A recently published study has finally been able to put numbers to a widely accepted concept in reef science: that the materials in seawater (phytoplankton, organic matter, suspended sediment, etc.) can impact how much light, as well as the wavelength of light, reaches the seafloor.
As a result, the ecology of organisms, including corals and algae, that live on the seafloor and rely on that light for photosynthesis is impacted.
“Given that reef ecosystems are driven by photosynthesis, there should really be a greater interest in light ecology on reefs,” said lead author, a Bermuda Institute of Ocean Sciences (BIOS) senior scientist and coral reef ecologist Eric Hochberg. “In order to do that, you need to have numbers, so this paper is a start in that it generates the first reasonably large data set on water clarity on reefs.”
In order to gather the necessary data, Hochberg and his team collected and analysed 199 water column profiles across the reefs and deep waters of Hawaii and Bermuda using an instrument called a profiling reflectance radiometer (PRR).
The PRR is a 76cm finned metal tube that can measure both the spectrum (intensity for each colour of the rainbow) of light in the water column coming down from the surface, as well as the spectrum of light reflected up from the bottom. By deploying the PRR over the side of the boat while tethered to a computer onboard via a data cable, scientists can monitor it in real-time as it drifts to the bottom. It can collect data profiles on its descent at a rate of 15 measurements per second.
Hochberg hopes to use this data to conduct models to address fundamental ecological questions, such as how much light reaches the various reef zones (fore-reef, reef flat, and lagoon) or how ecological zoning on reefs might be driven by light absorption. For example, while the outer reef area is generally more clear and allows more blue light to penetrate to deeper depths, lagoon areas are more turbid (cloudy) and allow more green light to penetrate to deeper depths.
“Different colours of water reach different depths in different zones, which matters for the communities that live on the bottom,” Hochberg said. “The pigments that organisms have might change depending on light availability—not just how much light is available, but what colour of light is available.”
For information regarding co-authors and to read the full study, “Trends and variability in spectral diffuse attenuation of coral reef waters”, click here.
Photograph by Eric Hochberg.
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