The Science of Harmful Algal Blooms

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Building knowledge to protect ecological and human health

Cyanobacterial harmful algal blooms (cyanoHABs) are increasingly a global concern. CyanoHABs can threaten human and aquatic ecosystem health; they can cause major economic damage.

The toxins produced by some species of cyanobacteria (called cyanotoxins) cause acute and chronic illnesses in humans. Harmful algal booms can adversely affect aquatic ecosystem health, both directly through the presence of these toxins and indirectly through the low dissolved oxygen concentrations and changes in aquatic food webs caused by an overabundance of cyanobacteria. Economic damages related to cyanoHABs include the loss of recreational revenue, decreased property values, and increased drinking-water treatment costs.

Nationwide, toxic cyanobacterial harmful algal blooms have been implicated in human and animal illness and death in at least 43 states. In August 2016, at least 19 states had public health advisories because of cyanoHABs (USEPA, 2016).

Chart - National status for cyanoHABs, August 2016
Toxic cyanobacterial harmful algal blooms (cyanoHABs) have been implicated in human and animal illness and death in at least 43 states in the United States (Hudnell, 2008; Graham and others, 2009). In August 2016, at least 19 states had public health advisories because of cyanoHABs (U.S. Environmental Protection Agency, 2016).

What are cyanobacteria?

Cyanobacteria are naturally occurring microscopic organisms. Although they are true bacteria, they function more like algae in aquatic ecosystems. Thus they are typically considered to be part of algal communities (this explains why they often are called blue-green algae). Cyanobacterial blooms may appear as discolorations in the water or paint-like scums at the water-surface. Typically, the blooms are blue-green in color, but they also may be yellow, red, or brown.

Cyanobacteria are notorious for producing a variety of compounds that cause water-quality concerns. Cyanobacteria produce taste-and-odor compounds that people are sensitive to at very low concentrations (even parts per trillion) in drinking water. Taste-and-odor compounds may accumulate in fish flesh making it unpalatable, an important concern for the aquaculture industry.

Of greater concern is the production of the toxins that affect human health. Human ingestion, inhalation, or contact with water containing elevated concentrations of cyanotoxins can cause allergic reactions, dermatitis, gastroenteritis, and seizures.

Recent science insights

USGS scientists are leading a diverse range of studies to address cyanoHAB issues in water bodies throughout the United States, using a combination of traditional methods and emerging technologies in collaboration with numerous partners. However, despite advances in scientific understanding of cyanobacteria and associated compounds, many questions remain unanswered about the occurrence, the environmental triggers for toxicity, and the ability to predict the timing and toxicity of cyanoHABs.

Advanced warnings at time scales relevant to cyanoHAB management (hours to days) would allow proactive, rather than reactive, responses to potential events. Sensors that measure cyanobacteria in near real-time show promise for use in early warning systems.

The ability for cyanobacteria to produce cyanotoxins as well as taste-and-odor compounds is caused by genetic distinctions at the subspecies level of the bacteria. By analyzing those distinctions, we can gain a greater understanding of the world of cyanoHABs, an understanding that may lead us to new ways to combat this threat. USGS is developing field and laboratory methods to quantify cyanobacterial and associated compounds that include field protocols, field guides, sample preparation techniques, development of assays, and molecular tools.   

Nutrient enrichment: A key factor in occurrences of cyanoHABs

One of the key causes of cyanoHABs is nutrient enrichment. When nutrients from agricultural and urban areas are transported downstream, they can cause cyanoHABs in reservoirs, which can impair drinking-water quality and result in closures of recreational areas.

The USGS, in cooperation with local, state, federal, tribal, and university partners, is pioneering new monitoring, assessment, and modeling approaches to better understand nutrient sources, their transport, and their role in toxic cyanoHABs.

Illustration of nutrient sources of Harmful Algal Blooms.
Illustration of nutrient sources of cyanobacterial harmful algal blooms. USGS image.

Tracking the water quality of the nation’s streams and rivers

The USGS monitors nutrient concentrations and flux at key sites nationwide. Annual data are featured at the website, Tracking Water Quality of the Nation's Rivers and Streams.

In addition, the USGS uses advanced optical sensor technology to track nitrate levels in real time at about 135 sites nationwide. These data provide real-time information, improve load calculations, and advance our understanding of processes controlling nutrient variability. The data are publicly available at the website, WaterQuality Watch.

Identifying Nutrient Sources and Hotspots

USGS models of nutrient concentrations and loads in streams provide an important tool for identifying nutrient sources. Estimates derived from these models provide insights into which areas and sources are contributing the largest amounts of nutrients to local streams, lakes, and reservoirs. The models also enable the tracking of nutrients and their sources from local streams to the Nation’s estuaries and the Great Lakes. See website, Tracking the Source and Quantity of Nutrients to the Nation's Estuaries.

Information sources for USGS Cyanobacterial Harmful Algal Bloom science

Cyanobacterial Harmful Algal Blooms and U.S. Geological Survey Science Capabilities — USGS factsheet, 2016

Real Time Water-Quality Watch

Real time measurements nationwide for water temperature, specific conductance, pH, dissolved oxygen, turbidity, nitrate, and chlorophyll.

Field and laboratory guide to freshwater cyanobacteria harmful algal blooms for Native American and Alaska Native communities (USGS, 2015)  Press release

Recognizing that Native Americans and Alaska Natives who depend on subsistence fishing have an increased risk of exposure to cyanotoxins, the USGS has produced a special field guide for these communities. 

Slimy Summer Swimming: Harmful Algal Blooms in Lakes, Rivers and Streams — USGS podcast, 2010

USGS Publications on Cyanobacteria

Harmful Algal Blooms and Hypoxia Comprehensive Research Plan and Action Strategy – An Interagency Report to Congress (Feb. 2016)

A comprehensive research strategy that outlines the roles and responsibilities of federal agencies for evaluating and managing HABs and hypoxia. USGS is the principal agency for scientific research in this field.

Information on Federal Agencies' Expenditures and Coordination Related to Harmful Algae (GAO report, Oct. 2016)