Have you ever wondered why the lakes in southern Minnesota are generally shallower and “greener” than the lakes in northern Minnesota? This difference is mainly based on a lake’s ecoregion. An ecoregion, is a geographical area where the land use (agriculture, forest, prairie, etc.), underlying geology, potential native plant community, and soils are relatively similar.
Many of these differences in soil fertility and underlying geology are from where the glaciers advanced and where they scraped and deposited till. Northern Minnesota was scraped fairly clean down to the bedrock, with boulders, sand and clay left behind, while southern Minnesota was left with a rich, fine prairie (now agricultural) soil.
Minnesota is divided into seven ecoregions, but most of our lakes are in four of them. Minnesota Pollution Control Agency (MPCA) researchers have developed a way to compare lakes within and between ecoregions. They studied the watershed characteristics, land use, and water quality of reference lakes in each of the ecoregions and derived an average range of water quality typical for each ecoregion. These reference lakes are not considered pristine, but are considered to have little human impact and therefore are representative of the typical lakes within the ecoregion.
For example, the lakes in the Northern Lakes and Forests Ecoregion (Hubbard County and east to Lake Superior) have characteristically low phosphorus and algae concentrations due to the abundance of forests, and sandy, relatively infertile soil. Lakes in the Western Corn Belt Plains Ecoregion (southern Minnesota) tend to have higher phosphorus and algae concentrations due to the fertile black soil, agriculture and the Minnesota River Valley.
The MPCA discovered through lake-user surveys that user perception of water quality varied by ecoregions. This has led to ecoregion-specific criteria for phosphorus, and in general helped to clarify expectations and goals for protecting lakes in Minnesota.
You can look up your lake phosphorus, chlorophyll-a and secchi disk levels at the DNR Lakefinder site: http://www.dnr.state.mn.us/lakefind/index.html. Once you have the average phosphorus, chlorophyll-a and secchi disk for your lake, you can compare it to the other lakes in your ecoregion by using the table below.
| Ecoregion | Red River Valley | Northern Minnesota Wetlands | Northern Lakes and Forests | North Central Hardwood Forest | Northern Glaciated Plains | Western Cornbelt Plains |
| Abbreviation | RRV | NMW | NLF | NCHF | NGP | WCBP |
| TP (ug/L) | 23 – 50 | 14 – 27 | 14 – 27 | 23 – 50 | 130 – 250 | 65 – 150 |
| CHLA (ug/L) | 5 – 22 | <10 | <10 | 5 – 22 | 30 – 55 | 30 – 80 |
| Secchi (ft) | 5 – 10.5 | 8 – 15 | 8 – 15 | 5 – 10.5 | 1 – 3.25 | 1.5 – 3 |