Evaluating Innovative Treatment Strategies to Control Nearshore HABs in Lake Hopatcong

In collaboration with the U.S. Army Corps of Engineers, Engineer Research and Development Center (USACE ERDC), Princeton Hydro designed and is currently implementing a technology demonstration project to evaluate the cost effectiveness of innovative treatment strategies for controlling nearshore harmful algal blooms (HABs) in Lake Hopatcong, New Jersey.

HAB management efforts have traditionally focused on reducing external watershed nutrient inputs and applying in‑lake techniques, such as nutrient inactivation and aeration, to limit internal phosphorus loading from deep, anoxic areas. However, these approaches do not adequately address cyanobacteria growth originating from shallow, nearshore zones, where nutrient‑rich sediments and frequent sediment–water interactions can serve as significant sources of HAB development. As a result, additional and complementary strategies are needed to effectively manage nearshore cyanobacteria blooms.

The objective of this technology demonstration project is to evaluate whether non‑pesticidal products and early‑season, benthic algaecide applications can control cyanobacteria populations originating from nearshore sediments. These approaches are designed to target overwintering cyanobacterial cells prior to the onset of bloom‑favorable conditions, with the goal of reducing the magnitude and persistence of HABs during the growing season.

In 2025, six treatment strategies were implemented across multiple near‑shore coves and bays within Lake Hopatcong. Pre‑ and post‑treatment incubation studies are being conducted under controlled environmental chamber conditions using sediment and water samples collected from treatment and control sites to quantify cyanobacteria growth potential from overwintering cells.

In addition, a comprehensive field monitoring program was established to collect physical, chemical, and biological data prior to treatment and at immediate (one week), short‑term (one month), and long‑term (one year) post‑treatment intervals. Multiple lines of evidence are being used to evaluate treatment effectiveness spatially and temporally, and a cost–benefit analysis will be developed to inform future management decisions.

Preliminary results from immediate and short‑term monitoring indicate promising outcomes in the two coves that received proactive, early‑season benthic algaecide treatments in spring 2025. These sites exhibited reductions in cyanobacteria abundance and related water quality metrics within treatment zones relative to control areas, suggesting that early‑season targeting of overwintering cells may provide an effective and cost‑efficient complement to existing HAB management strategies. Post‑treatment monitoring continued through the 2026 growing season.