Trophic State of Lakes

Having a general understanding of the different trophic states of lakes is useful for a number of reasons. In an unknown water body, it can be a good indicator, along with geographic location, of what species are likely to be present. It can give also an indication as to the likely productivity, or numbers of fish that are present. It can help us understand how fast a fish may be expected to grow and as to how likely it may be that larger fish are likely to be present, provided we take into account other factors – a major one being fishing pressure.

With this in mind the following set of trophic state qualities are a useful guide:

Oligotrophic

• Low nutrient content
• Deep lake with steep shoreline and bottom gradient
• Little planktonic growth and few aquatic plants
• Sand or rock along most of shoreline
• Likely to support coldwater fishery
• High dissolved oxygen content

Mesotrophic

• Moderate nutrient content
• Moderate planktonic and plant growth
• Some sediment accumulation over most of lake bottom
• Usually supports coolwater and/or warmwater fish species

Eutrophic

• High nutrient content
• Much planktonic and extensive plant growth
• Shallower lake with much sediment accumulation on bottom
• Low dissolved oxygen on bottom
• Many times only warm water fish species present

Hypereutrophic

• Extremely nutrient rich
• Subject to frequent and severe algal blooms
• Can support many plants and fish but can also cause dead zones
• Depleted in oxygen with subsequent fish/plant kills

Of course waterbodies can fall into many points along this continuum. Water temperature plays a large roll here as can many man-made influences – particularly runoff from agriculture and industry. Indexing the different trophic states using chlorophyll and phosphorus amounts in the water are possible (see Carlson). Total accumulated phosphorus is often times a good measure of trophic state and will be expanded upon in a separate post.

References:
1. Carlson, R.E. (1977) A trophic state index for lakes. Limnology and Oceanography. 22:2 361-369.
2. http://des.nh.gov/organization/commissioner/pip/factsheets/bb/documents/bb-3.pdf