Feature ArticleNew Window Opens For Coral Reef Research
By Evan Lubofsky
Research over the years has shown that the structural complexity of forests has a direct impact on bird species diversity and abundance. The greater the complexity, the greater the biodiversity.
Similarly, a number of ocean studies have indicated that structure is also an important component of the diversity and overall health of coral reef systems. In 1965, a marine scientist compared fish collections in reefs, ranging from few to many coral species, and found that the latter allowed for a greater number of fish species. In 1972, graduate student Mike Risk used a fine-link chain, which could be draped and applied over a length of 1 meter, to create the chain-to-tape ratio gold standard measure of reef rugosity, which is a measurement of a reef’s structural complexity. This was done eight times for each meter-square quadrat; in some cases 4 meters of chain were required to equal 1 “straight” meter.
Instrumental to this study was the coral reef rugosity technique, which has been widely used over the past several decades. It involves calculating the ratio of a straight-line transect to the distance of a contour-following rugosity chain draped along a reef.
While the technique is able to provide a measure of a reef’s structural complexity by showing the variation of its vertical profile, it has some limitations.
For example, chains can easily become tangled as they are laid out on the reef, which in turn may compromise measurement accuracy and make the surveying process very time-consuming. Also, the technique is typically only able to provide a measurement resolution of 4 to 5 meters, making it unsuitable for studies where a smaller-scale spatial resolution is needed.
Remote visual sensing techniques have also been used to measure reef structure; however, they are not well-suited to the fine-scale measurements that coral ecosystems require, often at the scale of individual transects or quadrats.
A Modern, Noninvasive Approach
To overcome these limitations and improve the accuracy with which a reef’s topographical variability and scale can be measured, Dr. Phillip Dustan from the College of Charleston developed an innovative new technique, referred to as digital reef rugosity (DRR).
“Instead of relying on rugosity chains, DRR is a diver-operated technique where the diver swims along a transect with a pressure-based water level data logger,” he said. “The instrument continuously logs level and temperature at one-second intervals for the duration of the dive. The raw measurement data are then downloaded and exported to a spreadsheet, and analyzed to more precisely measure a reef’s structural complexity.”
Dustan has been studying reef rugosity to help determine the ecological tipping point of reefs. If reef structure degrades to a certain point, it loses its ability to support fish communities. Predicting this critical tipping point can help reef managers and others determine the most appropriate course of conservation.
“One of my friends who owns a dive shop mentioned that the coral was bleaching about 10 miles offshore here in South Carolina and asked me to come out and take a look,” said Dustan. “I had used a digital level gauge previously in a project to estimate the flow of water through an estuarine swamp. I brought it out to the reef to see if we’d be able to trace the reef and get a profile of it digitally. We took it to the bottom of the reef, traced alongside a tape, and I lifted the gauge over my head every 5 meters as I swam. Inverting the pressure measurement and converting it to depth gave us a really nice profile of the reef.”
In looking further at the data, Dustan noticed that due to a combination of the one-second sampling rate and swimming at a slow, even pace, he was able to see measurement points every 10 centimeters—a much finer scale than was possible with traditional rugosity chain methods.
Field Study in Bali
Having realized the potential of this proof-of-concept for reef studies, Dustan and the Biosphere Foundation set out to Menjangan Island off the northwest corner of Bali, Indonesia, to study five reef sites—four on the island and one on mainland Bali—utilizing the new technique. More than 125 species of reef fish and 40 genera of corals have been found in the region, despite the fact that some of the reefs have been degraded by blast fishing, overfishing, anchor damage and bleaching from elevated water temperatures.
“One of the degraded areas where the coral was mined to make cement was dubbed the ‘Killing Fields’ because the distribution was near complete with no signs of coral recovery and/or recruitment observed on the rubble substrate,” said Dustan. Other sites showed signs of intermediate damage from blast fishing, Crown-of-Thorns [starfish] predation and/or severe bleaching due to climate change. “All of these disturbances highlight the need for more assiduous ecosystem-based management,” he said.
In general, the study sites in Bali presented a range of benthic conditions, which were estimated by point counting consecutive digital photos of each transect. This includes reefs with both stony and soft coral cover, those with rich undamaged coral and some with barren, rubble-like coral.
In addition to widely varying amounts of coral cover, fish populations varied considerably across the different sites in terms of abundance, biomass and diversity of species. In general, the studies demonstrate that there are both physical and biological components of coral reef community structure.
Tracing the Reef
Prior to a first dive, Dustan configured the water level logger—an Onset (Bourne, Massachusetts) model HOBO U20-001—for deployment.
Slightly larger than a cigar, the instrument measures to a resolution of 0.41 centimeters and an accuracy of ± 1.5 centimeters over its 30-meter depth range.
Initial logger configuration involved using accompanying HOBOware software to set the sample rate and logging start time. The delayed start feature eliminated the need to bring a laptop out to the site. Based on the logger’s 42,000-measurement capacity and one-second sample rate, it would be able to record continuously for a period of four to six hours before having to be downloaded, more than enough time for two or three dives during a day. To continue this article please click here.
Evan Lubofsky earned a B.A. in journalism from the University of Massachusetts and has been writing about the use of technology in ocean science for more than 10 years. His work has appeared in a broad range of trade journals and magazines throughout the United States.