Researchers with NOAA’s National Centers for Coastal Ocean Science (NCCOS) are expecting a low, gulf-wide red tide for the Gulf of Maine this spring and summer, continuing the pattern of smaller blooms observed in the region over the last few years.

Gulf of Maine red tide, caused by the algae Alexandrium catenella, produces a toxin that can accumulate in shellfish, which can result in serious or even fatal illness in people who eat contaminated shellfish. The magnitude and severity of Alexandrium blooms, and the subsequent need for shellfish harvesting closures to protect human health, vary considerably from year to year and between decades. Shellfish toxicity was severe and widespread from 1978 to 1988 and again from 2003 to 2009, but has been lower since then. The causes of the decadal variations are the subject of ongoing research.

Figure 1. Concentration of Alexandrium cysts in Gulf of Maine bottom sediments (cells/cm²), collected in October 2020. Credit: NOAA NCCOS.

NCCOS and its partners produce a yearly red tide forecast for the Gulf of Maine that helps state shellfish managers gauge the severity of the season’s harmful algal blooms before they occur, reducing the possibility of recalls and miscommunication that can lead to reduced seafood consumption. The forecast is based on data collected about Alexandrium’s dormant stage, when it exists as seed-like cysts in ocean sediments before growing in the spring into the swimming cells that can produce red tide. Scientists can estimate the size of red tides in the spring and summer by counting the number of cysts in bottom sediments the preceding fall.

In October 2020 aboard NOAA Ship Henry B. Bigelow, researchers from NCCOS and the Woods Hole Oceanographic Institution collected the data needed to map the presence of red tide cysts in the gulf (Figure 1), and to predict the size of the red tide in 2021. The scientists and crew observed strict COVID-19 anti-viral transmission protocols to complete the cruise safely and successfully.

The cyst abundance for Fall 2020 ranked 16 out of 17 years of cyst sampling, representing the second lowest abundance on record. NOAA will provide coastal stakeholders with weekly updates of modeled bloom extent, trajectory, and intensity throughout the spring and summer, and guidance to states monitoring harmful algae and shellfish toxicity along the coast.

This marks the fourteenth year that NOAA and its partners have issued the seasonal forecast, which enables proactive responses to protect coastal economies, making the region more resilient to red tide outbreaks. To learn more, visit the forecast webpage.