By Hillary Kenyon and Dr. George Knoecklein
Essential long-term lake water quality monitoring
Lake management requires ongoing monthly water quality monitoring for water clarity, nutrient concentrations of at least Top-Middle-Bottom samples, (data shown here indicate more depths are necessary for necessary characterization), dissolved oxygen profiles, temperature profiles, and regular inlet-outlet sample collection and nutrient analysis. Measurements of bottom waters in October and November need to include oxygen demand constituents as well as Redox potential, and pH.
In order for a volunteer monitoring program to replace professional sampling, volunteers must commit to collecting monthly data from April through November. Weekly clarity readings and dissolved oxygen and temperature profile measurements would further increase the value of the dataset and provide a greater understanding of the internal lake dynamics during the months of October and November. The Internal
Loading versus External Loading Dilemma
Internal loading of phosphorus is a concern during the observed seasonal anoxic periods. The lake was monitored into October and November for the first time in 2015, and due to a combination of the benthic oxygen demand and the depth of West Hill Pond, anoxia peaks in October. The trend was similar in 2016, which means that the prior years, where summer dissolved oxygen content and bottom phosphorus were measured in August, do not represent the worst case scenario of internal loading. Instead, we now know that the fall season has the potential to load more phosphorus into the hypolimnion (bottom layer) than was previously expected.
The large hypolimnetic nutrient mass may be blended with upper water when the lake mixes to the bottom in November. Based on lower percent saturation of oxygen in November some amount of oxygen demand is circulated lake-wide at time presumably from entrainment of hypolimnetic waters. At this there, is only partial characterization of the hypolimnetic water beneath the anoxic boundary in October when anoxia is most intense. The lake phosphorus loading model used a common internal release rate of 1mg/m2/day to estimate 60kg/yr TP load. However, that release rate needs to be refined to better reflect West Hill Pond.
External nutrient load from appears to be different in base-flow vs. stormwater flows. Stormwater flows had lower phosphorus and nitrate than the base-flow values suggesting that largest contribution of water to base-flows of streams is from septic systems. These concentrations appear to be diluted by rainwater, suggesting that stormwater runoff drains minor impervious areas and is collecting only minimal nutrients from these surfaces.
o Continued monitoring of streams during both base-flows and storm events is recommended.
o A perimeter study of seepage water during the fullest extent of winter drawdown is recommended.
o A Very Near Shore Conductivity Sweep during late summer is suggested.