Lake Alexandrina (Māori: Whakatukumoana) is a lake located in the Mackenzie Basin of New Zealand's South Island. It lies immediately to the west of the much larger Lake Tekapo and further to the east of Lake Pukaki, located to the north of Lake Tekapo township. It is a shallow lake with distinct indications of glacial origin and is spring fed with an outlet on its eastern shore midway down the lake. The outlet feeds into a smaller lake, Lake MacGregor before feeding into Lake Tekapo described as “Opaque and milky blue” in colour.
Geography
thumb|left|Landscape near the lake
Categorised as a Canterbury High County lake in the Mackenzie Basin, it lies a short distance from Lake Tekapo while also being connected to it. Lake Alexandrina is approached from the State Highway 8 via the Godley Peaks Road. covers an area of with width of and extending to a length of . Its shores are flat. The maximum water depth in the lake is reported to be . The lake catchment has landscape that provides for plentiful wildlife but also has extensive cultivation which brings in a lot of nutrients, enriching the lake's with phosphorus. as well as black swans and ducks. Six species of waterbirds have been recorded in total. Paradise shelducks flock here during the summer season, seeking sanctuary when they become flightless during their moult. Freshwater snails Potamopyrgus antipodarum have also been analysed in the basin area. For example, studies in 1994 found that there are more male snails in the lake's shallow water than females, and that snails are sicker in shallow water than deep water.
thumb|Australasian crested grebe
In December 2021, the Department of Conservation reported that over 60 breeding pairs of Australasian crested grebes had established nests in the first 50 metres of the outlet stream, and urged holidaymakers to keep their distance, to avoid disturbing the birds.
Vegetation
The breeding of the Elodea canadensis species has been inhibited at Lake Alexandrina because of the steep slopes on the east and west sides of the lake, as well as turbulence caused by high winds of 10–14 m/s.
Aqua fauna
The lake has plentiful stocks of brown trout, rainbow trout and salmon, which are well known to fishing enthusiasts. The trout are not found in abundance because of overfishing and inadequate spawning duration. However, the fish do grow to a large size, especially the trout, each weighing as much as on an average. Some of prize catch could be of weight for rainbows and for brown variety trout (which generally are found along the shores of the lake where flies and insects abound).
;Threats
The threats to the lake waters that were identified in the 1980s were the regular occurrence of algal (Anabaena) blooms, which were a result of high nutrient levels of phosphorus, survival of wildlife, and recreational fishery. It was assessed by measurements that nearly 50% of the phosphorus was contributed by groundwater, with 32% from surface water inflows and only 3–9% from hut settlements. This resulted in poor quality of the lake waters and subsequently, measures to check the phosphorus content in the lake received priority attention of the Government of New Zealand. After the trout grew and prospered, fishing licenses were issued. Lake Alexandrina, now known for its attraction to fisherman, has a small number of fisherman's huts clustered at each end of the lake and near the outlet. Rowing boats are the only mode of transport on the lake as sail and motor boats are prohibited.