The Freshwater algae Didymoshenia geminata (Didymo) has been identified in Massachusetts waters. Sites with sporadically visible blooms of Didymo include the Green River in Southern Berkshire County and locations on the Upper Branches of the Westfield River.
Once thought to be an introduced invasive, Didymo is now believed to be native to North America and the New England region. MassWildlife does not know how many streams in Massachusetts contain Didymo, as it is visually undetectable unless in bloom. Blooms may appear gray, brown, or white and has a texture of wet wool or cotton balls. Blooms, which happen only when certain conditions (including flow, nutrients, light intensity, and water chemistry) are present, can produce a dense covering on rocky substrate and eventually result in long stalks. Extensive Didymo blooms can temporarily cover river bottoms almost entirely. Didymo generally occurs in cold, clear, nutrient-poor waters with a neutral or slightly basic pH.
The frequency and intensity of Didymo blooms vary widely between watersheds because of differing environmental conditions. In the Northeast, Didymo blooms have been observed in NH, VT, CT, NY, PA, VA, MD and WV. In Massachusetts, there have been no reported changes to fisheries resulting from the few Didymo blooms. There is no known method for eliminating or controlling Didymo, although blooms lasting more than a few weeks are uncommon.
It is unknown if Didymo has been transferred in Massachusetts by human activity or if it historically found statewide. However, it is good practice to Clean, Drain, and Dry equipment between uses, particularly when moving between waterbodies, to prevent the transport of any plant or animal species. All recreational users should always thoroughly wash equipment, clothing, waders, and boats in hot, soapy water. Boats and other non-absorbent materials should be scrubbed. Soft, absorbent materials should soak in hot, soapy water for a minimum of 30 minutes and dried thoroughly before reuse.