Algae From Above: Mapping the Rocky Shore by Drone

By Avani Fachon, science communication assistant, San Francisco Bay Area Inventory and Monitoring Network.

For decades, biologists of the National Park Service's San Francisco Bay Area Network have monitored small patches of rocky intertidal shore along the central California coast, to see how they respond to changing conditions. They learn a lot — but cannot be sure how far their findings hold across a whole site. So they are looking for efficient ways to track change at a wider scale, using new technology and artificial intelligence. One answer may be aerial mapping.

From up close to up high

The team's long-standing methods — biodiversity surveys, sea star counts, surfgrass line transects and photo plots — all mean going out each year onto slippery rocks at extreme low tide. They give a detailed picture of small areas within the wider habitat.

As Dr. Ben Becker, an NPS marine ecologist, puts it, monitors have always knelt over plots perhaps 1 metre square, counting and classifying every organism. That gives great detail, but not a picture of a whole reef, or of hundreds of metres of coast.

In May 2022, a team flew a drone in a lawnmower pattern over study sites in Golden Gate National Recreation Area and Point Reyes National Seashore, taking many overlapping high-resolution photos. Stitched together, they make an orthomosaic map — the same idea as the satellite view on an online map, according to Jennifer Muha of Front Range Community College, one of the image-collection leads, only at a much smaller scale and a much higher resolution.

Becker likens the difference to a whole-body scan versus a blood test. Ground monitoring shows how many mussels are in a plot and how big they are, or whether a snail sits at a point on a transect. An orthomosaic shows the whole landscape and how much of each organism it holds. Drone imagery, he says, now gives nearly the detail of kneeling at a quadrat, but lets scientists say far more confidently whether particular algae are declining or mussel beds growing.

Learning from light

The drone also recorded near-infrared light, invisible to the eye.

• Spectral signatures. Every feature in a landscape absorbs and reflects a particular mix of light — blue, green, red, near-infrared. Plants absorb visible light and reflect most infrared.
• A wartime start. Aerial near-infrared mapping began in World War II, when the U.S. military used infrared film to detect camouflage: real vegetation glowed hot pink on the film, while things painted to look like plants did not. After the war, the interpreters trained to find the enemy turned the skill to science.
• On the shore today. Near-infrared can tell different kinds of algae apart. Nathan Duggins, a geospatial sciences student at Front Range Community College who analysed the imagery for his associate degree capstone, found that classmates could not tell two algae apart in normal colour, but the infrared bands made them look completely different.

Aerial photos of the coast around Slide Ranch, one of the NPS's regular intertidal monitoring sites, taken on colour infrared film by NASA's Johnson Space Center in 1972. Healthy vegetation, reflecting plenty of near-infrared light, shows red or hot pink. Today drones can capture the same kind of view. U.S. Geological Survey.

In preliminary near-infrared images of Slide Ranch, mussels stood out in dark red and algae in yellows and oranges — information invisible to the naked eye that can help track how communities change, in numbers.

Why it matters

Rocky intertidal zones are highly vulnerable to climate change — rising seas, warming air and water, and ocean acidification — and to pollution such as oil spills. Tracking how mussel beds, sea stars, algae and other life shift across whole sites would help guide restoration and conservation.

What comes next

The 2022 project was a proof of concept: the team worked out a method for collecting the images that future teams can refine. The maps have gone to researchers at the University of California, Santa Cruz, who are developing the best way to use artificial intelligence to sort the organisms into species groups. The pilot looks promising for adding aerial mapping to the annual monitoring, alongside the plot-level work — for Becker, the next generation of using technology to watch over the environment.

Sources

• National Park Service, San Francisco Bay Area Inventory and Monitoring Network, "Algae from Above: Scientists Pilot Aerial Mapping of Park Rocky Intertidal Zones," by Avani Fachon. https://www.nps.gov/articles/000/algae-from-above-scientists-pilot-aerial-mapping-of-park-rocky-intertidal-zones.htm
• NPS rocky intertidal monitoring: https://www.nps.gov/im/sfan/rocky-intertidal.htm · Multi-Agency Rocky Intertidal Network (MARINe): https://marine.ucsc.edu/index.html
• The original's drone photographs, maps and field photos are credited to individual researchers and partners rather than the NPS alone, and are not reproduced here.
• Words on this page from people and organisations outside the federal government are paraphrased; rewritten in hubnx's own words.