Changes in the Water, Soil, and Vegetation of a Wetland after a Decade of Receiving a Sewage Effluent

The impact of discharging an oxidation pond effluent into a wetland in the Waitangi Forest (Northland) was assessed by comparing the water, soil, and vegetation of this wetland (the sewage wetland) with that of an adjacent wetland not receiving effluent (the reference wetland). The hydroperiod of the two wetlands differs markedly with the sewage wetland now permanently flooded whereas the reference wetland is subject to summer drawdown. Marked differences were found in gross chemical indicators such as pH and redox potential between the soils of the 2 wetlands. Extreme differences were also found in the nutrient chemistry of the soils and this was related to differences in water chemistry. This was especially the case for the plant available forms of nitrogen and phosphorus. Typically the sewage wetland soil contained ammonium and inorganic phosphorus two orders of magnitude greater than that of the reference wetland. Major differences were noted also in the plant communities of the two wetlands. The sedge Eleocharis sphacelata and the bulrush Typha orientalis dominated communities covering a large part of the sewage wetland, whereas the reference wetland contained a much more diverse Baumea-Isachne grassy sedge community. Historical information indicated that the sewage wetland had similar hydrological, chemical, botanical characteristics to the reference wetland before discharge of the effluent commenced. It is concluded that most of the vegetation community changes that have occurred may be interpreted in terms of allogenic succession but that future community changes will have a strong autogenic component.