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Lake Oscawana Advisory Committee Invites Public to Share in Consultants Report on Lake and Watershed Monitoring and Preservation
by Edward Paul Greiff
On Saturday March 11, 2007, at 10am in the Putnam Valley Town Hall, Dr. George W. Knoecklein from "Northeast Aquatic Research" will present the results of his lake and watershed monitoring activities for 2006. The general public is invited to attend what promises to be an educational, informative, and from a lake preservation viewpoint, an important meeting. Putnam Valley has many lakes and although each lake community is different, there are commonalities that apply to all lakes.
The area studied by Dr. Knoecklein is Lake Oscawana, a 368 acre lake with an average depth of eighteen feet and a maximum depth of thirty-seven feet. His samplings were taken from seven different streams that feed into the lake as well as from the lake itself. The sampling process began in April and ended in October. The streams are located on a 2,875 acre watershed that surrounds Lake Oscawana. Dr. Knoecklein estimates that it takes 423 days for the average annual stormwater runoff from the watershed to flush the total volume of water in the lake.
On a regularly scheduled basis Dr. Knoecklein monitors both the watershed streams and lake water clarity, phosphorus levels, dissolved oxygen concentrations, bacterial indicator organisms, and nitrate concentrations. The results are compared to previous years' monitoring. Having this kind of factual data at their disposal enables the Lake Oscawana Advisory Committee and the Lake Association to make informed decisions regarding the preservation efforts required to maintain a lake water quality that is healthy and safe.
In his summary report, Dr. Knoecklein points out that the lake's water clarity was relatively consistent during the season with a high of 4 meters visibility and a low of 1.8 meters visibility. "The season trend was similar to prior years as characterized by moderate clarity in the spring, good clarity in late June through early July and poor quality after that time. Uncharacteristically good clarity was noted in late August."
Dr. Knoecklein's report confirmed a high build-up of phosphorous levels as a result of the high build-up of sediment at the bottom of Lake Oscawana. He said, "Phosphorus levels in the lake were generally as seen in prior years except for significantly higher phosphorus levels at the bottom during July and August. Monitoring station #1 had a peak concentration of almost twice that observed from the last two years at 735 ppb. Station #2 had a peak concentration of more than twice the prior highest concentration observed at that station at 430 ppb. Station #3 also had high deep water phosphorus concentrations comparable to maximums observed at that station in prior years.
When the water over the bottom sediments is devoid of dissolved oxygen (a condition known as anoxic; without oxygen) phosphorus is no longer chemically bound to the sediments and is free to rise out of the sediments into the over laying water. This results in algae bloom.
Options for addressing in-lake phosphorus are limited to adding oxygen to the bottom of the lake, applying a chemical form of aluminum called alum, or removing the sediments through dredging the entire lake bottom.
Dr. Knoecklein expressed a concern about the depleting and lack of oxygen in the waters of Lake Oscawana. He reported, "Dissolved oxygen concentrations were zero in waters below about 6 meters during most of the summer. Although this is typical for Lake Oscawana, the maximum ascent depth of the zero concentration was 5.75 meters, higher than has been observed in more than 10 years."
The sediments and phosphorus in the lake are attributed to many years of watershed runoff from a diversity of sources; development within the watershed, land clearing, erosion, street runoff, animal waste, septic systems, precipitation, fertilizers, car washing . . . etc.
Dr. Knoecklein raised yet another red flag when he reported that the, "Watershed sampling showed that streams #3, #4, and #7 continue to contain bacterial indicator organisms and nitrate concentrations, sometimes at high levels."
High phosphorous, low oxygen, the wrong type of bacteria and high nitrate concentrations are all indicators that help support the call for "In-Lake Management" a term explained in the "Lake Oscawana Aquatic Growth Control District" spring 2007 newsletter from the Lake Oscawana Advisory Committee.
The newsletter states "To have the greatest impact on reducing external phosphorus all discharging of storm water directly into the lake should be stopped. Stormwater should be redirected to detention basins or some other method of filtration."
It is important to note, says a committee member, that the water quality in a lake is a result of many years of build up and that correcting any problems found will also take many years of constant vigilance. "We can all do simple things to reduce the amount of external phosphorous running into a lake such as keeping litter, pet wastes, leaves, and debris out of street gutters and storm drains - these outlets drain directly into lakes, streams, rivers, and wetlands."
You can find out more from Dr. Knoecklein at the Saturday meeting.
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