What’s the best way to monitor kelp? The fun/efficiency trade-off

Kelp occupies the subtidal of our shores. In coastal ecosystems, kelp form habitats, providing food and a home for countless species. Their importance in coastal ecosystems is great, as is their commercial value to harvesters. HOWEVER, kelps face a wide range of pressures from harvesting to invasive species and climate change. Rising sea surface temperature (SST) is forcing many species of seaweed to move towards the poles, as they track suitable temperatures.

Ocean Approved farms kelp

Picture: Gregory Rec

The Macroalgal Research Group has previously highlighted changing distributions in large brown seaweeds in response to rising sea surface temperature, and indicated a need to monitor these changes consistently. Though, kelps tend to occupy difficult-to-access areas of the shore (in the rocky subtidal) and monitoring these vitally important habitats is therefore not easy…

Poor visibility, difficulty navigating slippery rocky shores and their sometimes expansive spatial coverage, means that recording how much kelp forest is actually present (accurately) is almost impossible from the shore.

That brings me to remote sensing…

Remote sensing offers new avenues to map kelp habitats quickly and accurately. Aside from that, some remote sensing tools can be really, really fun. So from a data gathering stand-point remote sensing is exciting but for an excuse to get into the field, some remote sensing tools (i.e. UAVs or ‘drones’) provide the opportunity for grown adults to live out their childhood dreams… and call it ‘work’.

Besides the clear advantages of flying drones up and down the coast, they can be really effective tools for monitoring kelps and other seaweeds. However, drones are fundamentally an optical tool (light-based) and can encounter difficulty in murky waters with poor visibility. The use of multispectral sensors can help, but missing some (particularly deeper) kelp is likely. Aerial images gathered by drones and other aerial vehicles can be used to map seaweed habitats, as demonstrated by members of the Macroalgal Research Group recently (more info -> HERE).

sea-rock-ocean-stone-underwater-biology-80583-pxhere.com

Picture: pxhere

Satellite-based sensors offer a similar avenue to drones, but at a much greater coverage. The trade-off is resolution, but this isn’t so much of an issue when your target species floats, like giant kelp. Floating Forests make great use of satellite images and citizen science to map massive areas of Macrocystis pyrifera. I really recommend Floating Forests as a coffee-break exercise, dragging your mouse around clumps of kelp is surprisingly therapeutic, and at the same time you’ll be making a valuable contribution to the science community, win-win.

Light detection and ranging (LiDAR) is another optical tool that has successfully been used to map kelp (example -> HERE). LiDAR uses a pulsing laser to obtain bathymetric data which can be used to create bathymetric derivatives and aid the creation of habitat suitability models, I know what you’re thinking… that sounds like way less fun than drones… well never fear! LiDAR devices have been developed that can fit nicely onto a drone so you now have TWO excuses to get your hands on a drone.

Terrestrial_Lidar_Scan

Credit: Toby Minear, USGS. Public domain

Looking for more excuses to do more fun kelp monitoring? Look no further, underwater imagery is another useful technique for monitoring kelp habitats. Images can be gathered from drop and towed cameras from boats or by scuba divers or by snorkel. There are advantages to underwater imagery, namely, biotic interactions can be examined and taxonomic information can be gathered. HOWEVER, and it’s quite a considerable HOWEVER, even though it could arguably the most fun method for monitoring and mapping kelp habitats, it’s probably the slowest (aside from traditional shore monitoring). Underwater imagery is really only applicable over a small spatial scale, it’s slow and data processing is even slower.

Speed is a major concern when considering a viable large scale monitoring procedure. This is why I’m now going to mention SONAR (sound navigation and ranging). One major plus for sonar is that most of the English coastline has been mapped already, by the United Kingdom Hydrography Office, the data of which is available upon request. Most importantly, when acoustic data is gathered, bathymetry data (depth) is recorded but so too is backscatter information. Acoustic backscatter is vital for subtidal habitat mapping, described “as the amount of energy received by a sonar device after a complex relationship with the seafloor”; it can be used to identify seafloor substrate.

Although, as with any other remote sensing method, there are certain drawbacks associated with multibeam sonar and its application to kelp monitoring. Issues surrounding the harmonisation of backscatter data limit its wide spread use (see HERE for more info). Another major consideration is that sonar is ineffective in waters <2m, which means some kelp habitat is going to go unaccounted for. However, this means that you’ll need to combine acoustic methods with optical methods to ensure all possible habitat is accounted for, which could mean… DRONES + LASERS!!

There is always going to be a trade-off between different remote sensing techniques based on desired resolution, spatial coverage, cost and target species… to name a few. Multiple tools and methods will most likely be needed to achieve consistent monitoring, but the rapid advancement of technology means that the tools will probably become less expensive and potentially even more fun.

Happy kelp hunting!

yawning-otter_1973063i

Picture: Matt Maran / Nature Picture Library

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