Wednesday July 18, 2018

Secchi Readings – Measuring our Water Clarity

The Ministry of Environment and Energy (MOEE) monitors Ontario’s recreational lakes in partnership with cottagers and residents. The goal of the Lake Partner Program is to create a valuable, long-term database to evaluate the nutrient status of Ontario’s inland lakes. The lakes’ nutrient status will reflect land use changes in the watershed and assist in our efforts to conserve and protect our inland lakes.  HHLPOA participates in this program

Secchi Readings

A Secchi disk is a black and white coloured disk used to determine water clarity. The disk is lowered into the water. The point, at which you can no longer distinguish the black and white, is called the Secchi depth.

Secchi Reading and Lake Nutrient Status:

  • Over 5 metres Oligotrophic – unenriched, few nutrients
  • 3.0 to 4.9 metres Mesotrophic – moderately enriched, some nutrients
  • Less than 2.9 metres Eutrophic – enriched, higher levels of nutrients

Oligotrophic lakes contain very low concentrations of those nutrients required for plant growth and thus the overall productivity of these lakes is low. Only a small quantity of organic matter grows in an oligotrophic lake; the phytoplankton, the zooplankton, the attached algae, the macrophytes (aquatic weeds), the bacteria, and the fish are all present as small populations. It’s like planting corn in sandy soil, not much growth. There may be many species of plankton and many different types of other organisms, but not very many of each species or type. There may be some big fish but not very many of them. With so little production of organic matter, there is very little accumulation of organic sediment on the bottom of oligotrophic lakes. And thus, with little organic food, we find only small populations of bacteria. Moreover, with only small numbers of plankton and bacteria, we have very little consumption of oxygen, from the deeper waters. One typical measure of an oligotrophic lake is that it has lots of oxygen from surface to bottom. Other measures are good water clarity (a deep Secchi disk reading, averaging about 10 meters or 33 feet), few suspended algae, the phytoplankton, which yield low chlorophyll readings (average about 1.7 mg/m3), and low nutrients, typified by phosphorus (average about 8.0 mg/m3). There are other chemical characteristics, but these are the ones most often mentioned. The bottom of oligotrophic lakes are most often sandy and rocky and usually their watersheds are the same, resulting in few nutrients entering the lake. Oligotrophic lakes have nice clean water, no weed problems and poor fishing. They are often deep with cold water. They are seldom in populated areas — too many people and heavy use tends to eventually shift them out of the oligotrophic category. They are seldom in good agricultural areas; rich soils needed for agriculture do not allow nutrient poor drainage water needed for the oligotrophic lake.