Harmful Algal Bloom Moves Inexorably Down The Hudson

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A comparison of satellite photos taken two days apart shows the advancement of the Harmful Algal Bloom (HAB) (Photo courtesy of USGS)

September 16, 2025

By Elizabeth Tucker–

A Harmful Algal Bloom (HAB) observed in the Hudson River in recent days has been expanding. Satellite images show it currently extends from Kingston to south of Croton Point on the Hudson’s eastern shore and to a point opposite Tarrytown on the western shore. According to Riverkeeper’s press release, “this appears to be the most extensive HAB in the estuary in living memory.”

The visible bloom of cyanobacteria—“algal” is a misnomer—can appear in green streaks, green dots, or blobs, or it may resemble spilled paint on the water surface. HABs can produce toxins harmful to the health of people and animals, also wildlife, so any contact with contaminated water should be avoided. Dogs that swim in affected waters have died.

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HABs should not be confused with non-toxic blooms of true green algae, which appear as floating rafts on the water. Pictures of both types can be found here:

https://dec.ny.gov/environmental-protection/water/water-quality/harmful-algal-blooms

According to Riverkeeper, climate change is creating conditions that make harmful algal blooms more likely and more severe. Warmer water temperatures, longer periods of low river flow, and more frequent droughts create an ideal environment for HAB-forming cyanobacteria to thrive. Heavy rains can also increase runoff of nutrients such as phosphorus and nitrogen into the river, fueling blooms. Treated sewage, fertilizer, agricultural runoff, and insufficiently maintained septic systems are common sources of nutrient runoff. HABs can pose a threat to drinking water.

It is unknown how long the condition may persist. The public can help by sending photos and reports to the DEC using this form: https://survey123.arcgis.com/share/66337b887ccd465ab7645c0a9c1bc5c0

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