As Oceans Change, HABs Invade

Original Paper: Gobler, C.J., Doherty, O.M., Hattenrath-Lehmann, T.K., Griffith, A.W., Kang, Y. and R.W. Litaker. 2017. 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 Sciences114(19), pp.4975-4980.

Featured Image Source: Lake Eerie HABs Aug 2017. Credit: NOAA, Aerial Associates Photography, Inc. by Zachary Haslick. Flickr.


What words come to mind when you think of the ocean? One of its quintessential descriptors is “blue.” Columbus sailed it, and there’s even a color named after it. However, sometimes the ocean changes colors, from red to green, brown to yellow. Those colors aren’t just random; they are due to a very specific group of organisms called phytoplankton.



Phytoplankton are microscopic marine algae. They are similar to terrestrial plants in that they use chlorophyll to capture sunlight and turn it into energy using the process of photosynthesis. They can be hugely beneficial to the earth, as they provide food for a number of marine creatures, such as whales, shrimp, and jellyfish. Take two big breaths – one of those was possible because of phytoplankton, as they produce half the world’s oxygen as a photosynthesis byproduct.

About 1% of the global biomass is due to phytoplankton. This might not sound like a lot, but it means that there are some of these tiny organisms in most surface waters around the globe. Since they are so small, you often need a microscope to tell if they are there, but in some cases spotting them is no challenge.

Algal bloom in the Baltic sea. Credit: European Space Agency. Contains modified Copernicus Sentinel data from 2019, processed by ESA, Contains modified CC BY-SA 3.0 IGO. Flickr.

Something called an algal “bloom” occurs when certain environmental conditions allow phytoplankton to grow extremely well. During a bloom, the water containing the bloom will often turn a certain color, which is determined by the species of phytoplankton. Though more phytoplankton can be a positive thing for ocean life, in certain cases, an excess of it can actually cause more harm than good.


What are “HABs”? 

Despite the generic name, the types of phytoplankton found on earth are extremely diverse. There are thousands of species that come in a variety of colors, shapes, and sizes. Some of these species produce toxins. HABS, or Harmful Algal Blooms, are blooms of certain phytoplankton species that produce toxins or are otherwise harmful to the surrounding environment. The consequences of a bloom depend on the species of HAB present, but they can include wildlife mortalities, shellfish poisoning, public health issues, beach closures, and loss of fishing revenue.

As a result of climate change, many areas of the world have experienced warming water temperatures. Scientists have found that ocean temperatures are one of the most important environmental factors affecting phytoplankton growth rates. We also know that, to some degree, the structure of phytoplankton communities is affected by other environmental factors. Since urbanization, or the increased population of cities, has intensified over the last few decades, things like nutrient runoff into coastal water systems and ocean acidification also play a role in influencing phytoplankton growth rates.

Though phytoplankton overall are highly influenced by temperature, it is unknown how different species of HABs will respond to the changing climate since there are a multitude of other environmental factors that could possibly affect HABs. Coupling this with the idea that climate change-driven warming is unevenly distributed across the globe means that it is extremely difficult to determine trends in HAB growth. In addition, every HAB species reacts differently to its environment. Interaction of factors begs the question of whether we can come up with a way to predict future HAB events?

Picture of Gloeotrichia echinulata, a harmful algal bloom species, under an epifluorescent microscope. Credit: NOAA, USGS. Flickr.
Modeling HAB Trends

Since global ocean temperatures are currently rising and have been for decades, a group of scientists were interested in how HABs were being affected by the changing climate over time. Specifically, they wanted to see the effects of changing ocean temperatures on the geographic ranges (or realized niches) of HABs. This information could help scientists predict where HABs and toxin events could be found in the future and better understand why HABs occur in certain places.

To assess this relationship, the researchers decided to look at two of the most toxic and widespread HAB species native to the North Atlantic and North Pacific oceans, Alexandrium fundyense and Dinophysis acuminata. They compiled data from 1982 to 2016 on sea-surface temperatures in these areas, as well as the growth rates and the length of the bloom seasons for each HAB species. Using these data, the scientists first made a model for each species that included the ocean-wide sea-surface temperatures, the average growth rates, and the bloom season duration of the species. These models were used to determine the overall trends in the relationship between these variables over time. Then, they compared these models to records of local HAB sightings over the years to see whether the trends from the models correlated with local (in situ) observations documenting the emergence of blooms.

Overall, the scientists found that ocean warming has expanded the geographic ranges of HABs in the North Atlantic and North Pacific oceans. In many North Atlantic regions, the average annual growth rate and bloom season duration increased significantly over time for both HAB species. Recordings of incidents of new blooms in these areas correlated with the models, indicating that blooms in areas that did not have blooms in the past were also areas that had increased sea-surface temperatures. In the North Pacific ocean, it was more difficult to identify trends relating sea-surface temperature to increased HAB ranges from the models, since there was more variation in growth rate and season duration in HABs between different regions. However, certain regions did follow a trend, and there were blooms that recently emerged as waters warmed. Many areas also experienced significant increases in the average growth rates and bloom season duration of HABs over time as sea-surface temperatures rose.


Why Is This Significant?

The results of this study are important because they demonstrate that ocean temperatures are very important for HAB growth. This means that, as ocean temperatures rise, we could see an intensification and expansion of HABs, which could threaten human and wildlife health. However, different species of HABs are likely to respond in different ways, so some may experience geographic range expansions, whereas some may just move to new areas. More long-term research is needed on HABs and climate to parse out the effects and better predict future HAB events.

Luckily, scientists are well on their way to discovering the mysteries behind HABs, but they still have a long way to go. Not a phytoplankton scientist? No problem! If you suspect a bloom in your area, get the word out to your state health or environmental departments. Take precautions by staying safe, and make sure to keep pets and children away from the water! Preventing nutrient runoff by being mindful of fertilizers or other chemicals present around your home is another way you can make a difference.

Citizen science is a powerful tool that can be used to further scientific research and turn scientific knowledge into public knowledge. Even if you don’t have the skills or equipment to analyze a bloom, you can still help out! The EPA has a site dedicated to HABs and places you can volunteer as water monitors in your area.



NOAA. What are phytoplankton? National Ocean Service website,, 10/09/2019.

Lindsey, R. and M. Scott. What are phytoplankton? NASA Earth Observatory website, 2010

Woods Hole Oceanographic Society. What are harmful algal blooms (HABs)?, 2019.


Reviewed By:

Mary Grace Lemon
Lindsay Green-Gavrielidis

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Jessica Espinosa

Hi! My name is Jessica Espinosa and I am a PhD student at UConn in the Ecology and Evolutionary Biology Department interested in the bird conservation. I received my masters degree in Conservation Biology from Columbia University where my thesis focused on the effects of coastal pollution on the behavior and morphology of hermit crabs in Fiji. I am also a Mount Holyoke College and City Year Alum. In my free time, I enjoy reading, writing, hiking, doing martial arts, and playing music.

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