that ecological factors probably were influencing the DO cycle in Dunnville Marsh whereas both ecological and weather factors influenced the cycle in the Grand River.;Monthly delta18ODO data from the river revealed a shift towards atmospheric equilibrium compared to the wetland. These data exhibited less photosynthetic and more respiration activity in the fall and more photosynthetic activity during the summer. The wetland showed higher photosynthetic activities in the summer than the river; however, there was discrepancy between the low delta18ODO signature and the accompanied low DO saturation levels in some of the wetlands sites especially in the summer of 2008. The ground water input or the small isotopic changes on overlying water of delta18ODO (small sediment respiration fractionation-factor) could be an explanation for this difference.;Nitrogen input from the agricultural areas was low at most of the time and had minimal influence on the DO in the Dunnville Marsh. Despite low nitrogen input the attenuation ability of the Dunnville Marsh was apparent, presumably due to plant uptake, especially in the northern part of the marsh. The Grand River showed a significant hydrological effect on Dunnville Marsh during the spring when runoff is high. delta18O-water proved to be an effective tool for differentiating river and wetland waters. Based on the delta 18O-water signature in late April (after the flood season) it appears river water extended about two-thirds along the main stream well into Dunnville Marsh. River water, probably inundates a significant part of the Dunnville Marsh in early April (flood peak), when water flow was more than 10 fold higher than later in April following the peak flood season.;At most of the river sites, DO data exhibited subsaturation levels in fall 2007, around saturation in spring 2008, and supersaturation in summer 2008.The opposite was the case for most of the wetlands sites in summer 2008 demonstrating subsaturation levels of DO, around saturation in spring 2008, and DO records ranged from subsaturation to supersaturation in fall 2007.