[Lake Manager's Handbook: Land-Water Interactions. June 2002. Prepared for the Ministry for the Environment by Sandy Elliott and Brian Sorrell, National Institute of Water and Atmospheric Research, June 2002 by the Ministry for the Environment, New Zealand, http://www.mfe.govt.nz/publications/water/lm-land-water-jun02.html.]
Lake Alexandrina is a relatively shallow lake, with a maximum depth of 27 m, situated in the Mackenzie basin. It is 7.2 km long and 0.9 km wide with an area of 640 ha, and shows strong visual signs of its glacial origin. It has high landscape, wildlife and recreational values (fishing, birds, aesthetic), and is situated in a catchment where farming has been the principal land use. Aquatic macrophyte communities have grown to 10-11 m with 100% cover, and Chara coralline continues sparsely down to 16 m.
Sources of water to the lake (cumecs/year x 106) include:
Water is retained in the lake for approximately four years.
The lake has mestrophic status (Vant and Davies-Colley 1984). Hayes (1980) recorded chlorophyll at levels of 0.4-3.8 mg/1 between November 1978 and March 1979, and total phosphorus at 1 m of 0.009-0.015 mg/l in March 1979. Nutrients from the catchment enter the Lake attached to soil particles, as organic matter and in solution from inflow streams, groundwater and diffuse sources. Nutrients originate from:
The main management concern for the Lake has been the frequency of algal (Anabaena) blooms caused by increased nutrient levels in the Lake, which began to cause concern after 1980 due to their potential effects on wildlife and recreational fishery.
Hoare (1982) modelled the phosphorus in the Lake and concluded that the nutrient status was probably the result of nutrient loads coming in with the bulk of the water rather than from sewage, which is a relatively minor contributor. Lovegrove (1985) estimated that up to 50 percent of the phosphorus entering the lake may come from groundwater, 32 percent from surface water inflows and only 3-9 percent from hut settlements. Water from springs entering the bottom of the Lake was sampled in 1987 and found to contain phosphorus and nitrogen in higher quantities than in water nearby (Ward 1989).
The Lake Alexandrina Steering Committee was set up in 1984 following a meeting of interested parties. Phosphorus was identified as the major factor causing deterioration of water quality (Ward-Smith et al 1985). Two main sources of phosphorus were identified: one from the hut settlements at the outlet and south end, the other from agricultural sources.
A report by the Taranaki Catchment Commission to the Waitaki Catchment Commission (1987) states that while inflows exceed outflows, much will be permanently lost to the sediments when the annual mass contribution to the lake is low. They recorded total phosphorus levels in the Lake of 0.014-0.032 mg/l from June 1984 to June 1985. Concerns over the blooms and the potential effects on wildlife and the recreational fishery resulted in interim guidelines for management of the Lake (Ward-Smith et al 1985).
To control phosphorus from agricultural sources, the interim guidelines of the steering committee (Ward-Smith et al 1985) recommended:
Subsequent studies suggest that the Cass fan to the northeast of the lake catchment is the source of the groundwater inflow (Waitaki Catchment Commission and Regional Water Board 1987).
In 1987 the Mt John Station run that occupies the southern part of the Lake catchment was purchased by the New Zealand Defence Force (NZDF), so stock were removed from this land. However, the NZDF are in the process of negotiating with a runholder to exchange this land for land adjacent to their live-fire area, so it is likely that grazing will be re-established on the Mt John run. The Department of Conservation, however, now have a reserve/buffer strip around Lake Alexandrina on the Mt John run, so stock intrusion to the southern lake margins is controlled. Around the northern margins, on the Glenmore run, stock intrusion continues to be a problem as sheep are drawn to the lake edge by the shade of the willow trees.
To control phosphorus deriving from the settlements, the interim guidelines (Ward-Smith et al 1985) recommended that:
Lake Alexandrina Management Guidelines, prepared by the Mackenzie District Council, were adopted in August 1999 following a consultative process with stakeholders at the lake. These provide assistance to applicants seeking resource consents in relation to building development. All liquid waste must be discharged into a holding tank, which is to be emptied by an approved septic maintenance contractor. The holding tank installation has been very successful, with 90 percent completed by June 2001 and a cut-off date for the remainder by January 2002 (A Shuker, Outlet Hut Holders Committee Chairman, personal communication).
The Mackenzie District Council has included a Lake Side Protection Area around Lake Alexandrina in the Proposed District Plan 1997, to protect the visual amenity of the lake-side environment from inappropriate building development. It has discretionary status on new buildings and extensions to existing buildings around the lake.
Although management guidelines and controls in the lake catchment have been progressively implemented since 1985 to reduce phosphorus levels in surface inflows, an increase in total phosphorus from 1992 to 1996 of about 6 percent has been recorded (Burns and Rutherford 1998). Total nitrogen and chlorophyll values showed little change over this period. However, the heavy algal blooms of the 1980s seem to have disappeared and are now seen on the Lake only about every three years (A Shuker, Outlet Hut Holders Committee Chairman, personal communication).