Monitoring chlorophyll a in lakes

Chlorophyll a is a photosynthetic pigment found in plants and algae, including in phytoplankton (microscopic algae suspended in a water column). High concentrations of chlorophyll a indicate high phytoplankton biomass and, sometimes, the proliferation of potentially toxic cyanobacteria blooms.

Phytoplankton or cyanobacteria blooms are a symptom of nutrient enrichment (eutrophication) of a lake and can cause poor water clarity and anoxia of bottom waters as phytoplankton cells die and sink. They can also impact the recreational and aesthetic values of a lake. Chlorophyll a is used as a proxy for phytoplankton biomass in lakes as it can be readily measured through chemical or fluorescence methods. In contrast, direct measurements of phytoplankton biomass usually involve laborious microscopy methods.

Monitoring chlorophyll a 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 indication of how much growth or productivity occurs in the lake, with productivity directly related to the availability of nutrients. Chlorophyll a is one of four attributes (along with total phosphorus, total nitrogen and Secchi depth) used to determine the 'Trophic Level Index' for New Zealand lakes.