Secchi disk transparency (SDT) is an indicator of water clarity. To measure water clarity, a black and white disk is lowered into the water and the reading is taken at the depth at which it is no longer visible. Factors that affect water clarity include algal growth, zooplankton densities, natural water color, and suspended silt or sediment particles. For more information about temperature and dissolved oxygen in lakes and why this information is important, see our water quality monitoring page for more information.

As part of our advanced water quality data collection on Androscoggin Lake as a response to the recent algal blooms, phosphorus profile grab samples are collected approximately every month. By collecting this data, we can measure the volume- weighted mass of phosphorus in the lake and provide an estimate of how much phosphorus is being internally loaded from bottom sediments each year.

Phosphorus is the nutrient that most influences the growth of algae in lakes. Because its natural occurrence in lakes is very small, phosphorus “limits” the growth of algae in lake ecosystems. Even small increases in phosphorus in lake water can cause substantial increases in algal growth, which hinders not only the overall health of the lake system, but also the economic, recreational, and aesthetic values. Generally speaking, in lake phosphorus concentrations less than 10 ppb are ideal. Lakes with in-lake phosphorus concentrations of ~13 ppb or more are known to sustain algal blooms, and blooms become frequent as average concentrations approach 20 ppb.

Grabs are taken every other meter down through the water column.