Algal Blooms Monitoring

Remote Sensing Know-How

Last edited: April 24, 2023

Published: August 17, 2022

Ariel Calle

Ariel Calle

GIS Analyst

From a distance, all that could be seen for miles was an endless stretch of the color red draped over the steady waves. It looked like something out of an episode of Goosebumps.

There was an unnatural and chemical-like odor that permeated the summer breeze. I observed an abundance of “Red Tide” warning posts scattered along the shoreline to ward off beachgoers. Having attended school on Long Island, these events were no rare occurrence. But this was my first time seeing a red one, which was even more visually striking than I’d seen in my textbooks.

Being a marine biology major, I knew exactly what this was. This was a Harmful Algal Bloom.


What are Harmful Algal Blooms?

Algae is present in virtually all bodies of water, both marine and freshwater. While usually posing no serious threat, these photosynthetic phytoplankton fulfill a vital niche at the bottom of the food chain.

However, with the right set of environmental conditions in play, rapid ‘bloom’ events may occur in which algal population growth explodes and causes a Harmful Algal Bloom, or HAB for short. These HABs may severely destabilize and devastate surrounding ecosystems, with aquaculture and human livelihoods also taking a hit as well.

Why are HABs so bad exactly?

Let’s look at Aureococcus anophagefferens, a common algae found in the waters of Long Island. This species, like many, happens to produce a neurotoxin that shellfish bioaccumulate in large concentrations as they filter feed. Upon consumption, mild symptoms ranging from headaches, fever, and disorientation turn to more severe ones including seizures, paralysis, respiratory failure, and death. Fisheries that regularly supply seafood to the many New York City markets and restaurants year round are forced to close up shop when toxins are detected in aquaculture populations.

Large-scale fishery operations and small-scale local farmers alike face huge financial blows as a result of shutting down. In 2016, about 23 million salmon in a Chilean fishery died from a bloom event which was accompanied by a net loss in revenue of about $800 million USD.

Furthermore, numerous oxygen-depleted areas known as dead zones from the uncontrolled population growth. In these waters, sunlight is also blocked by the booming algae and prevented from penetrating deeper waters where eelgrass and other vegetation are fully reliant on the minimal amount of light they can get.

Without adequate amounts of oxygen and light, biodiversity becomes crippled in these hypoxic zones. Dead zones only continue to grow and expand with time, rarely ever recovering. Massive species die-offs are not uncommon either, affecting both marine and terrestrial species- fish, dolphins, whales, birds, and even livestock.

If blooms are naturally occurring in nature, how can they be so bad?

Harmful Algal Blooms are a global phenomena, able to occur in every continent including Antarctica. While it was previously believed blooms were not possible that far south and in such cold climates, global warming has permitted algal blooms to flourish in otherwise impossible areas such as beneath ice sheets.

Not to mention the increase global surface water temperatures as a result of climate change directly promotes HABs, as warmer waters are favorable to bloom conditions. Another example of human activity exacerbating bloom conditions is seen in urban runoff. Nutrient overloading from residential lawns, agricultural fields and faulty septic systems causes phosphorus and nitrogen to leech into the sea where it is fully available to dormant algal populations.

This influx supports even larger populations of algae than would normally be sustained by the environment, and may allow blooms to last even longer (in some cases over a year).

HABs sound like a real concern, but what can we do?

The answer comes not from below, but above! Satellites to be specific. Right now, there are hundreds of satellites orbiting the planet capturing near real-time satellite imagery on the ground. Equipped with various high-resolution sensors, these satellites are capable of analyzing, modeling, and forecasting HAB occurrences sometimes before they even happen.

Thanks to these remote sensing technologies, we are able to quickly and reliably monitor HAB activity and acquire insight to better understand conditions suitable for blooms. In doing so, we can better predict when and where they may occur so that resources and mitigation efforts can be allocated accordingly.

Earth observation is cool and all, but how exactly do satellites help us forecast HABS?

There are many types of Harmful Algal Blooms and the exact causes of any one are impossible to say with infallible certainty. However, they are likely the product of multiple and compounding environmental conditions being met at a certain time in space. Forecasting requires us to have a thorough understanding on the local and regional conditions that may act as a precursor to HABs.

In the simplest of instances, we can visually interpret satellite imagery for discoloration in bodies of water to identify the type of bloom and extent. This is made easy since blooms reflect their actual color in a simple true-color composite. Usually, this simple identification method is not reliable on its own. It is when we combine information from different satellites and datasets that we can form logical explanations and meaningful solutions that prove useful for HAB forecasting.

The sensor MODIS, present on satellites Terra and Aqua, is particularly useful in identifying harmful algal blooms in optically-complex waters like in the Gulf of Mexico. It’s also capable of estimating primary productivity in phytoplankton infested waters. The Ocean and Land Color Instrument (OLCI) on Sentinel-3 provides data on chlorophyll a concentrations that, when coupled with Lagrangian particle tracing models, enable us to calculate water parcel paths and trajectories. This method seems ideal for tracking algal bloom dispersal routes.

With two Sentinel-3 satellites in orbit, OLCI has a revisit time of less than two days. We can compare this with the hyperspectral Hyperion, launched in 2000 on the Earth Observing 1 (EO-1) satellite, which has a revisit time of 16 days. In addition, Hyperion provides data for 196 bands, compared to MODIS’ measly 36 bands. The latter also has a max resolution of 250 meters versus Hyperion’s 30 meters.

While it was designed for terrestrial monitoring, Hyperion has proven to be the superior method of HAB detection especially for cyanobacterial blooms (Blue/Green Tides). The drawbacks of Hyperion are irrelevant when combined with data from the Compact High Resolution Imaging Spectrometer (CHRIS), another hyperspectral sensor that provides high-resolution data with a revisit time of 7 days and an even higher spatial resolution of 18 meters.


There is no one satellite that is singularly better than any other for detecting and monitoring HAB activity. The relativity of remote sensing and earth observation varies on a case by case basis, and in some scenarios it may be better to use one over another. Only when we apply these tools in conjunction with one another do we make space for ideas and models to fully fledge.

By having a deeper understanding of the conditions that promote Harmful Algal Blooms, we are better equipped to monitor their incidence, impacts and solutions. Companies like Orbify put the power of earth observation into the hands of the people. They streamline the process of learning, understanding, and applying remote sensing concepts and tools for clients. Small-scale fishery owners, for example, can save a lot of time and many resources by employing platforms like Orbify to monitor HABs.

Related blog posts

6 mins read

November 20, 2023

Remote Sensing Know-How

10 mins read

October 4, 2022

Remote Sensing Know-How

11 mins read

July 28, 2022

Remote Sensing Know-How






Customer StoriesPricing