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Mapping underwater habitats

Many challenges remain when it comes to mapping underwater environments. Dr Javier Leon explain more using coral reefs as an example.
In this section you will learn about a few different technologies that help us map underwater environments. Hi, I’m Dr. Javier Leon from the University of the Sunshine Coast, Australia. I’m a geographer and most of my work focuses on mapping and monitoring coastal environments. Since the 1970s, we have been using Earth observation systems such as the Landsat satellite to monitor the land and ocean surfaces, biosphere, solid Earth, and atmosphere. For example, we can monitor vegetation health, through seasons and years all around the world. But what about underwater, more than 70% of our planet is covered with water after all. Well, we can see underwater reflections down to around 30 metres, but that depends on water turbidity, of course.
We can use satellite images in shallow, clear waters to cost effectively map underwater ecosystems, such as coral reefs. Coral reefs harbour over 9 million species worldwide, and provide important ecosystem goods and services such as fish production and coastal protection.
And here is an interest fact: Only as of 2021 has the first ever detailed map of all coral reefs in the
world been produced: the Allen Coral Atlas. Why only now you might be asking? That is 50 years after the first Earth observation satellite was launched. Well, there are a few challenges when using remotely sensed data when mapping underwater. The first limitation is image resolution. Spatial resolution refers to the size of the smallest feature that can be detected by a sensor. For example, satellite imagery with a spatial resolution of 100 metres will cover a soccer stadium with only one pixel. That is pretty coarse to observe smaller objects like small reefs. Very high resolution imagery can resolve features of only a few metres, but this imagery can be quite expensive, and the higher the resolution, the more imagery you need to analyse.
The real problem is that the perfect sensor does not exist. There is always a compromise between resolution, spatial extent and cost. A further limitation with mapping coral reef from space is the pervasive cloud cover along the tropics. So, what changed for the Allen Coral Atlas team? Well, a few things. The multidisciplinary team used imagery from Planet Labs microsatellites, which produce daily, four metre resolution imagery. This allowed them to choose cloud free images and resolve reefs with greater detail. Plus, they used machine learning to analyse the half a million images. But the completion of the Atlas is by no means the end of the journey.
There are many biophysical characteristics of habitats within the reef that are very important to monitor but require even higher resolution data. For example, structure complexity of the reef is important when it comes to biodiversity and wave dissipation. To understand this, we need imagery and 3D topographic data at a centimetre or even at a millimetre spatial resolution. Fortunately, technological advances from the last few decades are helping us to do so. For example, we can use overlapping imagery taken from drones or underwater cameras to reconstruct 3D surfaces. This is done using what is known as Structure-from- Motion algorithms. And we can only do this now because computers are powerful enough to process the hundreds to 1000s of images that are required.
Drones allow us to map many hectares at great detail. And drones fly below the clouds so there are no issues with cloud coverage, unless of course it is raining or very windy. Wind also produces chop, and it gets harder to see through choppy waters as sun reflection creates sun glint. Sun glint is a big issue when mapping through water. Underwater cameras allow us to reconstruct imagery and surfaces at millimetre resolution that helps us assess changes in micro topography. This can help us assess the impact of cyclones or coral bleaching, for example. Plus, there are no issues with rain or wind or sun glint, although strong currents and waves can be challenging for automated underwater vehicles.
But the biggest challenge when mapping underwater is covering large areas. Remember, there is always a compromise between resolution, spatial coverage and cost. So, is there a perfect method to map underwater environments? Well, that depends on the required information or research question you are asking. We have come a long way when it comes to mapping underwater environments, but there is still much to do. The oceans are huge, and we have only scratched the surface

Since the 1970’s, we have been using Earth Observation Systems, such as the Landsat satellite, to monitor the land and ocean surfaces, biosphere, solid Earth and atmosphere. We can use satellite images in shallow, clear waters, to cost-effectively map underwater ecosystems such as coral reefs. However, satellite imagery has usually lacked the detail and frequency to efficiently map these habitats. Issues such as water turbidity and cloud cover add to the challenges.

More recently, higher-resolution satellite imagery and machine learning algorithms have been used to map all shallow coral reefs in the world. Moreover, technological advances have made other tools such as drones and underwater cameras readily available to map coral reefs in unprecedented detail. Nevertheless, the ocean is huge, and the perfect sensor does not exist as there is always a compromise between resolution, spatial extent, and cost. The quest for mapping underwater environments still has a long way to go.

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Life Below Water: Conservation, Current Issues, Possible Solutions

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