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Harmful Algal Blooms

Report Algal Blooms: call (410) 260-8630, or use the Algae Bloom Sighting Report.

To learn more about Harmful Algal Blooms (HABs) visit the Phytoplankton Monitoring Program. We also have a basic factsheet on harmful algae in the Chesapeake and Coastal Bays. To learn about the state-wide harmful algal bloom surveillance program, check out this factsheet on Harmful Algal Bloom Management.

Read recent HAB stories below the map, or find archived HAB stories in our searchable database.

Exploring the HAB Map:

*Due to recent Google map changes, we suggest clearing your cache before browsing our maps.*

  • Change Dates: click the arrows to search forward or backward by 2 weeks
    OR enter a date range (< 31 days), click the 'Go!' button
  • Filter/Find Species: turn data points for HAB species on or off
  • Show Legend: mouseover this menu to display the legend
  • Station markers: click a marker to view the array of all algal events for each station; clicking each symbol displays details for each HAB.

For a full-screen map view click here Eyes on the Bay - Interactive Mapping of Current Year Long-Term Station Data
Show Legend
Relative algal abundance:
blue circle map icon
Site Sampled
Algae were sampled, no harmful species identified
green circle icon
Algae Bloom
High abundance of algae that are not known to produce any toxins
orange map icon
HAB Present

A species of concern was identified below bloom thresholds

red triangle map icon
HAB Bloom
Potentially harmful algae detected above bloom thresholds
Algae are a natural part of the Bay ecosystem; however, some algae may be harmful when they bloom in unnaturally high abundance or they may produce a toxin that can impact people, animals and/or bay life.  Toxin testing is initiated when cell densities exceed set thresholds.

Visit DNR's Harmful Algal Bloom page for more information.

Filter/Find Species
Change Dates
Samples from

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Recent HAB Stories:

  • 2019: Coastal Bays Set State Algae Record - Prorocentrum minimum bloom in mainstem Chesapeake Bay.
  • 2019: Why is the water green? - coccoid chlorophyte bloom in multiple Chesapeake Bay areas.
  • 2019: Mystery material found in Smith Creek identified - Diatom bloom in Potomac River.
  • 2019: Maryland Coastal Bays experience unusual winter Mahogany Tide - Prorocentrum minimum bloom in mainstem Chesapeake Bay.
  • 2018: Spring Blooms in the Bay - Prorocentrum minimum bloom in mainstem Chesapeake Bay.

  • View the Archived HAB Map (qualitative data)
    Note: Archived HAB Map data from samples prior to 2015 should not be used for analytical purposes
    as these were qualitative assessments based on live cell counts.

    If you would like to request algae data, please email us.