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Water bodies, watersheds and storm water
Quick reference

Find tips for what you can do to reduce algae and water pollution.

For information about swimming in waters with algae, visit the Florida Department of Health. That site contains links to your local health department.

To report a fish kill or for information about potential impacts to wildlife, contact the Florida Fish and Wildlife Conservation Commission.

Algae information is available from the World Health Organization (WHO).

Understanding algal blooms


Spring through early fall are the times of year that water bodies typically exhibit the most visible response to water quality problems. Algal blooms can be dramatic and are a result of excess nutrients from fertilizer, wastewater and stormwater runoff, coinciding with lots of sunlight, warm temperatures and shallow, slow-flowing water.

What is an algal bloom?

Some species of algae grow in clumps, stick together in large surface scums or form thick mats.

Some species of algae grow in clumps, stick together in large surface scums or form thick mats.

Algae are photosynthetic microorganisms that are found in most habitats. Algae vary from small, single-celled forms to complex multi-cellular forms.

An algal bloom is a rapid increase in the density of algae in an aquatic system. Algal blooms sometimes are natural phenomena, but their frequency, duration and intensity are increased by nutrient pollution. Algae can multiply quickly in waterways with an overabundance of nitrogen and phosphorus, particularly when the water is warm and the weather is calm. This proliferation causes blooms of algae that turn the water noticeably green, although other colors can occur. Some species of algae grow in clumps covered in a gelatinous coating and have the capability to float, allowing cells to stick together into large surface scums in calm weather. Other algae form thick mats that float on or just below the surface along the shoreline. In the St. Johns River, blooms most often are composed of cyanobacteria, also known as blue-green algae.

Cyanobacteria are bacteria that, like plants, use solar energy and carbon dioxide to grow (photosynthesis). Cyanobacteria occur naturally in both freshwater and marine (salt) water bodies. Blooms also can be caused by dinoflagellates (single cell microorganisms [phytoflagellates] that include luminescent forms) in marine or estuarine water bodies (red tides are an example of a dinoflagellate bloom).

Why do algal blooms occur?

The potential for blooms comes from nutrient pollution, an overabundance of the essential plant nutrients nitrogen and phosphorus. These elements enter waterways from point sources (such as industrial and wastewater treatment plant discharges), nonpoint sources (such as septic tanks and stormwater runoff from farms, urban areas and residential areas), and from nutrient-enriched rainfall. When the concentrations of nitrogen and phosphorus increase in a water body, the right combination of temperature, sunlight and low flow can trigger an algal bloom. Though nitrogen and phosphorus occur naturally and are essential plant nutrients, an overabundance of these nutrients can cause significant imbalances in the water body’s ecology, and blooms are one symptom.

An overabundance of nitrogen and phosphorus can cause blooms of algae that turn the water noticeably green.

An overabundance of nitrogen and phosphorus can cause blooms of algae that turn the water noticeably green.

Masses of algae often are observed in stormwater ponds during Florida’s warmer seasons. Stormwater facilities are designed to capture polluted runoff. The ponds help remove nutrients and sediments collected by storm water before these pollutants can reach fragile waterways. Stormwater ponds serve important environmental purposes — they treat pollutants and hold floodwaters.

What is being done to reduce algal blooms?

Because the problem is an overabundance of algae fueled by excessive nutrients, the St. Johns River Water Management District targets its work on the reduction of nutrient pollution entering water bodies. Reduction in nutrients will reduce the occurrence and intensity of harmful algal blooms.

The district, in partnership with other agencies, local governments and water utilities, has designed and constructed dozens of tailor-made projects to restore degraded water bodies.

Projects for nutrient reduction have included re-directing wastewater discharges from water bodies, stormwater collection and retention improvements, sanitary sewer collection improvements, and repairing or removing malfunctioning septic tanks. At the same time, the district encourages best management practices and has constructed regional stormwater treatment facilities to reduce nutrient runoff from agricultural areas.

