Changes in the Emergent Plant Community of Netley-Libau Marsh Between 1979 and 2001
Grosshans, Richard E.
Wrubleski, Dale A.
Goldsborough, L. Gordon
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We used aerial photography combined with field observations to develop a detailed aquatic vegetation map for Netley-Libau Marsh in south-central Manitoba. This report describes the creation of a new geographically accurate map (georeferenced for use in a Geographic Information System - GIS), based on aerial photos taken in 2001, and construction of a detailed vegetation map for evaluating the changing state of Netley-Libau Marsh. This provides a basis for comparison with a 1979 vegetation map enabling a quantitative assessment of changes in the marsh over a 22-year period. Comparisons between 1979 and 2001 reveal several significant changes in Netley-Libau Marsh. Loss of emergent vegetation and the erosion of separating uplands between adjoining water bodies has been extensive, resulting in the amalgamation and expansion of many marsh bays and ponds. Currently, half of the entire marsh (13,125 ha, 51%) is open water, compared to 35% (8,884 ha) in 1979. Cattail (Typha spp.) continues to be the dominant emergent plant in the marsh, showing little change between surveys. However, hard- and soft-stem bulrush (Schoenoplectus spp.) have declined ten-fold in abundance, from 3,247 ha (13%) to 317 ha (1%). The mixed river bulrush and sedge community, along with the wet meadow communities, have also declined in abundance. Plant communities at drier sites, however, have remained relatively unchanged. Reasons for the observed changes in the marsh are not well known or understood, but change is not a recent development. Maps of the marsh from the 1920s to the present show a pattern of increasing open water area and loss of upland and island habitats. These changes are likely related to a number of factors, but the influence of Lake Winnipeg and the Red River are likely the most important. Lake Winnipeg dictates water levels within Netley-Libau Marsh. Since the droughts of the 1930s and 1940s, water levels on Lake Winnipeg and the marsh have included few intervening dry periods. Without extended dry periods, to periodically allow the germination of new emergent vegetation, there has been a slow but consistent loss of emergent vegetation in the marsh. As this vegetation is lost, the protection that it provides for the soft sediments that make up island and upland habitats is also lost, and these habitats are slowly being washed away. The current management of Lake Winnipeg for hydroelectric production works to prevent low water levels on the lake and the marsh. The Red River passes through Netley-Libau Marsh and it has likely contributed to some of the observed changes. High flow events on the river result in the erosion and collapse of weak points in the levees that border the river and other channels. Netley Cut, which was originally dredged in 1913, has been gradually eroded to a point where it now carries a substantial portion of the Red River flow into Netley Lake. The end of dredging on the Red River in 1999 has also likely contributed to the alteration of Red River flows through the marsh. High nutrient loads in the Red River, along with the arrival of common carp, may be contributing to enhanced algal growth and loss of submersed vegetation within the marsh. Loss of submersed vegetation results in the destabilization of bottom sediments and increased wind-induced wave action, which further helps erode island and upland habitats. Without an ability to manage marsh water levels independently of Lake Winnipeg, only a prolonged drought will help restore the emergent plant communities of Netley-Libau Marsh. Dry conditions experienced in 2003 helped re-establish some of the emergent plant communities of the marsh, but the recent return to wet conditions may make this reversal short-lived. We conclude that Netley-Libau Marsh resembles a shallow turbid lake more than a healthy coastal wetland. Any benefits to Lake Winnipeg which the marsh could provide as wildlife and fisheries habitat, and in removing and storing nutrients that would otherwise enrich the lake, have probably been degraded or lost.