Harmful Algal Blooms

"Harmful algal bloom" is one of the most-used phrases in freshwater science and one of the most misunderstood. A bloom is a lot of cells; a harmful bloom is one that causes harm; a toxic bloom is one that contains toxins at concentrations that matter. Those are three different claims, and each requires its own evidence. This chapter separates them, describes the major cyanobacterial toxins, explains why the numbers you might measure — cell counts, color, pigments and toxin concentrations — are related but not interchangeable, and shows how health agencies, including Kentucky's, turn measurements into advisories.

5.1What is a bloom?

A bloom is a rapid increase in the biomass of phytoplankton or attached algae, often to the point that it changes how the water looks: green, blue-green, brown or red discoloration, floating flecks, surface scums, or mats on the bottom or shoreline [1, 2]. There is no single universal threshold — scientists define blooms using cell counts, biovolume, chlorophyll or visual criteria, depending on the purpose. In fresh water, most problematic blooms are formed by cyanobacteria, which is why "cyanobacterial harmful algal bloom" (cyanoHAB) is often used, even though cyanobacteria are not algae [3].

Planktonic blooms develop when cyanobacteria grow faster than they are lost to grazing, sinking, flushing and death. Warm temperatures, nutrient enrichment, calm stratified water, long residence times and abundant light all push the balance toward growth, and buoyant species can concentrate at the surface in calm weather [4, 1, 5]. Chapters 3 and 4 develop these drivers.

5.2Why some blooms become harmful

A bloom becomes harmful through one or more of these routes [3, 1]:

• Toxins. Some strains produce compounds that damage the liver, nervous system or other organs of animals and people (Section 5.4).
• Oxygen depletion. Dense blooms produce oxygen in daylight but consume it at night, and when a bloom dies, bacterial decomposition can strip oxygen from the water, killing fish and invertebrates (Section 5.6).
• Taste and odor. Compounds such as geosmin and 2-methylisoborneol give drinking water an earthy or musty taste, increasing treatment costs even when no toxin is present [6].
• Ecological and economic losses. Blooms shade out other producers, provide poor food for grazers (Chapter 6), close beaches, and reduce recreational and property value. One national estimate put the potential annual cost of freshwater eutrophication in the United States in the billions of dollars [7].

5.3Toxin-producing and non-producing strains

Toxin production is a property of genotypes, not species. Within a single Microcystis population, cells that carry the genes for microcystin synthesis grow alongside cells that do not, and the two cannot be told apart by appearance [8, 9]. Studies that measured toxin genes directly found that the abundance of toxic genotypes predicted microcystin concentrations better than total cyanobacteria, total Microcystis or chlorophyll [10]. And because bloom communities often contain several genera, a bloom dominated by one genus can contain toxins made by another [6].

5.4The major cyanotoxins

Cyanotoxins are grouped by the organ or process they target. Four groups dominate freshwater monitoring [11, 12, 13]:

Benthic cyanobacteria are not a separate, safer category: mats in rivers and lakes produce most of the same toxins, including microcystins, anatoxins, cylindrospermopsins and saxitoxins, and they have been involved in many animal poisonings [14, 15]. Other cyanobacterial compounds — skin irritants and lipopolysaccharides from the cell envelope, for example — can cause rashes and other symptoms after recreational contact [14, 16].

Toxin mixtures are common. In blooms sampled from 23 Midwestern lakes, microcystins occurred in every bloom, and many blooms contained several toxin groups along with taste-and-odor compounds [6]. Nationally, the 2007 EPA National Lakes Assessment detected microcystins in 32% of samples, saxitoxins in 7.7% and cylindrospermopsins in 4.0% [17].

5.5Intracellular versus dissolved toxin

Most microcystin in an actively growing bloom is held inside living cells (intracellular). It is released into the water (dissolved or extracellular) when cells age, die or are broken — by natural decline, by grazing, or by treatments such as algaecides that rupture cells. Cylindrospermopsin, in contrast, is often found largely dissolved [13, 16]. This distinction has practical consequences:

• Sampling. A "total" toxin analysis requires lysing cells (for example, by repeated freeze–thaw) before measurement; filtering the sample first measures only the dissolved fraction. Results must say which was measured.
• Treatment. Conventional drinking-water treatment can remove intact cells and low levels of toxins, but severe blooms require planning, because treatments that break cells can move toxin into the dissolved phase, where it is harder to remove [18].
• Timing. Dissolved toxin can persist after a visible bloom has disappeared, so "the water looks clear now" is not evidence that it is safe [13].

5.6Oxygen depletion

A dense bloom swings dissolved oxygen through a daily cycle: photosynthesis can supersaturate surface water in the afternoon, while respiration by the same cells and by bacteria can drive oxygen very low before dawn. When the bloom collapses, decomposition of the dead biomass consumes oxygen further, especially in warm, stratified water where the bottom layer is cut off from the atmosphere. The result can be fish kills and the release of phosphorus from oxygen-poor sediments, which feeds the next bloom [3, 19]. For field researchers, this means a single daytime oxygen reading can badly misrepresent conditions; time of day must be recorded with every measurement.

