Algal Bloom Hotspots: Where Are the Most Affected Areas in the U.S.?

A severe algal bloom covers Lake Erie in 2014, illustrating an environmental crisis that impacted local water quality and ecosystems.

Algal blooms have become an increasingly common sight across the United States, disrupting ecosystems, harming public health, and affecting economies dependent on clean water and recreation. But some regions are hit harder—and more frequently—than others. In this post, we’ll explore the top algal bloom hotspots in the U.S., the reasons behind their vulnerability, and what’s being done to monitor and mitigate these growing threats. 

Top Algal Bloom Hotspots in the U.S. 

1. Lake Erie (Ohio, Michigan, Pennsylvania) 

Perhaps the most notorious algal bloom hotspot in the country, Lake Erie experiences large-scale blooms almost every summer. The western basin, in particular, is plagued by nutrient runoff from agricultural fields in the surrounding watershed. The 2014 bloom contaminated Toledo’s water supply, leaving over 400,000 people without safe drinking water for days. 

2. Gulf of Mexico (Florida, Texas, Louisiana) 

The Gulf is home to frequent red tides, caused mainly by Karenia brevis. These blooms can devastate marine life and cause respiratory problems in humans. Florida’s west coast, especially around Tampa Bay and the Florida Panhandle, experiences regular outbreaks, often tied to nutrient-rich runoff and warming coastal waters. 

3. California Coast (Monterey Bay, Santa Barbara) 

California has seen repeated blooms of Pseudo-nitzschia, a diatom that produces domoic acid, a neurotoxin harmful to marine mammals and humans. These events have led to mass sea lion strandings, fishery closures, and disruptions to the tourism industry. 

4. Chesapeake Bay (Maryland, Virginia) 

Chesapeake Bay suffers from both hypoxic zones (low oxygen caused by algal decay) and toxic blooms. Urban stormwater and agricultural runoff make this estuary particularly vulnerable. Species like Microcystis and Alexandrium have been detected in increasing numbers. 

5. Great Lakes (Michigan, Wisconsin, Minnesota) 

Beyond Lake Erie, Lakes Michigan, Huron, and Superior are seeing more frequent and intense blooms. While historically colder and less nutrient-laden, changing climate conditions and invasive species have altered these lakes’ ecosystems, making them more susceptible. 

6. Inland Lakes in Florida and the Southeast 

Florida’s Lake Okeechobee is infamous for explosive cyanobacterial blooms that feed into both coasts via manmade canals. These blooms impact everything from property values to public health and have sparked heated political debates over water management. 

7. Pacific Northwest Estuaries (Oregon, Washington) 

Coastal estuaries here are subject to toxic shellfish poisoning due to algal blooms like Alexandrium catenella, which produce paralytic toxins. While the region benefits from strong monitoring programs, blooms still disrupt fisheries and public access to shellfish. 

What Makes These Areas Hotspots? 

• Nutrient Runoff: Fertilizers from agriculture, lawn care, and wastewater feed blooms. 

• Warmer Temperatures: Climate change is warming waters, creating better conditions for algae to thrive. 

• Water Flow and Circulation: Stagnant or slow-moving waters trap nutrients and allow algae to grow unchecked. 

• Shallow Water: Lakes like Erie and Okeechobee are shallow, which promotes rapid temperature changes and nutrient cycling. 

Monitoring and Solutions 

Addressing HABs requires a mix of technology, policy, and public awareness. Many states and research institutions use tools like: 

• Remote sensing & satellite imagery 

• In-situ fluorometers to detect chlorophyll in real time 

• Citizen science apps for bloom reporting 

• Policy reform around nutrient management and water infrastructure 

Organizations like NOAA and the EPA have ongoing HAB forecasting and mitigation programs, but local monitoring remains critical. 

Looking Ahead 

As algal blooms become more frequent and widespread, recognizing the most impacted areas—and understanding why they’re vulnerable—is the first step toward change. By combining technology, research, and responsible land-use practices, communities can work to protect vital water bodies and the people who depend on them.