Mires and Peat (Dec 2015)
Quantifying the water balance of Mfabeni Mire (iSimangaliso Wetland Park, South Africa) to understand its importance, functioning and vulnerability
Abstract
Peatlands occurring in regions with high rates of total evaporation (ET), matching or exceeding precipitation (P) during seasonal dry periods or longer-term dry spells, are dependent on sustained groundwater flows to ensure peat accumulation. The objective of this study was to quantify the water balance of Mfabeni Mire in South Africa over one year, and thereby define its contribution to downstream and adjacent ecosystems and identify risks and consequences likely to arise from future shifts in the water balance. P (1,031 mm) and ET (1,053 mm) dominated the water balance measured from May 2008 to April 2009. These were followed by groundwater inflows (14 mm), stream outflow (9 mm) and storage change (-3 mm, a net loss in water stored in the mire) with the smallest flux being groundwater outflow (0.3 mm). There were differences in the seasonal patterns of ET from the two dominant plant communities (swamp forest and sedge/reed fen), which probably resulted from their significantly different canopy structures. Limiting factors for ET were low vapour pressure deficit and cloud cover. Although the water balance of Mfabeni Mire was dominated by and equally split between ET and P, it still contributed a small efflux to downstream ecosystems by streamflow. Its value in a landscape where seasonality and long-term dry periods are major ecological drivers lies in its damping effect on climatic variability. This creates a more stable environment for adjacent aquatic ecosystems by contributing to a steady groundwater condition. Mires occurring in areas that experience dry periods, where water stress frequently threatens biodiversity, should be recognised as assets in natural resource management; and their potential to support adjacent ecosystems should be protected through planning and conservation practices. Management of the area should include careful consideration of any proposed changes in land use or encouragement of one plant community at the expense of another, as such changes will alter the equilibrium of the water balance. Mfabeni Mire is particularly vulnerable through the impact of adjacent commercial forestry, which has impacted river and estuary management over the last 80 years and depleted groundwater levels in the surrounding aquifers.
