Trophic State

The trophic state of a lake is a general concept with no precise definition and no well defined units of measure. In general it refers to the biological production, both plant and animal life, that occurs in a lake. The level of production that occurs is determined by several factors, but primarily by the phosphorus supply to the lake and by the volume and residence time of the water in the lake.

Many different indicators have been used by scientists to describe trophic state. A few of the more commonly used indicators are presented below, along with ranges of values that depict the three trophic categories for New Hampshire lakes. A given lake may fall into more than one trophic category, depending on the indicator used.

Trophic Categories

Oligotrophic:

larger, deeper lakes with clear water, rocky or sandy shorelines, low phosphorus enrichment, limited rooted plant growth, low algal growth and adequate dissolved oxygen throughout.

Mesotrophic:

an intermediate category with characteristics between the other two groups.

Eutrophic:

smaller, shallower ponds with mucky bottoms, extensive rooted plant growth and depleted dissolved oxygen in the bottom waters; often tea-colored and .sometimes murky from planktonic algal growth.

1. Phosphorus

   In New Hampshire lakes, phosphorus is the plant nutrient that controls aquatic plant growth. Phosphorus values in NH lakes range from less than .001 mg/L to .121 mg/L, with a median value of .011 mg/L.

   Phosphorus (mg/L)				Trophic Category
   < .010				oligotrophic
   .010 - .020				mesotrophic
   > .020				eutrophic

2. Water Clarity

   The water clarity or Secchi disk transparency is a measure of the depth one can see into a lake. It ranges from less than a foot to over 40 feet in NH lakes with a median value of 11 feet.

   

   Water Clarity (ft.)				Trophic Category
   > 13				oligotrophic
   6 - 13				mesotrophic
   < 6				eutrophic

3. Chlorophyll

   Chlorophyll is a measure of the amount of planktonic algae in the water. Chlorophyll values in NH lakes range from less than one to over 100 ug/L with a median value of 4.4 ug/L.

   Chlorophyll (ug/L)				Trophic Category
   <4				oligotrophic
   4 - 15				mesotrophic
   >15				eutrophic

4. Rooted Plant Growth

   In general, rooted plant growth is more reflective of substrate type and water depth than it is of in-lake nutrient levels. Most rooted plants obtain most of their nutrient requirements from the sediment and not directly from the water. However, to the extent that shallow, weedy, mucky-bottomed lakes are considered eutrophic, rooted plant growth can be used as a trophic indicator. A subjective evaluation of the amount of plant growth is used for the evaluation below.

   Plant Growth				Trophic Category
   sparse to scattered plant growth around the shore with perhaps a few small patches				oligotrophic
   plants present along most of the shoreline with some thick patches				mesotrophic
   floating or emergent plants covering over 1/3 the surface area and/or submerged plants over most of the visible bottom				eutrophic

5. Dissolved Oxygen

   This criterion can be used only for lakes that are deep enough to develop a cold bottom layer of water (hypolimnion) during the summer. The extent of dissolved oxygen depletion in the hypolimnion is a measure of decomposing organic matter in the bottom waters and in the sediments, and represents an indirect measure of the biological production in the lake.

   Dissolved Oxygen (mg/L)				Trophic Category
   > throughout the lake				oligotrophic
   < 1 in less than half of the hypolimnion volume				mesotrophic
   < 1 in more than half of the hypolimnion volume				eutrophic

   

   Changing Trophic State

   Lakes generally change trophic state very slowly, gradually becoming more eutrophic over time, where time is measured in thousands of years. This process is called natural eutrophication. Often the process is greatly accelerated due to human activity (called cultural eutrophication). Cultural eutrophication can be controlled by managing human activity within the watershed and on the lake. Watershed controls that reduce phosphorus runoff and erosion into a lake will help protect the lake and slow its movement toward a more eutrophic state. See fact sheet WSPCD-1989-12 for lake protection tips.