7.2 Nutrients

Nutrients are elements and compounds needed to sustain life. Healthy ecosystems contain nutrients in small quantities from natural sources. The more nutrients an aquatic ecosystem contains the more fertile (Eutrophic) it is. Trophic State Index (TSI) is a standard measurement used by the Minnesota Department of Natural Resources and the Pollution Control Agency to measure fertility.

Phosphorus is the limiting nutrient in Pelican Lake. That is, if all phosphorus is used, plant growth will cease, no matter how much nitrogen is available. Phosphorus typically functions as the "growth-limiting" factor because it is usually present in very low concentrations. Any unattached or "free" phosphorus (OP) is quickly removed from the aquatic system by algae and larger aquatic plants.

Fig 7.1 Natural verses Cultural Eutrophication.

Phosphorus enters the stream as either dissolved phosphorus or particulate phosphorus. Dissolved phosphorus is comprised mostly of orthophosphate (OP) that is immediately available for uptake by algae. Particulate phosphorus is attached to soil and organic matter particles and as such is not immediately available for plant use. The soil and other solid matter are measured as total suspended solids or TSS. The fate of particulate phosphorus is dependent on the characteristics of the receiving water body. If the existing sediment is low in phosphorus, a portion of the dissolved phosphorus may be absorbed to the sediment. However, if the sediment entering the water body is high in particulate phosphorus, a portion of it may be desorbed into dissolved phosphorus over a prolonged period of time.

In the plant, phosphorus is essential for a number of physiological functions that are involved with energy transformations. Phosphorus is a component of many cell constituents and plays a major role in several key processes, including photosynthesis, respiration, energy storage and transfer, cell division, and cell enlargement. Adequate phosphorus is needed for the promotion of early root formation and growth.

7.2.1 Effects

Extra inputs of nutrients (primarily nitrogen and phosphorus) disrupt the balance of lake ecosystems by stimulating population explosions of undesirable algae and aquatic weeds. The algae sink to the lake bottom after they die, where bacteria decompose them.

The bacteria consume dissolved oxygen in the water while decomposing the dead algae. Fish kills and foul odors may result if dissolved oxygen is depleted.

Excessive concentrations of phosphorus can quickly cause extensive growth of aquatic plants and algal blooms.

Fig 7.3 Algae

7.2.2 Sources

Pelican Lake is a development surrounded by mostly agriculture and wetlands. Fertilizers from lawns and farms have the most potential to be a source of nutrients. Other sources around Pelican include construction runoff, hydrologic modifications, shoreline removal, compost, feedlot waste, and septic system failure.

7.2.3 Recommended Phosphorus Concentrations

Fig 7.4 Total Phosphorus (ppb) related to Lake Trophic State.

The USEPA has recommended a limit for controlling eutrophication of 0.05 mg/ L for total phosphorus in streams that enter lakes and 0.1 mg/ L for total phosphorus in flowing streams (USEPA, 1986). Acceptable levels of phosphorus in surface runoff from agricultural fields have not been established. The critical concentration of phosphorus for eutrophication that is frequently cited is 0.01 (mg/ L). Total phosphate concentrations of non-polluted waters are usually less than 0. 1mg/ L.

Fig 7.5 Phosphorous indicator.

7.2.4 Comparison by Ecoregion

The North Central Hardwood Forests ecoregion is an area of transition between the forested areas to the north and east and the agricultural areas to the south and west. The terrain varies from rolling hills to smaller plains. Upland areas are forested by hardwoods and conifers. Plains include livestock pastures, hay fields and row crops such as potatoes, beans, peas and corn.

The ecoregion contains many lakes, and water clarity and nutrient levels are moderate. Land surrounding many of these lakes has been developed for housing and recreation, and the densely populated metropolitan area dominates the eastern portion of this region. Water quality problems that face many of the water bodies in this area are associated with contaminated runoff from paved surfaces and lawns.

Below are typical measurements one might find for lakes and streams in this ecoregion.

Lakes

Streams

7.2.5 Ecoregion Average Phosphorus

This region typically sees a summer average value of Total Phosphorus (TP) of .023-.05 mg/L for lakes, and .100-.108 mg/L for minimally impacted streams (no point sources).

Fig 7.6 Phosphorous indicator with Lake and Stream average levels.

Next page: Chapter 7.2.6 Historical Phosphorus levels