What We Know about Toxic Algae Blooms

from Maryland Department of Natural Resources

Local water systems, such as the WSSC Water Rocky Goege Reservoir in Maryland and Lake Anna in Virginia, have experienced a dramatic rise in frequency and intensity of Harmful Algal Blooms (HABs) over the last two decades. Let’s take a look at the current science on Hazardous Algae Blooms, HAB’s. Science evolves, but this is what we know, or believe we know now. Cyanobacteria (blue-green algae) has proliferated in Maryland and Virginia reservoirs in the last few decades due to a combination of factors.  While we know of many factors that contribute to HABs, how these factors come together to create a “bloom” of algae is not well understood. HABs occur naturally, but human activities that disturb ecosystems seem to play a role in their more frequent occurrence and intensity. 

The U.S. Geological Survey (USGS), the U.S. Environmental Protection Agency (EPA) and several Universities have studied the phenonium. When you dive into the data from the U.S. Geological Survey (USGS) HAB Research, and others it becomes clear that Harmful Algal Blooms (HABs) are primarily driven by mankind. Human-engineered landscapes, severe nutrient imbalances, and altered water physics, rather than being a vague byproduct of global climate change. The true, measurable causes of these toxic outbreaks break down into four distinct structural pillars:

1. Artificial Hydrological Engineering (Stagnation)

Natural, undisturbed river systems feature high turbulence and rapid flushing speeds that prevent single-celled organisms from accumulating.

When mankind dams rivers to construct drinking water reservoirs we create massive, slow-moving pools with extended water residence times. The result of this is that the water column naturally stratifies in the summer. This rewards the unique evolutionary architecture of cyanobacteria, which use internal gas vesicles to float to the sunny surface while harmless native algae sink and starve in the dark. This is exacerbated by the presence of microplastics which the cyanobacteria exploit .

2. Saturated Phosphorus "Batteries" (Biomass Reserves)

The raw physical volume of a bloom is dictated by phosphorus, which functions as the primary growth limiter in freshwater. Decades of heavy agricultural fertilization, livestock operations, and suburban lawn management have overloaded watershed soils with phosphorus.  This historic surplus creates internal sediment loading. When summer stagnation depletes oxygen at the reservoir bottom, chemical bonds in the mud break, releasing decades of "legacy phosphorus" back into the water to fuel massive blooms from the bottom up—even during dry years with zero active farming runoff.

3. Elevated Nitrogen Inputs (The Toxicity Trigger)

While phosphorus controls how much algae grows, dissolved inorganic nitrogen directly dictates how dangerous the bloom becomes.  Municipal wastewater plants, urban stormwater networks, and failing residential septic infrastructure continuously release nitrates and ammonium directly into local tributaries.  Microcystin and other dangerous cyanotoxins are peptide molecules structurally packed with nitrogen. When human infrastructure floods a system with nitrogen, it alters the water's chemical ratios and acts as a genetic switch, forcing the algae to aggressively synthesize toxins.

4. Microplastic and Chemical "Plastispheres" (Incubation Hubs)

The modern, 21st-century chemical footprint has introduced synthetic materials that alter the physical and biological dynamics of reservoir water. Trillions of microscopic plastic fragments from synthetic clothing, consumer waste, and urban runoff settle into regional water basins.  Floating microplastics absorb organic pollutants and trace metals like iron. Cyanobacteria attach to these particles to form highly resilient biological communities called plastispheres. These plastic rafts keep the colonies floating in the optimal sunlight zone and physically shield them from natural zooplankton predators.

Spatial heterogeneity in mechanisms of internal phosphorus loading from sediments in a subtropical plateau lake - ScienceDirect

What is a harmful algal bloom? | National Oceanic and Atmospheric Administration

A REVIEW OF WATER QUALITY RESPONSES TO AIR TEMPERATURE AND PRECIPITATION CHANGES 2: NUTRIENTS, ALGAL BLOOMS, SEDIMENT, PATHOGENS - PMC