The Environmental History of Te Waihora – Lake Ellesmere
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Te Waihora – Lake Ellesmere is an expansive, shallow, turbid, brackish, hyper-eutrophic, lowland lake located on the east coast of New Zealand’s South Island. The catchment and lake are in a highly modified state, with much of the catchment used for intensive agriculture and the lake’s level artificially controlled by cutting a channel through the barrier separating the lake from the sea. Although it is known that Waihora is highly modified, it is difficult to determine the factors contributing to the current lake state and what constitutes a natural state for this lake. In order to plan management strategies, it is important to have this information. This study aims to provide insight into these matters using paleoecological techniques, in particular, analysis of sediment characteristics, palynology and diatom analysis, on cores obtained from the lake bed.
The results of these analyses show that Waihora has had a diverse history, beginning as a freshwater lake, low in nutrients, not long before c. 7500 years ago, following the fusion of Kaitorete ‘Spit’ with Bank Peninsula. This freshwater state was interrupted by the discharge of a large river into the basin, causing a permanent barrier opening and tidal, brackish conditions to prevail. A second brackish state formed after this, caused either by a shift in the discharge point through the barrier or, more likely, a second avulsion event of the Waimakariri River to a discharge point into Waihora. Upon the avulsion of this river to a discharge point north of Banks Peninsula, a freshwater, nutrient rich lake formed. Subsequently, human influenced lake changes became evident, with a hypereutrophic, shallow, brackish lake forming. This research provides evidence that modern lake management has led to decreased lake levels and increasing salinity within Waihora. Intensive agriculture, particularly since the 1970’s has led to an increase in nutrients within the lake and its current hypereutrophic state. A combination of lake level management and the ‘Wahine Storm’ (1968) has led to the lake’s current turbid, phytoplankton dominated state. Therefore, sediment characteristics, palynological and diatom data suggest that a natural condition for the lake is one with lower nutrient levels, lower salinity with greater depth and area than the current lake, with a large distribution of freshwater riparian vegetation and little halophytic vegetation.
If restoration of the lake is a target then (1) the lake should be opened to the sea less frequently, allowing a decrease in lake salinity and conditions conducive to the prevalence of freshwater riparian vegetation to prevail, and (2) a transition from a phytoplankton dominated state to a macrophyte dominated state should be targeted, by maintaining the lake at greater depths, the use of riparian planting practices and decreasing nutrient input. However, the latter will be costly and involve questionable trade-offs between lake values and stakeholders. Regardless of whether or not restoration of Waihora to something resembling a natural state is, or will be, a management aim, a decrease in nutrient input catchment wide and riparian planting in the area surrounding the lake should be a priority and may present a more realistic, short term management objective.