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Mercer County Park, spanning over 2,500 acres across the Townships of West Windsor, Hamilton, and Lawrence, is a treasured natural resource. Like many waterbodies throughout New Jersey, some of the lakes within Mercer County Park have been increasingly affected by harmful algal blooms (HABs) in recent years. In response to the growing frequency, duration, and severity of these blooms, the Mercer County Park Commission (MCPC) has intensified its efforts to enhance the overall health of its lakes.
To address these challenges, the County of Mercer tasked the MCPC with developing a comprehensive Lake and Watershed Management Plan. The ultimate goal is to ensure the health, stability, and sustainability of the park’s aquatic ecosystems, thereby enhancing the recreational experience for park users. In this endeavor, the MCPC has partnered with Princeton Hydro to bridge gaps in the existing data and create a thorough management plan.
The plan documents the current conditions of waterbodies within the park, including Mercer Lake, which is the largest, and its surrounding watershed; identifies and prioritizes existing and potential water quality challenges; and provides targeted recommendations for treatment and restoration.
Princeton Hydro conducted a detailed analysis of the lakes’ ecological health, including water quality monitoring, bathymetric mapping, and assessment of hydrologic and pollutant budgets. These comprehensive efforts have culminated in a robust management plan designed to protect and improve the lakes’ ecological balance and recreational value.
While Mercer Lake is a key focus, Princeton Hydro’s commitment extends beyond this single waterbody. Recognizing the interconnected nature of the county’s aquatic ecosystems, the team conducted similar analysis and developed Lake and Watershed Management Plans for three additional lakes in other parks within Mercer County.
These lakes are:
Each of these lakes, like Mercer Lake, faces unique challenges related to maintaining water quality, protecting ecological balance, and mitigating HABs. By applying a comprehensive approach tailored to the specific conditions and needs of each lake, Princeton Hydro aims to enhance the overall health of these vital resources.
Let’s dive into the details of Mercer Lake’s plan!
The first crucial step in developing Mercer County Park’s comprehensive lake management plan involved a thorough review of historical data obtained from various sources, including the County, New Jersey Department of Environmental Protection (NJDEP), New Jersey Department of Transportation (NJDOT), and U.S. Geological Survey (USGS). This review was essential for capitalizing on established water quality trends, identifying recurring problems, and evaluating the success of previous restoration efforts.
The historical data review spanned an impressive range of years from 1963 to 2016, though it did contain some significant gaps. Despite these gaps, the long-term data provided invaluable insights into the lake’s ecological history. By integrating reliable data from past studies, the team could complement their field efforts with supplemental information.
Princeton Hydro examined data on Mercer Lake, a key focus of the management plan initiative, and on all streams within each watershed that feed into the lake. This included any available surface water data from the USGS, a standard approach in aquatic system studies. By analyzing these data, the team identified trends in water quality, highlighted persistent issues, and assessed the effectiveness of past restoration efforts.
This comprehensive historical data review set the stage for a robust watershed assessment, ensuring that the management plan would be informed by a solid foundation of past knowledge.
A bathymetric survey is a scientific method used to map the depths and topography of waterbodies, providing detailed information about the underwater terrain and the distribution of sediments. This survey is crucial for understanding various aspects of a lake’s ecosystem, including sediment thickness, water volume, and potential areas for dredging. The data gathered from a bathymetric survey helps in making informed decisions regarding the restoration and protection of lakes.
Princeton Hydro conducted the bathymetric survey using a calibrated sounding rod for shallow areas and a dual-frequency echo sounder with GPS for deeper regions. The sounding rod was employed in areas with water depths of 12 inches or less and where sediment composition hindered echo sounding. The echo sounder, a Knudsen Engineering model 1612, used high and low frequencies to distinguish the top and bottom of sediment layers. Data points were collected along predetermined transects spaced 150 feet apart, running from shoreline to shoreline in a north-south direction.
Once fieldwork was completed, the collected data was processed using Hypack Max software. This involved editing the raw sounder data to correct errors such as double reflections and interference from aquatic vegetation. The cleaned data was exported to ArcGIS for further analysis and mapping.
