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Memorial Day Weekend Water Testing
Posted on May 28th, 2019
I took our 2019 Memorial Day water samples on Wednesday, 5/22 Between 10:38 and 10:52 am.
#1 Dock - 3 cfu
Lake water temperature was 81F/27C at all sites. The SECCI clarity reading was 36 inches. These are all very good readings, in the normal range for our lake at this time of the year.
For the two weeks prior to this sampling, we had reports of an elusive dark and iridescent “slick” appearing in the morning along the South side of the long Booker Creek arm of the lake that extends to the left of the lake park, as seen when standing at the beach looking out across the lake. During that time, the lake water was 79F, the SECCI clarity was 38-44 inches, and the water was a greenish yellow with some distributed debris. Water flow through the lake was good, there was about ½ to 1 inch of water going over the full width of the top of the dam, with a couple of areas of “sudsy foam” below the dam, and one small transient area of the same foam along the South shore of the lake. Clarity at this time of the year is often closer to 20 inches.
From John Steffens - Biologist and LFA Member:
The substance seen on the lake is not an oil slick. The iridescent sheen on the water is actually a colony of microbes that have formed a biofilm on the water's surface. A biofilm is an extremely thin layer of microbes held together by a matrix of polysaccharides and protein. Like an oil slick, the biofilm refracts light, resulting in the silver or rainbow-like iridescence characteristic of a slick formed by oil or gas. Unlike petroleum, which spreads continuously on water, a biofilm exhibits edges, which allows you to readily distinguish it from an oil slick. Try this: you can poke a stick through a biofilm and the hole remains; an oil slick will just spread back over the hole. Agitate the stick a little and a biofilm will fracture into discrete rafts; an oil slick won't do that either. Usually we don't notice these biofilms on the lake because wind and waves break them up. With lots of calm days they're much easier to see. As an important part of the lake's food web, the microbes that form biofilms are also grazed upon by an array of other microorganisms, and some believe the microbes generate the biofilm as a way to protect themselves from grazing. Anyway, they're a harmless and interesting part of our lake's ecosystem.
John and I have both observed the iridescent biofilm upstream of the forebay in Booker Creek over the past several years.