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.

Issues affecting water quality are seen as one of the most important and pressing global problems of our era. In New Zealand, water bodies with the poorest water quality and ecological condition tend to be surrounded by pastoral land use. Lake Ellesmere/Te Waihora in Canterbury, New Zealand, is a typical example of the issues that nutrient and sediment run-off from pastoral land can create. The aim of this study was to determine the relationship between sediment concentrations, phosphate concentrations, ecological state and the degree of riparian restoration on drains that flowed into Lake Ellesmere/Te Waihora, and to calculate the load of phosphorus and sediment delivered by each of the drains to Te Waihora over the year, comparing this to the loads carried by larger, natural streams and rivers. Little research has been done on these small artificial tributaries of the Lake Ellesmere/Te Waihora catchment. Data collection was carried out on 10 drains with variable degrees of riparian planting, monthly in summer and autumn, and fortnightly in winter and spring, due to higher variability in drain flows during this time. Sites 1, 2 had low dissolved oxygen (DO) and high total phosphorus (TP), lack of flow and extremely high conductivity, and (with) Site 5, higher suspended particulate matter (SPM) concentrations. All these factors are consistent with the lack of ecology occurring in these drains. All drains failed to meet the Australian and New Zealand Environment and Conservation Council (ANZECC) guidelines for TP concentrations. All water chemistry parameters showed significant differences between seasons except conductivity. Mean water temperatures and pH were higher in summer and lower in winter, while mean DO levels were higher in winter (and spring) and lower in summer (and autumn). Macroinvertebrate analyses indicated moderate to severe pollution in all the drains, despite the amount of riparian planting present and the presence of macroinvertebrate community structure was related mainly to substrate size. The degree and type of riparian planting present on the drains studied did not appear to affect TP, SPM, macroinvertebrates or general water quality. This is likely to be due to the fact that little of the riparian planting had been specifically planted for restoration purposes. The highest loads of TP and SPM occurred in winter and spring, and in the larger (wider and deeper) drains. As flow increased in the drain, so did the load of phosphorus and sediment carried. Comparison with Environment Canterbury monitoring data for the river tributaries of the lake indicated that more TP and SPM is carried to the lake by natural rivers and streams, than by the drains, but the latter do make a significant contribution. The percentage of TP that is in dissolved form was higher than had previously been assumed, in both the drains and the larger, natural rivers and streams. It is recommended that future restoration work aim to reduce the amount of phosphorus and sediment entering the larger drains in winter and spring. More adequate riparian planting needs to occur on these drains, and it needs to be managed in a way that a reduction in dissolved phosphorus levels is also achieved.