Source: Gobler, C. J.; Doherty, O. M.; Hattenrath-Lehmann, T. K.; Griffith, A. W.; Kang, Y.; Litaker, R. W. Ocean warming since 1982 has expanded the niche of toxic algal blooms in the North Atlantic and North Pacific oceans. Proceedings of the National Academy of Sciences 2017, 114 (19), 4975-4980. DOI: 10.1073/pnas.1619575114
Do you know someone who has gotten sick from eating shellfish? Causing nausea, vomiting and diarrhea, it’s a nasty illness. The culprits are actually not the shellfish themselves, but harmful microscopic algae. When oysters and other bivalves filter feed, harmful algae (and the poisons they produce) can accumulate in the oysters. Two algae are responsible for the majority of diarrhetic (diarrhea-causing) or paralytic (paralysis-causing) shellfish poisoning (DSP and PSP) cases: Alexandrium fundyense and Dinophysis acuminate (Figure 1). These types of dinoflagellates can wreak havoc on local ecosystems and shellfish-dependent economies, not to mention human health.
Fortunately, toxin-producing algae are not usually in high enough concentrations in the ocean to cause PSP and DSP. But the picture changes when the algae experience a population surge known as a bloom, or in this case, a harmful algal bloom (HAB). HAB occurrences depend on a variety of factors, including water temperature and nutrient availability. In recent years, there have been reports of DSP and PSP in coastal communities typically too far north for HABs to occur. Christopher Gobler and his colleagues set out to explain this northward march of harmful algae and test their hypothesis that rising water temperature has been the main cause for recent shifts in HAB events.
The researchers focused on two characteristics of algal blooms: the growth rate of the algae and the duration of the bloom. They used an ocean model to study the effects of recent changes in sea surface temperature (SST) on HAB density and duration in the North Atlantic and North Pacific.
A wide region of the North Atlantic, spanning from the Bay of Fundy to the Baltic Sea, showed a significant increase in the bloom season length for both A. fundyense and D. acuminate (Figures 2, 3). Two zones in particular in the North Atlantic, marked by the black boxes in Figure 2, experienced above-average warming and showed increased HAB frequency (black boxes in Figures 2, 3). For both regions, the researchers’ model output showed increases in mean annual growth rate and that the duration of bloom season for A. fundyense and D. acuminate lengthened by 2-3 weeks since 1982.
To corroborate their findings, Gobler et al. compared the modeled regions of change to site-based in situ observations of HABs. According to observations, the first appearance of A. fundyense in the Bay of Fundy (leftmost of the two black boxes in Figures 2, 3) was on average three weeks earlier between 2000 and 2010 than it was between 1988 and 2000. Studies have also shown that the peak in D. acuminate populations in the North Sea now occurs several weeks earlier than it did in the 1950’s. Additionally, while no HAB-related shellfish poisoning cases had ever been reported in the United Kingdom before 1997, there have since been multiple shutdowns of shellfish beds in the U.K. due to PSP and DSP.
In the North Pacific, the changes were less pronounced, but a region of coastal waters from Alaska down to the Northwest United States showed statistically significant increases in bloom season length. Contained in this stretch are locations that have experienced new outbreaks of PSP and DSP, including sites along the southern coast of Alaska. Significant warming in the Pacific Northwest region of the United States was also accompanied by new outbreaks of DSP and an increase in PSP events.
The contrast in changes in the North Atlantic and North Pacific is likely due to the fact that the Pacific has been warming less – only by 0.1˚ Fahrenheit, whereas the North Atlantic warmed between 1.8 and 5.4˚ during the study period. Some regions in the North Atlantic, such as the Mediterranean Sea and the central North Atlantic, even experienced a decrease in HAB frequency, likely because the waters warmed beyond the livable temperature range for A. fundyense and D. acuminata.
While this study shows that temperature is an important control on the frequency and intensity of HABs, a variety of other factors can influence the occurrence of HABs. How much the water in the ocean moves around, the amount of nutrients locally available to A. fundyense and D. acuminata, and the rates that they are grazed upon by larger organisms all complicate the picture of predicting HABs. More work is needed to understand the response of HABs to changing temperatures in conjunction with these other parameters.
As our world warms and ecosystem dynamics change, we must understand how HABs have already reacted to changes in the ocean to predict how they will respond to future changes. This study shows that sea surface temperature is a primary control on the range and intensity of HABs. A better understanding of the effects of ocean changes on these algal blooms is essential for predicting HABs and planning accordingly to avoid harm to our society.
Julia is a PhD student at Scripps Institution of Oceanography in La Jolla, California. Her focus is on biogeochemistry, which, as the name suggests, centers on the combined effects of biological, geological and chemical processes on the earth system. She is advised by Dr. Ralph Keeling and is modeling the global carbon cycle to better understand how much carbon dioxide ends up in the atmosphere. When not at her computer writing code, Julia can usually be found reading and/or thinking about food.