Monitoring visual water clarity in lakes

Visual water clarity is a measure of the ability of light to travel through water. Visual clarity in lakes may be reduced due to suspended sediment or when there are phytoplankton or cyanobacteria blooms.

Visual water clarity is an important indicator of lake health. Reduced clarity can indicate high suspended sediment inputs to the lake from the surrounding catchment. Low visual clarity can also be a sign of algal (phytoplankton) or cyanobacteria blooms, which are an outcome of eutrophication (excess nutrients). Low visual clarity can impact the aesthetic and recreational values of a lake and adversely affect ecological health. For example, if light cannot reach the lake bottom the growth of aquatic plants will be affected.

Monitoring visual clarity is a key part of evaluating the trophic status of a lake, which is a common measure used to describe lake health in New Zealand. Trophic state provides an indicator of how much growth or productivity occurs in the lake, productivity being directly related to the availability of nutrients. Visual clarity is one of four variables (along with total nitrogen, total phosphorus and chlorophyll a) used to determine the ‘Trophic Level Index’ for New Zealand lakes.