Remote Sensing of Harmful Algal Bloom (HAB) events – case study

Phytoplankton bloom events are natural phenomena, typically occurring at spring and early fall by the coast of Norway. When conditions are favorable, i.e. sufficient light and nutrients (terrestrial and/or marine by upwelling processes) are present, phytoplankton thrives and a bloom will develop. However, in some cases potentially harmful phytoplankton species can dominate the algae population. This can result in significant economic and ecologic damages. Since phytoplankton are floating organisms, they are transported by the ocean currents. Hence a HAB event can impact areas far from its origin.

HAB events have occasionally been occurred along the Norwegian coasts in the past decades. Although the frequency of HABs is relatively low in Norway, the impact is significant once it occurs. The reason and timing of these events are still not well understood. Early warning and monitoring the extent and movement of HABs could mitigate the potential damages. Spaceborne and airborne remote sensing can contribute to monitor HAB events on large spatial coverage. At the 31^st of March 2025 the Norwegian Marine Institute has announced the occurrence of a harmful algae, Psudochattonella by the west coast of Denmark and the potential danger of being transport towards Norwegian waters.

A collection of satellite images acquired by the Sentinel 3 OLCI sensor are showing examples how information can be retrieved by multi-spectral optical sensors about a HAB event. Figure 1 shows Chlorophyll-a content, which is a proxy for phytoplankton biomass. It seems that the bloom originates from the near coast of West Denmark and being transported to the North.

Figure 2 shows the Total Suspended Matter content, which is an estimate for all scattering particulates in the upper water layers. Hence this includes phytoplankton constituents and other sediments that might come from the sea bottom or as likely in this case from land.

Figure 3 and 4 are showing the water leaving reflectance or remote sensing reflectancd at the wavelengths 490 nm and 681 nm, respectively. The 490 nm band is often used to estimate phytoplankton due to shifted absorption band of Chlorophyll-a during bloom events. The red band at 681 nm is the Chlorophyll-a fluorescence band, which is also a frequently used band in coastal waters, when water constituents besides algae are present.

Although satellite remote sensing provides an excellent possibility for large scale mapping of HAB events, it has certain limitation. These include cloud-free weather condition dependency, sufficient distance from land to avoid adjacency and possible limitations in the spectral domain regarding detection possibilities of potentially harmful algae species. As Fig. 1-4 reveals, the bloom is well defined, however close to shore the uncertainty increases due to the influence of other scattering particulates. It can be noticed, that the estimates show highest concentrations of both Chlorophyll-a and suspended sediments close to shore, as expected. This also supports the need of increasing the certainty of remotely sensed ocean color products.

A part of these limitations can be resolved by using hyperspectral airborne remote sensing, which has significantly improved spatial and spectral resolution, and acquires data at low altitude. Therefore, the combination of airborne and spaceborne optical remote sensing offers the possibility of early-warning and monitoring of HAB events.