top of page

Are Harmful Algal Blooms in Lake Hopatcong Fueled by Climate Change?

By Princeton Hydro

The Musconetcong River begins at New Jersey’s largest lake, Lake Hopatcong, and flows southwest for 42 miles before emptying into the Delaware River. At the headwaters in Lake Hopatcong, the community has been battling with harmful algal blooms (HABs). HABs can cause significant water quality issues in lakes and ponds, often forming a visible and sometimes odorous scum on the surface of the water. Blooms are primarily caused by warmer temperatures and increased amounts of nutrients (i.e., nitrogen and phosphorus) from stormwater runoff.

In 2019, the local community suffered immensely from HABs, which was the most prolific bloom the lake has experienced in over the last two decades, resulting in public health advisories to be issued for recreation on the lake. Because Lake Hopatcong is a popular summer vacation destination, this outbreak stunted the local economy, restricted recreational usage of the lake, and impacted fish and wildlife.

The Lake Hopatcong Commission and Lake Hopatcong Foundation, in partnership with municipalities, counties, the state, local groups like the Musconetcong Watershed Association, and Princeton Hydro, have been working to improve water quality for years by prioritizing stormwater mitigation and septic management policies within the watershed. So why was the summer of 2019 so intense?

Analysis of 30 Years Water Quality Data

Princeton Hydro scientists have been collecting water quality data in Lake Hopatcong for 30 years. This includes dissolved oxygen, pH, and temperature, as well as concentrations of total suspended solids, total phosphorus, nitrate‐N, ammonia‐N and chlorophyll a, and various biological factors. There are not many lakes in New Jersey that have such a robust and consistent public dataset, which presents a rare opportunity to study long-term trends. We dove a little deeper into this information to see what many have caused the 2019 blooms.

We analyzed a statistically significant dataset of surface water temperatures and found that the average July surface temperatures in Lake Hopatcong have been steadily increasing over time. We also have 20+ years of observational data that documents an increase in frequency, duration, and magnitude of HABs over the same time period. In fact, HABs have recently persisted all the way into the winter months, enabling “green ice” to form on the lake surface, as observed in December 2020.

During the summer of 2019, the Lake Hopatcong region was hit with a dramatic amount of rainfall. These weather patterns resulted in some of the highest early summer total phosphorus (TP) concentrations in Lake Hopatcong in over 20 years. The mean June TP concentration was 0.043 mg/L; the last time it exceeded 0.04 mg/L was in 1999. In order to have acceptable water quality conditions in the lake, the mean TP concentrations should be at 0.03 mg/L or lower.

It has been well documented that phosphorus is the primary limiting nutrient in Lake Hopatcong. Meaning, a slight increase in phosphorus can result in a substantial increase in algal and/or aquatic plant biomass. The water quality analysis identified the cause for the HABs (the high frequency of storms in June 2019 transporting nutrients, in particular phosphorus, to the lake) and identified why they persisted over the growing season (internal phosphorus loading).

Climate Change as a Driver for HABs

Climate change is leading to more frequent, more intense rainstorms that transport run-off pollutants into waterways, coupled with hotter days to warm the water. The latest Intergovernmental Panel on Climate Change (IPCC) report, “AR6 Climate Change 2021: The Physical Science Basis,” confirmed that human influence has warmed the atmosphere, ocean, and land, and that this human-induced climate change is already affecting many weather and climate extremes in every region across the globe. It predicts, “increases in the frequency and intensity of hot extremes, marine heatwaves, and heavy precipitation, agricultural and ecological droughts in some regions, and proportion of intense tropical cyclones, as well as reductions in Arctic sea ice, snow cover and permafrost.” In the Mid-Atlantic region of the U.S., most climate models indicate that the landscape will become warmer and wetter.

Looking at our observations and 30-year dataset for Lake Hopatcong, our preliminary analysis shows that climate change — increased precipitation (which flushed the phosphorus into the lake) followed by intense heat to warm surface water temperatures — was a significant variable that led to the devastating HABs at Lake Hopatcong in 2019.

Other communities have experienced similar trends too. According to the U.S. Environmental Protection Agency, HABs have now been observed in all 50 states, ranging from large freshwater lakes, to smaller inland lakes, rivers, and reservoirs. Our neighbors in Upstate New York suffered from 1,000+ HAB occurrences during the 2019 season, including a HAB that covered 600+ square miles of Lake Erie causing beach closures and fish kills.

