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 (Z_c). Vascular plants were encountered at Z_c in 25% of the 63 lakes investigated. Characean algae extended to greater depths and occurred at Z_c 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 Z_c predicted from water clarity, that was not consistent with their latitudinal displacement. In clear North Island lakes, Z_c 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 Z_c in South Island lakes was linear, with Z_c 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 Z_c in clear North Island lakes.

Keywords:

• attenuation coefficient
• submerged macrophytes
• charophytes
• water clarity
• water level