Harmful Algal Blooms in New Jersey’s Coastal Waters: Causes, Impacts and Nature‑Based Solutions

Posted on May 03, 2026 by Princeton Hydro

From tidal estuaries and back bays to nearshore marine waters, New Jersey’s coastal environments support fisheries, recreation, wildlife, and local economies. Increasingly, however, these valuable ecosystems are vulnerable to a wide range of harmful algal blooms (HABs). While algae are a natural and essential part of aquatic ecosystems, certain environmental conditions can cause some species to grow excessively, leading to ecological damage, public health risks, and economic losses.

Understanding what HABs are, what drives them, and how nature‑based restoration strategies can prevent or mitigate blooms is essential to supporting the long‑term resilience of New Jersey’s coastal environments.

What Are Algae and When Do They Become Harmful?

The term “algae” is ecological rather than taxonomic and encompasses a diverse group of organisms, including eukaryotic algae, such as diatoms and dinoflagellates, and prokaryotic cyanobacteria, commonly referred to as blue‑green algae. Algae are not inherently harmful. In fact, they provide critical ecosystem services, including:

Forming the base of aquatic food webs
Producing oxygen through photosynthesis
Sequestering carbon and contributing to climate regulation
Supporting fisheries and overall aquatic health
Offering potential applications in biofuel and pharmaceutical development

Phytoplankton are microscopic, free‑floating algae found in freshwater, estuarine, and marine environments. Scientists estimate there are 20,000 to more than 100,000 phytoplankton species, but only a small fraction—roughly 100 to 300 species—are capable of forming toxin‑producing harmful algal blooms. Problems arise when these species proliferate rapidly under favorable conditions. These blooms can become harmful when they produce toxins, deplete oxygen, shade submerged vegetation, or otherwise disrupt ecosystem function.

Most toxin‑producing HABs fall into three major groups:

Dinoflagellates (often associated with red tides)
Diatoms (commonly linked to brown tides)
Cyanobacteria (blue‑green algae)

While most harmful algal blooms are caused by phytoplankton, large, fast‑growing macroalgae can also create serious environmental and economic challenges when conditions allow them to proliferate. A well‑known example is Sargassum, a floating seaweed that can form extensive mats across the ocean surface. During periods of rapid growth, these mats can block sunlight from reaching coral reefs and other sensitive habitats. When Sargassum washes ashore in large quantities, it can deter tourism and recreation. As the algae decomposes, it releases hydrogen sulfide gas, producing strong odors that make nearby coastal areas unpleasant to visit. While Sargassum blooms occur most summers along the coast of south Florida, the severity and extent of these events vary considerably from year to year.

HABs can form in freshwater systems, brackish estuaries, and coastal marine waters, and they are particularly dangerous with myriad when they produce toxins that affect humans, pets, livestock, fish, shellfish, and wildlife.

A Deeper Dive into Marine HABs

Below is a closer look at the dominant types of marine HABs in the region, the organisms responsible, and the environmental conditions that influence their development.

Red Tides (Dinoflagellates)

Common toxin‑producing dinoflagellates include:

Red tide in summer season, Key West, Florida

Brown Tides (Diatoms and Related Groups)

Brown tides are associated with several diatom genera, such as:

Pseudonitzschia sp, known to produce domoic acid which is the marine biotoxin related to amnesic shellfish poisoning (ASP).
Pseudo-Nitzschia, cause harm through the production of the neurotoxin domoic acid (DA), which can be transferred to other trophic levels through bioaccumulation.
Amphora, can cause ASP in humans and marine mammals, and can accumulate in filter-feeding shellfish, leading to severe health issues.
Aureococcus (pelagophyte), a well‑known brown tide organism in mid‑Atlantic estuaries, can cause neurological damage in humans and wildlife.

Environmental Drivers of Red and Brown Tides

These blooms are influenced by a combination of physical, chemical, and climatic factors, including:

Increased water temperature and light availability
Reduced estuarine flushing or circulation
Water column mixing events
Elevated salinity
Mild winters and dry spring conditions
Elevated inorganic nutrients (for many, but not all, species)
Inputs of iron and organic nutrients

Green Macroalgal Blooms: Ulva

Ulva, commonly known as sea lettuce. Photo Credit: iNaturalist

Ulva, commonly known as sea lettuce, is a green macroalga that can form extensive blooms in shallow, nutrient‑rich estuaries. Another common bloomer, Enteromorpha, is now considered genetically equivalent to Ulva. Although Ulva blooms are non‑toxic, they can still cause serious ecological and social impacts:

Blooms generate strong odors that reduce recreational value
Decomposition consumes oxygen, stressing fish and invertebrates
Water clarity declines, further limiting seagrass growth
Dense mats can shade and smother seagrass beds
Seagrass loss weakens sediment stabilization and reduces habitat quality for many coastal species

Cyanobacteria

Common bloom‑forming Cyanobacteria genera include:

Microcystis
Dolichospermum (formerly Anabaena)
Aphanizomenon

Cyanotoxins should not be confused with taste‑and‑odor (T&O) compounds. Cyanotoxins are colorless, tasteless, and odorless whereas T&O compounds, such as geosmin and MIB, cause earthy or musty smells. Cyanobacteria can produce T&O compounds without toxins as well as toxins without noticeable odors.