5.7Cells, color and toxins are not interchangeable

Several different numbers are used to describe blooms, and each answers a different question [13, 17]:

The evidence that these differ is direct. In Northern Kentucky urban ponds monitored for a year, microcystin levels did not always coincide with high Microcystis cell counts [20]. In the national lakes survey, cyanobacteria were present in 98% of samples but microcystins were detected in about a third [17].

5.8Effects on fish, livestock, pets, wildlife and people

Animals are often the first victims. Dogs are especially vulnerable because they swim in, drink and lick bloom material and mats from their fur; U.S. records of suspected and confirmed canine cyanotoxin poisonings span nearly a century [27]. Livestock drinking from enriched farm ponds are also at risk — and two of the very few published Kentucky records are livestock cases: pigs poisoned by a pond bloom attributed to Anabaena (now Dolichospermum) in 1989, and 15 cattle that died around two ponds in August 2013, with microcystin-LR detected in one pond and in rumen contents [28, 29]. Because animal illnesses often precede or accompany human exposures, they can serve as sentinel events for public health [30].

Fish are affected mainly through oxygen depletion during bloom decline, and also by toxins [3].

People are exposed by swallowing water while swimming, by skin contact, by inhaling spray during activities such as water-skiing, and through drinking water [12, 31]. Symptoms range from skin rashes, eye irritation and gastrointestinal illness to, at high doses, liver or nervous-system effects. Young children are of particular concern, which is why EPA's drinking-water advisories set lower values for bottle-fed infants and pre-school children (Table 5.3) [32].

5.9Recreational and drinking-water guidance

Guidance values are concentrations at or below which health effects are not expected for a given exposure. They are not legally enforceable unless a state adopts them. For U.S. waters, the EPA has issued:

Sources: EPA recreational recommendations [33] and drinking-water health advisories [32]. The much lower drinking-water values reflect daily ingestion over many days, compared with occasional swallowing during recreation. Note that 1 µg/L equals 1 part per billion (ppb).

5.10Monitoring and advisories in Kentucky

In Kentucky, the Division of Water and the Department for Public Health issue harmful algal bloom recreational advisories jointly [34]. The state's bloom record in Cabinet releases begins in 2013, when the U.S. Army Corps of Engineers identified a bloom on Taylorsville Lake and then found elevated cyanobacteria in several Corps lakes, prompting the Division of Water to begin testing 14 other lakes [35, 21]. The largest events were on the Ohio River: in September 2015, advisories eventually extended from the West Virginia line downstream to the Cannelton Locks and Dam [36], and in 2019 an advisory covered the river from McAlpine Dam to Greenup Dam [34]. EPA-led modeling has since linked bloom occurrence and persistence on the river to antecedent flow conditions, calibrated on the 2015 and 2019 events [37], and USGS sampling has shown that cyanotoxins occur in large rivers even without visible blooms [38]. Chapter 9 reviews the Kentucky record in more detail.

For current conditions, use the official sources in the Resource Library: the Division of Water's HAB page and Water Maps Portal (which includes a HAB viewer), and the Department of Fish and Wildlife Resources guidance for anglers and boaters.

5.11Safety rules for student researchers

5.12Connection to local research

Key terms

Bloom

Rapid increase in algal or cyanobacterial biomass.

Harmful algal bloom

A bloom that causes harm through toxins, oxygen loss, taste and odor or habitat change.

Cyanotoxin

Toxic compound produced by some cyanobacterial strains.

Microcystins

Cyclic-peptide liver toxins; the most commonly measured cyanotoxins.

Intracellular toxin

Toxin held inside living cells.

Dissolved toxin

Toxin free in the water after cells release it.

Health advisory

Non-regulatory guidance concentration for a given exposure period.

µg/L (ppb)

Micrograms per liter; parts per billion.

Knowledge check

1. Two ponds have the same Microcystis cell count. Pond A has 12 µg/L microcystins; Pond B has none detected. Give two biologically plausible explanations.

   Show answer and explanation

   The ponds may contain different proportions of toxin-producing genotypes; or the populations may differ in how much toxin they are producing under current conditions. (Analytical differences — total versus dissolved toxin — should also be checked.)

2. A bloom was treated with an algaecide yesterday and the water looks clearer today. Why might toxin concentrations in the water be higher?

   Show answer and explanation

   Algaecides rupture cells, releasing intracellular toxin into the water as dissolved toxin, which can persist after the cells are gone.

3. A lake sample has 5 µg/L microcystins. How does that compare with the EPA recreational value and the 10-day drinking-water advisory for adults?

   Show answer and explanation

   It is below the 8 µg/L recreational value but above the 1.6 µg/L 10-day drinking-water advisory for school-age children and adults — water that may be acceptable for swimming is not acceptable to drink untreated.

4. Why should every dissolved-oxygen reading taken near a bloom include the time of day?

   Show answer and explanation

   Oxygen rises during daylight photosynthesis and falls at night through respiration, so the same site can range from supersaturated in the afternoon to very low before dawn.

5. Kentucky releases from 2013 describe cell-count thresholds; releases from 2015 describe a microcystin threshold. Why might an agency make that change?

   Show answer and explanation

   Health risk depends on toxin exposure, and cell counts do not reliably predict toxin concentrations; toxin measurements relate directly to health-based guidance values.