Monitoring Algal Blooms with Complementary Sensors on Multiple Spatial and Temporal Scales

Climate change, and other human-induced impacts, are severely increasing the intensity and occurrences of algal blooms in coastal regions (IPCC, 2022). Ocean warming, marine heatwaves, and eutrophication promote suitable conditions for rapid phytoplankton growth and biomass accumulation. An increase in such primary producers provides food for marine organisms, and phytoplankton play an important global role in fixing atmospheric carbon dioxide and producing much of the oxygen we breathe. But harmful algal blooms (HABs) can also form, and they may adversely affect the ecosystem by reducing oxygen availability in the water, releasing toxic substances, clogging fish gills, and diminishing biodiversity. Understanding, forecasting, and ultimately mitigating HAB events could reduce their impact on wild fish populations, help aquaculture producers avoid losses, and facilitate a healthy ocean.

Monitoring Algal Blooms with Complementary Sensors on Multiple Spatial and Temporal Scales By David R. Williamson, Glaucia M. Fragoso, Sanna Majaneva, Alberto Dallolio, Daniel Ø. Halvorsen, Oliver Hasler, Adriënne E. Oudijk, Dennis D. Langer, Tor Arne Johansen, Geir Johnsen, Annette Stahl, Martin Ludvigsen, and Joseph L. Garrett Climate change, and other human-induced impacts, are severely increasing the intensity and occurrences of algal blooms in coastal regions (IPCC, 2022).Ocean warming, marine heatwaves, and eutrophication promote suitable conditions for rapid phytoplankton growth and biomass accumulation.An increase in such primary producers provides food for marine organisms, and phytoplankton play an important global role in fixing atmospheric carbon dioxide and producing much of the oxygen we breathe.But harmful algal blooms (HABs) can also form, and they may adversely affect the ecosystem by reducing oxygen availability in the water, releasing toxic substances, clogging fish gills, and diminishing biodiversity.Understanding, forecasting, and ultimately mitigating HAB events could reduce their impact on wild fish populations, help aquaculture producers avoid losses, and facilitate a healthy ocean.
Phytoplankton respond rapidly to changes in the environment, and measuring the distribution of a bloom and its species composition and abundance is essential for determining its ecological impact and potential for harm.Here we present an integrated approach to observing blooms-an "observational pyramid"-that includes both classical and newer, complementary observation methods (Figure 1).We aim to identify trends in phytoplankton blooms in a region with strong aquaculture activity on the Atlantic coast of mid-Norway.Field campaigns were carried out in consecutive springs (2021 and 2022) in Frohavet, an area of sea sheltered by the Froan archipelago (Figure 2).The region is a shallow, highly productive basin with abundant fishing and a growing aquaculture industry.
Typically, there are one or more large algal blooms here during the spring months.We use multi-instrumentation from macro-to a microscale perspectives, combined with oceanographic modeling and ground truthing, to provide tools for early algal bloom detection.
Satellite remote sensing of chlorophyll concentration has been used extensively to observe the development of algal blooms.Although this tool has wide spatial and temporal (nearly daily) coverage, it is limited to surface ocean waters and cloud-free days.Microscopic analyses of water and net samples allow much closer examination of the species present in a bloom and their abundance, but this is a time-consuming process that collects only discrete point samples, sparsely distributed in space and time.Neither of these methods alone captures the rapid evolution of algal blooms, the spatial and temporal patchiness of their distributions, or their high local variability.In situ optical devices and imaging sensors mounted on mobile platforms such as autonomous underwater vehicles (AUVs) and uncrewed surface vehicles (USVs) capture fine-scale temporal trends in plankton communities, while uncrewed aerial vehicles (UAVs) complement satellite remote sensing.Use of such autonomous platforms offers the flexibility to react to local conditions with adaptive sampling techniques in order to examine the marine environments in real time.

FIGURE 2 .
FIGURE 2. (inset) Fieldwork location relative to Norway.(upper panel) The larger Frohavet region is overlain here with the path and coverage of the plane equipped with a hyperspectral camera during 2021 fieldwork, and the path of the long-endurance USV in the weeks around 2022 fieldwork.(lower panel) The locations of net and water samples and the paths of the UAV, AUV, and USV missions in the main sampling area in 2022.Elevation data from Kartverket, satellite images from Norge i bilder/Kartverket and CNES/Airbus, Landsat/ Copernicus, Maxar Technologies via Google Maps

FIGURE 1 .Frohavet
FIGURE 1.The observational pyramid concept offers simultaneous, integrated monitoring of the marine environment from space to seabed and from scales of hundreds of square kilometers to the microscopic.AfterDallolio et al. (2019)

FIGURE 4 .
FIGURE 4. A collage of cropped images from the AUV-mounted silhouette camera includes copepods and a fish larva.The abundance of such grazers impacts the timing and sizes of algal blooms.Image resolution is approximately 30 µm per pixel.

FIGURE 3 .
FIGURE 3. A chlorophyll forecast made on April 19 during the 2022 fieldwork using SINMOD, a coupled physical-chemical-biological ocean model (spatial resolution 160 m).The white box shows the fieldwork area.