The results of the bathymetric survey revealed that Mercer Lake, a key focus of the lake management plan, covers a surface area of approximately 287 acres and is primarily an oval-shaped impoundment. The lake receives inflow from Assunpink Creek and its tributaries and discharges water westward, eventually reaching the Delaware River, Delaware Bay, and the Atlantic Ocean.
Mercer Lake was found to be relatively shallow, with a mean depth of 8.9 feet and a maximum depth of 18.5 feet. The total volume of water in the lake was estimated at around 2,560 acre-feet, or 834.2 million gallons. The survey also indicated significant sediment deposition in the eastern portion of the lake, with a total sediment volume of approximately 855,325 cubic yards. This pattern is likely due to the lake’s role as a settling area for sediment carried by tributary inflows and stormwater discharges, which transport debris, leaf litter, and other materials into the lake.
Below is an image of the Bathymetric Survey that provides a detailed view of the sediment thickness contours measured in feet throughout Mercer Lake:
By establishing a detailed understanding of Mercer Lake’s depth and sediment distribution, the bathymetric survey provides a robust foundation for the comprehensive lake management plan, informing long-term management decisions. The bathymetric data collected is also essential for evaluating the need for dredging, understanding aquatic plant colonization patterns, and predicting the lake’s response to incoming nutrients, helping to guide restoration and protection efforts.
Hydrologic and Pollutant Loading Analysis is crucial for identifying the sources and impacts of pollutants entering a waterbody. It involves delineating watersheds, assessing hydrologic data, and evaluating nutrient loads.
For Mercer Lake, Princeton Hydro conducted an extensive analysis using tools such as USGS StreamStats and Stroud Research Center’s Model My Watershed®. This study provided a detailed understanding of the water and pollutant dynamics within the Mercer Lake watershed. The map below offers an aerial view of the watershed, illustrating the various types of land cover present within the area:
Runoff varied considerably between different sub-watersheds due to factors like land cover types, land-use consumption, impervious surfaces, and topography. Variations in elevation change also determine the impact runoff has on soil erosion, with steeper slopes causing higher erosion rates, especially if little vegetation is present. The chart below shows the various types of soil coverage in areas throughout the Mercer Lake watershed:
Princeton Hydro also assessed other pollutant sources, including groundwater seepage, streambank erosion, and contributions from residential septic systems. Additionally, the impact of waterfowl, particularly Canada Goose, was evaluated using nutrient loading coefficients. The presence of these birds significantly contributes to phosphorus and nitrogen levels.
The hydrologic budget, representing the water balance of the lake, was calculated by considering inputs such as direct precipitation, overland runoff, tributary inflow, and groundwater seepage. This data is vital for conducting trophic state analyses and determining the feasibility of various in-lake restoration techniques. Internal loading of phosphorus, which occurs when anoxic conditions in the lake’s bottom sediments release bound phosphorus into the water, was also analyzed.
Results of the analysis revealed that Mercer Lake, covering 287.1 acres, is influenced by a watershed area of 20,551.4 acres, predominantly consisting of cropland and forested areas. The lake’s shallow nature coupled with significant sediment deposition in the eastern portion, underscores the importance of managing both external and internal nutrient loads.
Understanding the hydrologic and pollutant dynamics through this detailed analysis allows for the development of a lake management plan that helps to prioritize management efforts, target the primary sources of pollution, and effectively address HABs.
Monitoring water quality is essential for understanding the existing chemistry of a lake, identifying trends, pinpointing problems, and assessing nutrient levels. It provides critical data that informs management decisions and helps maintain the health and stability of aquatic ecosystems.
For Mercer Lake, Princeton Hydro conducted thorough water quality monitoring from 2021 to 2023. This involved analyzing in-situ, discrete, and plankton data collected over three growing seasons. The monitoring focused on various parameters, including hypolimnetic anoxia and associated phosphorus dynamics, which are key contributors to HABs. The data collected offered a current assessment of the lake’s trophic state and plankton community, providing a baseline to document shifts in water quality in response to future management measures.
The Princeton Hydro team performed 13 sampling events at two consistent stations in Mercer Lake: a deep water station near the dam (ML-1) and a mid-lake station (ML-2). Various parameters were monitored, including water temperature, dissolved oxygen (DO), pH, specific conductivity, chlorophyll a, and phycocyanin, using an In-Situ AquaTROLL 500 meter.
