Global ocean temperatures have been rising, but the consequences of these increases are not fully clear. A recent paper published in PNAS clarifies one of them by showing that harmful algal blooms have already become more intense.

Some types of algae naturally produce toxins. When these algae grow rapidly, they create a bloom that can kill off other species in the same ecosystem. A number of species (including Alexandrium fundyense and Dinophysis acuminate) produces toxins that accumulate in shellfish. People who eat these shellfish can experience paralytic or diarrhetic shellfish poisoning. So these blooms can hurt aquatic ecosystems, fisheries, and people.

The recent paper in PNAS used data from the National Oceanic and Atmospheric Administration to model sea-surface temperatures and track algal blooms. These models showed that warming in the North Atlantic since 1982 has significantly increased the growth rate of the two most dangerous species of algae. For both of these species, algal blooms have grown to cover a much larger area of the oceans in the last 35 years. Additionally, the length of bloom season has increased by as much as eight weeks over that same time period.

Compared to the North Atlantic, changes in algal blooms in the North Pacific were less obvious. There were significant increases in bloom season length, and these were associated with some increases in paralytic and diarrhetic shellfish poisoning. But the correlations in the North Pacific weren’t as robust as those from the Atlantic.

For both the North Atlantic and the North Pacific, the researchers found that algal bloom season was beginning earlier in the year and that more blooms reached maximal growth rates. However, in the North Atlantic, bloom season was also ending later, whereas, in the North Pacific, the end date for algal bloom season wasn’t strongly effected. The difference in when blooms ended may account for the weaker correlations in the Pacific.

Why the difference?

Differences between the North Atlantic and North Pacific may be due to differences in how much these oceans have warmed. The North Atlantic warmed between 1-3°C over the study period, whereas the North Pacific only warmed by less than 0.6°C. This marked difference in warming could explain why algal blooms are seeing more dramatic changes in the Atlantic.

While the authors only had the data to show a correlation here, other work has provided a clear mechanism to explain that correlation: oceanic temperature is known to be one of the most important environmental factors shaping the structure of ocean algae growth. Other important factors include availability of nutrients and presence of predators—these factors work together to control the occurrence of harmful algal blooms. But sea-surface temperature has emerged as the most important factor contributing to recent changes in algal blooms.

If these patterns continue into the 21st century, the consequences for oceanic ecosystems and human health could potentially be serious. This paper presents yet another reason that we should be doing all we can to attenuate the harmful effects of human-caused climate change.

PNAS, 2017. DOI: 10.1073/pnas.16195751140076 (About DOIs)