Analysis of relationships between maximum depth limits of aquatic plants and underwater light in 63 New Zealand lakes

Abstract

Data from 63 New Zealand lakes were analysed to determine latitudinal and other patterns in the relationship between water clarity and the maximum recorded depths for submerged aquatic macrophytes (Zc). Vascular plants were encountered at Zc in 25% of the 63 lakes investigated. Characean algae extended to greater depths and occurred at Zc in 71% of the lakes, with a maximum depth of 34.4 m. There was a difference between North and South Island lakes, in values of Zc predicted from water clarity, that was not consistent with their latitudinal displacement. In clear North Island lakes, Zc was predicted to be less than in lakes of similar water clarity in the South Island and to reach an asymptotic maximum depth of 21 m. In contrast, the relationship between water clarity and Zc in South Island lakes was linear, with Zc of more than 30 m in some lakes at c. 2% of subsurface irradiance. Although the availability of light explained depth limits in most South Island lakes, other factors (e.g., grazing impact from freshwater crayfish) appear to be contributing to the shallower Zc in clear North Island lakes.