Heat waves—like the one that blistered the Pacific Northwest last June—are also occurring underwater.
A new study in “Frontiers in Marine Science” paints a troubling picture of recent and projected trends in marine heat waves within the nation’s largest estuary, with significant implications for the marine life and coastal economy of the Chesapeake Bay and other similarly impacted shallow-water ecosystems.
The study’s authors, Drs. Piero Mazzini and Cassia Pianca of the College of William and Mary’s Virginia Institute of Marine Science, noted that they saw “significant upward trends in the frequency and yearly cumulative intensity of marine heat waves within the Chesapeake Bay.”
The pair based their analysis on long-term measurements of water temperature from six sites along the bay’s 200-mile length.
Like other researchers, they defined a marine heat wave as any period of five or more consecutive days with water temperatures warmer than 90 percent of those measured on the same date and in the same spot as in years past. They analyzed the record of Chesapeake Bay heat waves in terms of frequency, intensity, duration and cumulative temperature stress.
Based on those criteria, Mazzini and Pianca determined that the Chesapeake Bay experienced an average of two 11-day marine heat waves per year between 1986 and 2020, with an average intensity of 5.4°F. (3°C.) and a maximum peak of 14.4°F. (8°C.) above the climatic norm. This translates to an average yearly cumulative intensity of 130°F days (72°C. days), a measure of heat stress for marine systems similar to the “cooling degree days” used to determine the energy required to keep indoor spaces comfortable for people.
The most troubling finding was that the maximum frequency of marine heat waves occurred during the last 10 years, reaching six to eight events per year compared to only four to five events per year before 2010. That equals a gain of 1.4 annual heat waves each decade, with a corresponding increase in annual cumulative intensity.
The researchers also found that years without marine heat waves were fairly common in Chesapeake Bay waters prior to 2010, but have occurred baywide only once since, in 2014.
“If these trends persist,” said Mazzini, “the bay will experience heat waves on a monthly basis within the next 50 years, and by the end of the century will reach a semi-permanent heat-wave state, with extreme temperatures present for more than half the year.”
The authors warn this would have devastating impacts on the bay ecosystem, aggravating the effects of nutrient pollution, increasing the severity of low-oxygen “dead zones,” stimulating algal blooms, stressing or killing bottom-dwelling communities, causing shifts in species composition, and leading to declines in important commercial fishery species such as striped bass. Similar trends and impacts are likely in other shallow-water coastal systems worldwide given continued global warming.