Harmful Algal Blooms occur when toxin-producing algae experience excessive growth within bodies of water. These blooms have the potential to cause detrimental effects on both aquatic and human health and can sometimes even cause death, depending on the type of algae involved (NIEHS, 2021). Thanks to the use of space-based remote sensing technology to monitor water quality conditions in coastal areas and drinking water reservoirs, nations are becoming more aware of the quality of their water. One example of this technology is the Hyperspectral Imager for the Coastal Ocean (HICO) which is used to capture images of coastal areas globally to identify toxic algal growth (Gaskill, 2018). This remote sensing technology has inspired advances in remote hyperspectral sensing globally.

For many, the most valuable and dynamic parts of our planet are Earth’s coastal areas, where hundreds of thousands of people all around the world access water and quality is crucial. In recent years, NASA has used remote sensing technology known as the Hyperspectral Imager for the Coastal Ocean (HICO) to capture images of coastal areas, all while identifying harmful and toxic algal growth in drinking water reservoirs (Gaskill, 2018). This instrument uses remote sensing technology which, when mounted to the International Space Station (ISS), is able to identify water quality irregularities and toxins invisible to the human eye. HICO has been extensively used to help monitor Harmful Algal Blooms (HABs), which occur when toxin-producing algae experience excessive growth within bodies of water (see Figure 1). 

Algae live in aquatic environments as microscopic organisms, and they produce energy using sunlight- this process is known as photosynthesis. Depending on the type of algae, blooms can be observed as red, brown, green, and/or blue-green in colour (NIEHS, 2021). Not all algae are considered toxic. In fact, algae are constantly present in bodies of water including oceans, rivers, and lakes (NIEHS, 2021). Depending on certain environmental factors such as light, temperature, and nutrient levels, toxic algae production may or may not be stimulated (NIEHS, 2021). Currently, scientists believe that HABs result from warmer temperatures alongside fertilizers or sewage from runoff which then causes an excess in nutrients (NIEHS, 2021). Therefore, it is important to monitor these factors, as water contaminated with toxic algae (such as the blue-green Microcystis spp. and Karenia brevis) can cause several health concerns including irritation in the eyes and lungs, numbness, and even severe liver damage (Nimon, 2014). Not only are HABs responsible for harming humans, but they also represent a significant threat to the marine ecosystem.

On account of climate change, experts believe that HABs will become more frequent, wide-ranging, and severe (NIEHS, 2021). When HABs occur, people can become exposed to toxins through swimming and drinking the contaminated water, eating fish they catch, and even by breathing in the toxic air. In fact, the toxins remain even after boiling the contaminated water or cooking contaminated seafood (NIEHS, 2021). For example, if consumed through contaminated seafood, the algae known as Alexandrium can cause paralysis and even death. Moreover, algae called Pseudo-nitzschia is able to produce toxins that cause “vomiting, diarrhea, confusion, seizures, permanent short term memory loss, or death when consumed at high levels” (NIEHS, 2021) (see Table 1). It is believed by experts that children and the elderly are at the most risk as they are significantly more sensitive to the toxins caused by HABs (NIEHS, 2021). Other high-risk populations include those who rely on seafood for survival, as there is an increased risk of long-term health complications resulting from potentially frequent, low-level exposures to HAB toxins (NIEHS, 2021). These blooms have also been responsible for depleting oxygen levels in the water as well as blocking sunlight from entering the water, effectively killing fish and any other organism living deep beneath the surface (NIEHS, 2021). 

Benefits of Hyperspectral Imagers for the Coastal Ocean

By using HICO to observe and monitor HABs, researchers are able to see around 90 wavelengths of data that the human eye cannot see on its own (Nimon, 2014). This provides a great opportunity as the biological matter involved with HABs emit unique wavelengths not visible to the human eye. Hence, HABs have a unique spectral signature. This specific signature can be identified through fluorescence and backscattering, which occurs as sunlight is reflected off of the biological matter and back to HICO where the data is read and further analyzed by experts (Nimon, 2014).

The use of space-based remotely sensed data allows for a greater awareness of water quality conditions globally. Not only can hyperspectral imaging support water quality conditions at regional scales, but also at smaller sustainable practices that work to maintain or improve environmental conditions around the globe (Keith, 2013). By using hyperspectral imaging, scientists have been able to get a broader understanding of water quality and algal blooms. Mary Kappus, branch head for Coastal and Ocean Remote Sensing at the Naval Research Laboratory (NRL), comments that numerous papers on algal bloom incidents affecting many people, have been published. He adds that researchers have an idea on the locations these blooms are likely to happen but are finding it challenging to predict them near real time (quoted in Gaskill, 2018). According to Ruhul Amin, Ph.D., principal investigator for the HICO CASIS-NRL project, conventional multi-spectral ocean colour imagery does not have the ability to discriminate between bloom species, however, due to HICO’s contiguous bands, the instrument provides experts with data that can help identify the HAB species (quoted in Nimon, 2014). 

