Nektonic invertebrate abundance and distribution at the emergent vegetation-open water interface in the Delta Marsh, Manitoba, Canada

The emergent vegetation-open water interface plays an important role in the use of freshwater wetlands by many species of animals. The objective of this study was to monitor the distribution and abundance of free-swimming invertebrates at the cattail (Typha x glauca Godr.)-open water interface in two bays (sheltered and unsheltered) in the Delta Marsh in south-central Manitoba during 1985. Submersed vegetation densities were higher in the open water sites of the sheltered bay during most of the summer. Water temperatures were constant across the interface through much of the year. By mid-summer, hypoxic conditions prevailed within the water column in the cattail stand. Algae levels were similar across the interface throughout much of the year, with peaks occurring just inside the interface in June and July. Early in the season, there was a trend for higher nektonic invertebrate numbers within the cattail stands of both bays. By late August, there were similar total invertebrate numbers across the interface. Cladocera and ostracod numbers were higher within the cattail stands during June and July; during the same periods, corixid levels were higher in the open water sites. The low oxygen levels within the cattail stands during the summer may reduce predation by fish and other predators that cannot tolerate low oxygen levels.