A study recently published in Nature journal reviewed three decades of high-resolution satellite data for 71 large lakes globally and determined that “peak summertime bloom intensity has increased in most (68%) of the lakes studied, revealing a global exacerbation of bloom conditions.” The study called for water quality management efforts to better account for the interactions between climate change and local hydrological conditions.

We are witnessing these impacts firsthand at Lake Hopatcong and within the Musconetcong River Watershed. And, according to the IPCC report, these climate change-induced instances (i.e. intense rainfall followed by intense heat) may become even more frequent. To further understand the connection between climate change and HABs at Lake Hopatcong, Princeton Hydro is conducting a more rigorous study that includes more distinct data. We hope this will provide some insight on how to manage expected climate impacts in lakes and watersheds.

Taking Action in the Musconetcong River Watershed

While the IPCC report conclusions may be concerning, there is still much we can do at both a global and local level to limit future climate change. The key is globally limiting cumulative CO2 (carbon dioxide) and CH4 (methane) emissions and quickly reaching (at least) net zero CO2 emissions. And, to specifically reduce occurrences of HABs fueled by climate change in Lake Hopatcong, eliminating sources of phosphorus from entering the lake is critical. So what can we do in the Musconetcong River Watershed?

In 2019, NJ Department of Environmental Protection committed $13.5 million via their Water Quality Restoration Grant programs for local projects that aim to improve water quality and help prevent, mitigate and manage HABs in New Jersey’s lakes and ponds. The Lake Hopatcong Commission landed a $500k grant via the program to evaluate and implement a variety of innovative, nearshore projects to address HABs at Lake Hopatcong. Projects included performing an alternative non-copper-based algaecide treatment and one of the largest nutrient PhosLock treatments in the Northeast on the lake as well as the installation of Biochar bags, near-shore aeration systems, and floating wetland islands. This could not be possible without the help of all project partners including Lake Hopatcong Foundation, Morris County, Sussex County, Jefferson Township, Borough of Hopatcong, Borough of Mt. Arlington, and Roxbury Township, who collectively contributed over $330k in match support. The Lake Hopatcong Commission also landed a subsequent $206,000 grant via NJDEP’s 319 program a few months later, with $44,000 in match support from the four municipalities and Lake Hopatcong Foundation and Commission, for the design and implementation of four in-lake/watershed projects to prevent and mitigate HABs on Lake Hopatcong.

The results of these projects were significant. Over the last two years, the mean June TP concentrations were lower than 2019 (0.033 mg/L in 2020 and 0.020 mg/L in 2021). These in-lake and watershed efforts have had a positive impact on reducing available phosphorus and, in turn, reducing the size and magnitude of HABs on Lake Hopatcong.

Just this month, the Lake Hopatcong Commission landed another $480k from a National Fish and Wildlife Foundation Delaware Watershed Conservation Fund grant, which was backed with $489k more in match support from Lake Hopatcong Commission, Lake Hopatcong Foundation, Musconetcong Watershed Association, NJDEP, Borough of Hopatcong, Township of Roxbury, Mount Arlington Borough, Morris and Sussex Counties, Lake Hopatcong Historical Museum, Rutgers University, NJ Highlands Council, and Princeton Hydro. The project team will design and implement three streambank stabilization projects in the watershed, which were identified as priority projects in the 2021 Upper Musconetcong River Watershed Implementation Plan.

Managing loads of phosphorus in watersheds is even more important as the East Coast becomes increasingly warmer and wetter thanks to climate change. Community groups, local agencies, and individuals can have a real impact in reducing phosphorus concentrations at a local level,” said Dr. Fred Lubnow, Director of Aquatic Resources for Princeton Hydro.

Changes in hydrology, water chemistry, biology, or physical properties of a lake can have cascading consequences that may rapidly alter the overall properties of a lake and surrounding ecosystem, which can lead to negative consequences like HABs. Recognizing and monitoring the changes that are taking place locally brings the problems of climate change closer to home, which can help raise awareness and inspire environmentally-minded action.


Princeton Hydro is committed to improving our ecosystems, quality of life, and communities for the better. The firm provides integrated ecological and engineering consulting services with expertise in natural and aquatic resource management, water resources engineering, geosciences, and regulatory compliance. From concept to construction, they collaborate with clients to create resilient, sustainable, regenerative, and nature-based solutions to help meet their respective missions.


Featured Posts
Recent Posts
Search By Tags
Follow Us
  • Facebook Basic Square
  • Twitter Basic Square
  • Instagram
  • YouTube
bottom of page