This distinction can complicate detection and public perception of risk.

Environmental Drives of CyanoHABs

HABs are commonly driven by:

Warmer water temperatures
Reduced flushing and slow‑moving water
Stable, stratified water columns
Elevated phosphorus concentrations, which increase biomass
Increased availability of inorganic nitrogen, which can stimulate toxin production (e.g., microcystins)

Ecological, Human, and Economic Impacts

The impacts of marine and estuarine HABs extend far beyond discolored water.

Algal toxins can cause neurological, gastrointestinal, and respiratory symptoms in humans and animals, and in severe cases, death
Human fatalities, linked to consuming contaminated shellfish or finfish
Fish kills and deaths of birds, sea turtles, and marine mammals associated with toxin exposure
Blooms reduce dissolved oxygen and shade seagrasses, stressing or killing aquatic life
HABs cause aesthetic and economic losses, including reduced beach access and impacts to commercial fisheries
Even non‑toxic blooms can degrade habitat quality and diminish recreational and ecological value

These HABs, the region’s most common, illustrate the wide range of organisms, toxins, and ecological pathways through which algal blooms can affect coastal systems. Although they differ in form, from microscopic phytoplankton to expansive mats of macroalgae, they are often driven by a common set of environmental conditions that favor rapid growth and persistence. Climate change is intensifying many of these drivers. Rising water temperatures, altered precipitation patterns, and longer periods of stratification increasingly create conditions that favor bloom formation. At the same time, human activities continue to increase excess nutrients to coastal waters. Runoff from agricultural lands, chemicals transported by rainfall and irrigation, and discharges from wastewater treatment facilities all introduce nitrogen and phosphorus into rivers, lakes, and estuaries. These nutrients act as fertilizer for algae, accelerating bloom development.

Nutrient‑laden stormwater runoff does not remain localized, rather, it moves downstream through interconnected watersheds, ultimately reaching estuaries and coastal waters where it can contribute to marine blooms. Understanding these linkages between land use, climate, and algae growth is critical to identifying effective strategies for preventing and managing HABs in coastal environments.

Nutrient sources of HABs – Illustration created by USGS.

Nature‑Based and Nearshore Restoration Strategies

A range nature-based nearshore and shoreline restoration and management strategies are increasingly used in coastal systems to help mitigate HABs:

Nutrient Remediation in the Watershed: these measures should include sewer upgrades and septic system management as well as stormwater BMPs and green infrastructure, including bioretention basins, rain gardens, and naturalized stormwater features. Helping to reducing nutrients before they reach coastal waters is one of the most effective long‑term HAB mitigation strategies (NJDEP, 2026).
Biochar in Watersheds and Aquatic Systems: Biochar is a porous, carbon‑rich material that adsorbs contaminants, especially phosphorus. Its use in waterbodies, particularly nearshore, shallow areas has shown to improve water quality, reduce nutrient availability for algal growth, providing a relatively low‑cost, renewable management option.
Nutrient Inactivators in Nearshore, Shallow Sediments: Lanthanum‑modified clays, such as Phoslock, bind with dissolved phosphorus to form a stable mineral that settles into the sediment, preventing it from recycling into the water column. 1lb of phosphorus can generate up to 1,100lbs of wet algae biomass, and 1.1tns of Phoslock can remove 24lbs of phosphorus (SePRO Corporation, 2012).
Floating Wetland Islands (FWIs): A single 250‑square‑foot island can function like one acre of natural wetland, improving water quality by assimilating and removing excess nutrients that could fuel algae growth; providing valuable ecological habitat for a variety of beneficial species; helping mitigate wave and wind erosion impacts; providing an aesthetic element; and enhancing biodiversity within open freshwater environments. 1lb of phosphorus can produce 1,100lbs of algae annually; one 250‑square‑foot FWI can remove approximately 10lbs of phosphorus annually, potentially mitigating up to 11,000lbs of algae (Floating Island International, 2011).

Additional management approaches, depending on site conditions, may include:

Restoring riparian and shoreline buffers
Selective dredging of overwintering beds of resting akinetes, cysts, or vegetative cells
Reactive use of non-pesticidal products (i.e. clays, chitosan, etc.)
Addressing internal nutrient load through aeration and/or oxygenation
Stimulating the growth of beneficial submerged aquatic vegetation
Encouraging the growth of beneficial, non-pathogenic, heterotrophic bacteria

HABs represent a complex and growing challenge in New Jersey’s freshwater, estuarine, and coastal systems. They threaten public health, ecosystems, and coastal economies, but they are not insurmountable. Nutrient control, thoughtful watershed management, and nature‑based restoration strategies are central to preventing, mitigating, and controlling HABs. If you’re interested in learning more about our work to identify, assess and mitigate HABs, click here to read about our groundbreaking research project with Friends of Hopewell Valley Open Space to monitor HABs using drone technology, advanced data modeling, and community science across a 73-mile stretch of the Delaware River and 23 associated waterbodies.

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Posted on December 9th, 2025 by Princeton Hydro

Harmful Algal Blooms in New Jersey’s Coastal Waters: Causes, Impacts and Nature‑Based Solutions