Water samples were collected at both in-lake stations at the surface (0.5 meters) and near the bottom (0.5 meters above the sediment) using a Van Dorn water sampler. Samples were preserved appropriately and transported to the NJDEP-certified laboratory Environmental Compliance Monitoring (ECM) for analysis. The samples were analyzed for total phosphorus (TP), soluble reactive phosphorus (SRP), total dissolved phosphorus (TDP), nitrate-N, nitrite-N, ammonia-N, total suspended solids (TSS), and turbidity. Surface samples were also analyzed for alkalinity, chloride, and hardness.
Additionally, samples were collected for zooplankton and phytoplankton analysis, including species composition, dominant organisms, and relative density. Cyanobacteria (blue-green algae) genera were quantified to estimate cell counts, providing an approximate concentration of cyanobacteria cells per milliliter of water. Samples were also analyzed for the cyanotoxin microcystins using the Abraxis field testing methodology.
The team also evaluated local climatic conditions during the 2021 – 2023 seasons compared to the long-term average. These conditions, including temperature and precipitation, can have significant effects on water quality. The combination of increased precipitation and an increase in temperatures sets the stage for HABs proliferation. The charts below the monthly mean temperatures and monthly precipitation from 2021 – 2023 and the 30-year average; ‘normal’ refers to the monthly average over the 30-year period from 1991 – 2020.
The Water Quality Monitoring analysis revealed several key insights about Mercer Lake’s water quality, and indicated that cropland runoff was the most significant source of phosphorus, a key driver of HABs. Hypolimnetic anoxia (the bottom layer of the lake becomes devoid of oxygen) was observed during all three field sampling seasons, contributing to internal phosphorus loading. The water quality monitoring also provided valuable information on the lake’s trophic state and plankton community.
Trophic State Modeling is a method used to assess the productivity of a lake by measuring the levels of nutrients, such as phosphorus, and the resulting biological activity. This assessment helps determine the lake’s overall health and informs management strategies. The Trophic State Index (TSI) is a common tool used in this process, calculating index values based on phosphorus concentrations, chlorophyll a levels, and Secchi depths.
For MCPC, Princeton Hydro, utilizing data collected in the field and through lake and watershed modeling, estimated the nutrient status and biological activity of Mercer Lake. Here are a few examples of the models the team utilized:
By leveraging these sophisticated models, Princeton Hydro was able to gain a detailed understanding of Mercer Lake’s nutrient dynamics and productivity. Many models were run twice: once for the watershed-based phosphorus load and once for the total combined load. This allowed for a comprehensive assessment of both external and internal nutrient contributions.
To mitigate pollutant loading issues, the Lake and Watershed Management plan outlines a series of Best Management Practices (BMPs) recommendations for implementation throughout the watershed, which include bioretention systems, wetland buffers, riparian buffers, and lakefront aquascaping. Such measures are designed to reduce nutrient loads, improve water quality, and enhance the overall ecological health of the lake and its watershed. By addressing the root causes of nutrient loading and implementing targeted management strategies, the MCPC is continuing their commitment to providing a sustainable and enjoyable recreational experience for park users while safeguarding the lake’s ecological integrity.
Stay tuned for more updates as we continue to work with the MCPC on implementing the Mercer Lake and Watershed Management Plan, ensuring the watershed remains a vibrant and healthy resource for generations to come.
Regional watershed planning is crucial for maintaining the health and sustainability of interconnected waterbodies. By considering entire watersheds rather than individual lakes, we can develop more effective and comprehensive strategies to manage water quality, control pollution, and enhance ecological resilience. This holistic approach ensures that all elements within the watershed are addressed, leading to more long-lasting improvements.
Princeton Hydro’s efforts in developing and implementing management plans for Mercer Lake, Curlis Lake, Rosedale Lake, and Spring Lake demonstrate the power of coordinated, science-based planning. By leveraging detailed data and advanced modeling techniques, our team is able to create tailored solutions that meet the specific needs of each lake while contributing to the overall health of the region’s aquatic ecosystems.
To read about another project we’re working on in Mercer County, check out our blog about Miry Run Dam Site 21. Through a blend of engineering and ecological enhancements, we are working with MCPC to revitalize 279 acres. With each phase, we edge closer to a vibrant, inclusive space that harmonizes nature and community.
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