The high spatial and spectral resolution provided by HICO allows researchers to use hyperspectral data to innovate new approaches for HAB species identification. In fact, according to Richard Gould, Ph.D., head of the U.S. NRL bio-optical/physical processes and remote sensing division, hyperspectral imagery provides a cost-effective method of bloom detection and monitoring (quoted in Nimon, 2014). Hyperspectral imaging has also been made available to be used in response efforts, to provide online information to organizations in regions impacted by natural/unnatural disasters (e.g., oil spills, flooding, etc.) for the United Nations International Charter for Space and Major Disasters (Nimon, 2014). This way, organizations are able to share crucial information with ground responders to produce resourceful information that otherwise would be challenging to obtain in real-time (Nimon, 2014).

How Does Hyperspectral Imaging Work?

While a regular camera is only able to collect data from three spectral channels, HICO is able to collect the full spectrum of wavelengths spanning from visible to near-infrared (Gaskill, 2018). Hyperspectral imaging has the ability to provide an image of one scene spanning around 40 kilometres wide and 190 kilometres long during its 90 minute orbit. This means that each pixel observed in the image represents a 95-metre square (Keith, 2013).

Since hyperspectral imagery has the ability to observe changes in water quality much more efficiently than previously existing ocean colour satellites and traditional field-based monitoring (Keith et al., 2014), this technology provides a unique and unparalleled opportunity for the coastal environmental monitoring community. Through images provided by the ISS’s optical sensors, experts are able to better quantify and understand changes in coastal zones (Keith, 2013). In fact, by using these remote sensing devices, researchers have been able to identify the “presence of microscopic plants, organic compounds, suspended sediments and other factors controlling water quality” (Nimon, 2014).

Example Application

In 2009, HICO was used alongside the Remote Atmospheric and Ionospheric Detection System (RAIDS) in the HICO and RAIDS Experiment Payload, collecting around 10,000 images of Earth’s coastal areas during a five-year run aboard the ISS (Gaskill, 2018). While HICO was recording the visible to near-infrared spectrum, RAIDS was able to provide more detailed information on the temperature, density, and composition of both the ionosphere and thermosphere, as well as other regions of Earth’s atmosphere within an altitude of approximately 95-300 kilometres (Gaskill, 2018). By combining two experimental sensors, experts were able to observe a much clearer and in-depth image of some of the most crucial parts on Earth (see Figure 2).

Challenges and Limitations

Challenges associated with hyperspectral imagers include the unavailability of functional software to support atmospheric corrections, as well as instrument calibration for readings in the blue-green spectrum (Keith et al., 2014). Other limitations include the narrow swath width of the sensor, as well as the low altitude orbit of the ISS (Keith et al., 2014). This can make it quite difficult for researchers to obtain an accurate reading of larger coastal areas within a certain timespan.

While NASA releases remote sensing data to the public at no cost, the data can be quite challenging to understand and use. To combat this limitation, NASA's Earth Science Data Systems (ESDS) program and Earth Observing System Data and Information System (EOSDIS) Distributed Active Archive Centers (DAACs) have developed resources and tools to overcome this challenge. 

One of these resources is an online system called EarthData which has made several useful presentations/tutorials (e.g., NASA Data Made Easy) available on platforms such as YouTube. The ease of access to these presentations gives anyone interested to learn more about this system and how to use it an opportunity to explore this resource and have easy access to data/information. 

Future Outlook

Unfortunately, HICO’s computer had been irreversibly damaged during a severe radiation hit from a solar storm in September of 2014, resulting in its retirement in 2018 (Gaskill, 2018). Despite this, the work accomplished during its time in action lives on.

The majority of the images collected by the HICO investigation can still be accessed online. In fact, researchers and scientists are still studying large amounts of data and photos of coastal zones using an online web application known as The Hyperspectral Imager for the Coastal Ocean Image Processing System (HICO IPS) (Gaskill, 2018). 

Furthermore, HICO has been credited with inspiring advances in remote hyperspectral sensing as well as improved remote sensing data analysis tools. Einar Bjorgo, manager of UNITAR’s United Nations Operational Satellite Applications Program, comments,

“...we are encouraged by the wide range of new applications that hyperspectral imaging will allow us to provide to U.N. member states and sister agencies, [...] these images would allow better natural resource management and disaster response, allowing national actors relevant and timely data at no or low cost” - Einar Bjorgo (Nimon, 2014).

Conclusion

Space-based remote sensing technology for water quality monitoring provides researchers with a unique global resource. Due to the work accomplished through the HICO project, we have four years' worth of data that we can use to continuously enhance our understanding and management of water on Earth. Only through a deeper understanding of the science pertaining to water quality, will we be able to successfully implement different levels of support and sustainable practices for communities everywhere. Thanks to innovative technologies like HICO and RAIDS, experts are now better able to understand and monitor the behaviour of coastal water areas in order to maintain healthy water conditions globally.