What can you do to help reduce nutrients and algal blooms?

The public plays an important role in reducing nutrient pollution. Read tips on what you can do to reduce water pollution.

Who is keeping tabs on algal blooms?

District scientists routinely collect water and algae samples, particularly during periods when conditions are right for algal blooms. When an algal bloom is observed, samples are collected, and additional tests are conducted to determine if algal toxins are present.

District staff provide these results to other agencies, including the Florida Department of Health (which is responsible for sharing the information with county health units), the Florida Department of Environmental Protection (DEP) and the Florida Fish and Wildlife Conservation Commission (FWC). Since 2009, these agencies have operated under a coordinated plan to respond to potentially harmful algal blooms. Additional information is available in the Fish and Wildlife Research Institute (part of FWC) technical report Resource Guide for Public Health Response to Harmful Algal Blooms in Florida.

Are algal blooms harmful?

Algae in the St. Johns River near downtown Jacksonville.

Algae in the St. Johns River near downtown Jacksonville.

Algae are a natural component of the aquatic food chain and are typically not harmful to people. However, the overabundance of algae in a bloom can be aesthetically unappealing and harmful to the environment. If the types of algae that produce toxins reach high concentrations, then native aquatic organisms, livestock, pets, and perhaps even people who come in contact with the toxins, can be affected. In the St. Johns River, harmful algal blooms have occurred in past years. Dense, widespread blooms occurred in summer of 2005 and 2010 in the river’s lower basin (that portion between Welaka and Mayport). These blooms led to fish kills and numerous reports of skin rashes, unappealing odors, and accumulations of foam and shoreline scums.

When algal blooms block vital sunlight from reaching beneficial underwater plants that provide food and a place to live and grow for fish and other animals, the ecosystem can be negatively impacted. Algae become stressed and die when they deplete the nutrient supply or move from freshwater into saltier waters. Decomposition of dying algae can reduce levels of dissolved oxygen in the water, which fish and other aquatic animals breathe. Some fish species with little tolerance for low dissolved oxygen levels may die. In addition, some algal species can cause fish kills directly either by production of algal toxins or by clogging the gills.

While the possibility of algal toxins in the environment is a serious concern, the more common problems associated with harmful algal blooms are environmental damage and the impact on recreational activities and commerce due to the unsightly green scum and accompanying unpleasant odor.

Is it safe to swim or fish in waters with an algal bloom?

The district recommends contacting your local health department for direction and advice about human health issues associated with algal blooms. The Florida Department of Health provides links to the websites of county health departments across the state.

The Poison Control Hotline operates a 24-hour, 7-day-a-week, 365 days per year toll-free telephone number — 800-222-1222 — that is available for the public and health professionals. It is staffed by physicians, nurses and pharmacists.

For information about potential impacts to wildlife, contact FWC.

The World Health Organization (WHO) in its Guidelines for Safe Recreational Water Environments, Vol. 1, Coastal and Fresh Waters, offers guidelines to follow when algal blooms are present:

  • Avoid areas with visible algae and/or scums. Direct contact and ingestion are associated with the greatest health risk. If no scums are visible, but water shows a strong greenish discoloration such that you cannot see your feet when standing knee deep (after sediment has settled) avoid bathing (swimming), immersion of head, and/or ingestion.
  • Avoid waterskiing in visible scums or waters with a strong greenish coloration as described above because of the potentially substantial risk of exposure to aerosols.
  • If sailing, sailboarding or undertaking any other activity likely to involve accidental immersion, wear clothing that is loose fitting in the openings. Use of wet suits for water sports may result in greater risk of rashes, as the algal material trapped in the wet suit will be in contact with the skin for longer periods of time. After coming ashore, shower or wash to remove algal material.


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St. Johns River Water Management District
4049 Reid Street, Palatka, FL 32177
(800) 725-5922