Trophic status of twenty‐one New Zealand high country lakes

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Abstract
Between December 1975 and March 1976 base‐line data were obtained from 21 Ndrth Canterbury high country lakes located in three major river catchments to establish trieir relative trophic status. Included were two lakes (Coleridge and Sumner) on the New Zealand Officials Committee on Eutrophication base‐line study list, and two lakes (Marlon and Vagabond's Inn) in the IBP Project Aqua series. The following biological parameters were measured: heterotrophic potential (Vmax), total bacterial counts, total and microbial adenosine triphophate (ATP), chlorophyll a and pH. Chemical analyses included dissolved oxygen; alkalinity; free and total CO2; nitrogen and phosphorus compounds; chloride; calcium; and magnesium. On the basis of Vmas, the lakes were separated into mesotrophic and oligotrophic groupings, in the following decreasing order of relative trophic status: mesotrophic—Lakes Little Maspn, Marymere, Vagabond's Inn, Evelyn and Ida (mean Vmas 0.071 μg.l−1.h−1 ); Lakes Maribn, Letitia, Sarah, Grasmere and Sheppard (mean Vmax 0.031 μg.l−1.h−1). Oligotrophic ‐ Lakes Katrine, Catherine, Hawden, Pearson, Taylor, Lyndon, Georgina, and Selfe (mean Vmax 0.017 μg.l−1.h−1); Lakes Big Mason, Sumner and Coleridge (mean Vmax 0.002 μg.l−1.h−1). (Glucose was used as the substrate for Vmax determinations.) Total bacterial counts and concentrations of ATP, Chi a, total N, and total P for these four groupings substantiated the breakdown by the relative Vmax values. Lakes Coleridge ftnd Sumner had very low levels of microbial activity and nutrients characteristic of large, deep oligotrophic lakes, while Lakes Marion and Vagabond's Inn were higher in trophic stajtus than previously reported. The heterotrophic potential technique can be used to investigate the trophic status; of poorly accessible lakes, even when prolonged periods in the field delay the analyses.