Nutrient Budget Model
By David Ernes – Lake Management Committee
In order to reduce the incidence of algal blooms, one must reduce the nutrient load that feeds it. But first you need to know where to focus your efforts. That is where a Nutrient Budget comes into play, something we recently contracted for Lake Roaming Rock. This is basically a modeling study whereby the various sources of nutrients (primarily phosphorous and nitrogen) are characterized. This includes measurements for both the internal sources (mainly from legacy sediment), watershed sources (run-off and stream inflows) as well as waterfowl and precipitation contributions. The results indicated that the former two constitute the major sources.
The internal loading portion was determined by collecting surface sediment samples from three location in the deeper areas of the lake, followed by analysis of the types of phosphorous detected. The results found elevated levels of phosphorous bound to iron that can be released under low oxygen conditions such as observed for our lake. This means that by reducing this source of phosphorous we can substantially reduce the growth of algae. The overall load from the sediment can be used to make cost-benefit calculations for various remediation approaches available such as aeration, alum, and others. Several recent articles discussed these approaches. The data also showed that the organic content of this sediment was only 11-13%, considered to be low. This means that the level of lake “slime” (a term often used to describe the degrading mass of leaves, plants, etc. on the lake bottom) often targeted by some vendors, may not be as effective for our lake.
The external loading is primarily from the watershed. This represents over 40,000 acres of land, 86 times the size of the lake. Literature data indicates that 42% of the watershed is agricultural. In more detail, this area is subdivided into 86 defined sub-watersheds. From the modeling data for each, it is possible to identify sections of the watershed showing higher nutrient loadings. In this way, we can focus our efforts for nutrient reduction. This is important as the estimates suggest that 62% of the total phosphorous load to our lake is from the watershed.
With the recent appointment of our Lake Advisor, plans are advancing for them to make recommendations for short-term treatments for 2021 as we move to the ultimate long-term goal of improving our greatest asset. This Nutrient model is one more tool to be used to select the one(s) best suited for our lake. Remember…
Be Lake Responsible