Savery Pond Conservancy

In response to recent intensification of algal blooms, SPC and Plymouth Department of Marine & Environmental Affairs (DMEA) have worked diligently to collect and summarize water-quality data from Savery Pond. Recent monitoring began in 2010 and has continued through present, with emphasis on sampling for the nutrients that tend to fuel algal blooms (phosphorus and nitrogen), chlorophyll (a phytoplankton indicator), and depth-profiles of physical parameters (temperature, dissolved oxygen, clarity). The most up-to-date summary of available data is presented in our 2016 Cumulative Data Report, and is summarized below.

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Water Clarity

Algal blooms cause increased concentrations of chlorophyll and reductions in water clarity.  Most summer measurements of water clarity in Savery Pond do not meet the U.S. Environmental Protection Agency (EPA) reference threshold.

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Temperature

Savery Pond is considered to be a “warm water fishery”, and measured temperatures are within associated Massachusetts water-quality criteria. Summer vertical temperature profiling has shown both “stratified” and “well-mixed” conditions; with lower temperatures near the pond bottom likely reflecting groundwater inflow (springs). Stratified conditions can promote accumulation of anoxic conditions (lack of dissolved oxygen) at the pond bottom and associated release of sediment-bound phosphorus; whereas mixing can transport nutrients from deep into the shallower water.

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Dissolved Oxygen

Profiles of dissolved oxygen (DO) showed occurrences of depleted DO at the pond bottom (where concentrations fell below the Massachusetts water-quality standard of 5 mg/l – and sometimes approached anoxic conditions), along with other periods where DO exceeded the MA standard throughout the water profile. Most profiles with depleted DO near the pond bottom corresponded to algal blooms. Anoxic conditions can support an increased rate of legacy nutrient regeneration from lake-bottom sediments, and plankton would be expected to respond to increased nutrient availability.  Periods of warmer temperature also appear to reduce DO near the pond bottom, possibly due to increased algae production and subsequent algal decomposition on the pond bottom.

 

Nutrients in Pond Water

Nutrients fuel algal blooms, and sampling of 38 Plymouth ponds and lakes in 2014 showed that Savery Pond had the highest nutrient concentrations among local water bodies evaluated. In response to this finding, water samples were taken near both the top and bottom of the water column over 3 summers. All nutrient concentrations exceeded the “impaired water quality” reference thresholds for total phosphorus (TP) and total nitrogen (TN). The highest measured TP concentration was 20-times the associated reference threshold, and the highest measured TN concentration was over 5-times the associated reference threshold.

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Nutrients in Pond Sediments

Several water sampling events occurred where TP near the pond bottom was more than twice the TP near the pond top, suggesting sediment release of TP into the overlying water column. Preliminary nutrient measurement in pond sediments performed in 2012 showed moderately high phosphorus concentrations, as would be expected after decades of fertilizer applications to the adjoining cranberry bogs.  "Legacy nutrients" adsorbed to pond sediments tend to release back into pond water under anoxic conditions. In 2019, member donations are being used to better measure nutrients bound to the pond sediments as a cooperative effort between SPC, the Plymouth Department of Marine and Environmental Affairs (DMEA) and UMass Amherst School for Marine Science & Technology (SMAST).

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Nutrient Budget

Developing a management plan requires an accounting of nutrient sources and sinks for the pond. Nutrient inputs to pond are both natural (from wildlife, shoreline plants and groundwater inflow) and anthropogenic (from septic system discharge via groundwater, residential stormwater runoff, fertilizer applications to cranberry bogs and residential landscaping, and historic accumulation of legacy nutrients in pond sediments). Data collected to date suggest that cranberry bog operation may have contributed significant amounts of nutrients to the pond; however, the only remaining commercial cranberry bog on the pond ceased operation in 2017.