The Issue of Phosphorus

Most anyone who has spent time on or near the lakes in the Red Cedar River Basin during mid to late summer has seen the lakes turn green and become less attractive for swimming, fishing, boating and other recreational activities. The green color is caused by photosynthetic algae and cyanobacteria (also called blue-green algae) growing in the lakes and on the surface.
The following series of pages describe the phosphorus issue as it relates to the growth of algae in the waters of the Red Cedar River Basin. Follow the link at the bottom of each page to proceed to the next chapter. There is also a link on each page that takes you to a slide presentation on that chapter’s topic.

Causes and Sources

Causes of Algae Blooms
Just like photosynthetic plants, algae need nitrogen and phosphorus to grow. In an aquatic environment like a flowage lake (a dammed-up portion of a large river), where the water is not moving as much as on an open river, these organisms can thrive and multiply into what is known as an algal bloom. Phosphorus is the key nutrient affecting algae and plant growth in freshwater systems. It is the limiting nutrient, meaning that if you keep it out of the water, the algae and plants cannot live and grow. When this nutrient is provided and conditions are right, algae will thrive. One pound of phosphorus in a water body can grow up to 500 pounds of algae.

Sources of Phosphorus
The main source of phosphorus in a watershed is often runoff from land surfaces. Where soil erodes into water bodies, phosphorus moves with it, and is usually referred to as soil or particulate phosphorus. Some phosphorus in runoff water is not attached to soil particles and is dissolved in the water. This is called soluble phosphorus. Phosphorus does occur naturally in soils. It frequently binds to fine soil particles, which keeps it from leaching into groundwater. However, this binding capacity of soil is not unlimited and can be overwhelmed, such as in the soil beneath an old septic system. Also, some geologic rock formations will deliver groundwater high in soluble phosphorus, but this tends to be a minor source in most watersheds. Only soluble phosphorus grows algae and plants. However, once in a water body, chemical processes can release phosphorus from eroded soil particles, trapped in the sediments at the bottom of a lake, into the overlying water making it available for algal growth

Sources in the Red Cedar Basin
Potential sources of phosphorus in the Red Cedar basin are extensive. Reducing phosphorus release from all anthropogenic (human-produced) sources is needed to achieve water quality goals. It is also important to identify the most significant sources. These vary from one watershed to the next. Some general sources include:

  1. Rural residential land
  2. Agricultural land
  3. Urban land
  4. Point sources (such as waste water treatment plants)
  5. Background sources (natural landscapes, groundwater, precipitation)

Each of these sources contributes some amount of phosphorus to the watershed. Measuring the contribution from point sources is fairly easy, since you can take a sample of what comes from the “pipe” responsible for the point source flow. However, to measure the amount contributed by the other “non-point” sources, often times modeling is the only reasonable option. According to modeling done in the Red Cedar Basin using a model called SWRRB (Simulator for Water Resources in Rural Basins), the largest contribution (57%) of phosphorus to the waters in the basin comes from agricultural land, which comprises over one-quarter of the land area in the Basin. Due to other factors like slope of land and erodability of soil, models can also help determine what geographic areas may contribute more pollutant loads to a water body.

Measuring Lake Water Quality
There are several methods to measure the water quality in a lake. They include:

  1. Measuring chlorophyll a as a determination of algal biomass
  2. The frequency, or number of times an algal bloom occurs
  3. The algal toxins present during a bloom
  4. The number, variety and type of rooted aquatic plants present
  5. Water clarity
  6. Dissolved oxygen levels
  7. The pH of the water

Modeling for Tainter Lake has shown that, if phosphorus loads to the lake were decreased, the levels of chlorophyll a would also decrease, indicating a reduction in the amount of algae present in the lake.

(Click here for a slide show on this topic)

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