Measuring Water Quality

Measuring Lake George Water Quality


Water Quality Indicators

Water quality is a result of:

  • What goes in the water (nutrients, pathogens, contaminants, invasive species)
  • What stays there (increased productivity, turbidity, etc.)

There are many measurements that can be used to determine water quality. Water clarity, total phosphorus, chlorophyll A, dissolved oxygen, chlorides, calcium, total coliform, and e.coli are all parameters that might be measured.

The most common measurements are those used to determine trophic state: water clarity, total phosphorus, and chlorophyll A. Learn more about trophic states.

Trophic state index
Secchi disk comparison chart

Lake George's water clarity averages around eight meters, but you can still often see ten meters on a good day.

Water Quality

Water clarity, or transparency, is measured with the use of a Secchi Disk. This measurement provides an indication of algae growth, weed growth and suspended material in the water body.

Water clarity is the most important factor in public perception of water quality. In general:

  • Unproductive lakes (with little nutrients) have a clarity > 5 m
  • Moderately productive lakes are 2 - 5 m
  • Productive lakes are <2 m

New York does not have a state water quality standard for clarity. However, the NYS Department of Health does require 1.2 meters (4 feet) of clarity to locate a swimming beach.

Total Phosphorus

Phosphorus is usually the nutrient that controls (limits) algae growth. It is usually measured as Total Phosphorus (referred to as “TP”). There is no NYS water quality standard for phosphorus. However, there are State Guidance Values. (ppb stands for parts per billion and is equivalent with ug/l - which stands for micrograms per liter)

  • Highly productive lakes: 20 ppb
  • Moderately productive lakes: 10 - 20 ppb
  • Unproductive lakes: < 10 ppb
Leibig Law of Minimum

Liebig’s Law of the Minimum says that growth is controlled by the scarcest resource, not the total of resources available. Image credit: Wikipedia

Energy enters the lake via the sunlight that plants use for photosynthesis. The rate of photosynthesis determines how much life can exist in a lake. The element in shortest supply for photosynthesis limits the amount of photosynthesis that can occur. In most NYS freshwater lakes, that happens to be phosphorus because there are not naturally large amounts of it in NYS lakes.

Chlorophyll A

Chlorophyll A is found in all green plants, so it is used as a measure of the amount of algae in the water, or lake productivity.

  • Highly unproductive or oligotrophic lakes: Chlorophyll a Levels < 2 µg/l. These lakes have rare algal blooms although some may suffer from growths of benthic (bottom-attached) algae.
  • Moderately productive or mesotrophic lakes: Chlorophyll a Levels Between 2 - 8 µg/l. These lakes have Occasional Algal Blooms (Generally Not Blue-Green Algae and Sporadically Throughout the Summer)
  • Highly productive or eutrophic lakes: Chlorophyll a Levels > 8 µg/l. Frequent and Persistent Algal Blooms (Often Blue-Green Algae and Particularly Late In The Summer)