It’s time to stop using macroalgae cover as a metric of reef health

Written by Sara Cannon

When we imagine a healthy coral reef, we tend to picture a diverse ecosystem with high hard coral cover, lots of herbivorous fish and invertebrates, and a low prevalence of macroalgae. Indeed, scientists and marine resource managers often use indicators related to this image when attempting to quantify reef health on a local scale. Common metrics of reef health include the total cover of macroalgae on a reef, assuming that high macroalgae cover implies that a reef is less healthy. Similarly, reef surveys may measure the percent cover of hard coral, assuming that a high value means a more healthy reef. 

In reality, these metrics could lead to misleading results. In Pacific coral reefs, phase shifts on disturbed reefs do not seem to uniformly follow the expected trajectory to macroalgae dominance, as seen in classic studies from the Caribbean. Some degraded reefs are experiencing phase shifts to low-diversity sites with high percent cover of weedy coral species.  Coral reefs that are far away from humans may have had high macroalgae cover well before the industrial revolution. In the Indian Ocean, high cover of calcifying macroalgae may facilitate higher cover of live coral, countering the assumption that high macroalgae cover indicates a degraded reef. These studies together suggest that despite decades of studies on coral reefs, scientists still lack a clear understanding of what a healthy reef looks like, although we are hopefully getting closer to figuring it out.

Our recent study, The relationship between macroalgae taxa and human disturbance on central Pacific coral reefs, investigated these relationships on coral reefs in the Republic of the Marshall Islands. My advisor, Simon Donner, and I worked closely with collaborators from the Marshall Islands Marine Resources Authority (MIMRA) to conduct surveys in order to compare the benthic communities across two neighboring atolls (Figure 1). We also compared our surveys to those conducted by Dr. Maria Beger from the University of Leeds, Dr. Doug Fenner in American Samoa, and MIMRA to investigate changes over time across our sites. Majuro Atoll, on the left, is the capital of the Marshall Islands and home to about 30,000 people (roughly half of the country’s population), while Arno Atoll (right) has a population of approximately 2,000.

Figure 1: Research sites in the Republic of the Marshall Islands.

This paper offers two novel analyses and findings, in addition to providing a thorough overview of the state of the reefs in Majuro and Arno. First, it is the first paper studying human impacts on coral reefs that used the normalized difference vegetation index (NDVI) to quantify human disturbance. NDVI is a measurement of density of vegetation on land. Its values range between -1 (no vegetation) and +1 (highly dense vegetation). It is commonly used as a metric of human disturbance in terrestrial studies, but this is the first study of my knowledge to use it in marine systems. Instead, many studies of marine systems use human population, distance to fish markets, or some other metric that combines population and distance. 

Those metrics were not useful here for a few reasons. First, detailed population metrics are not available for all of the sites at the resolution that we would need. Also, many of the areas that had high levels of disturbance were far from places where most people lived (and therefore had low populations). For example, our study site MAJ02 (Figure 1) is located close to the airport, where there was a controversial expansion project happening at the time of our surveys,and construction crews were dredging nearby to build the runway. This site experienced heavy sedimentation as a result but would have been considered a site with “low human influence” had we used a population metric instead of NDVI. Using NDVI allowed us to measure areas that had lots of construction and less greenery. NDVI is a metric that will capture different causes of disturbance, including dense populations and dredging and construction that is farther away from where people live.

This paper is also the first (to our knowledge) to suggest a relationship between the genera of macroalgae and different levels of human impacts on coral reefs (but see this 2017 study from Kristen Brown et al). Our results cast doubt on a common metric for human disturbance in coral reef studies — the percent cover of macroalgae — and suggest that using this metric may give misleading results, even potentially masking human disturbance rather than revealing it. In fact, the most disturbed sites in our study had low macroalgae cover and instead had high cover of other benthic taxa, such as turf algae, sponges, and cyanobacteria (Figure 2).

Figure 2: An example of the macroalgae genera found at sites with low human influence and high NDVI (left), versus the encrusting sponge Terpios hoshinota and cyanobacteria more common at sites with high human influence and low NDVI (right).

We found that some genera of macroalgae grew on disturbed reefs and other genera were found on undisturbed reefs. For example, the macroalgae genera Halimeda was associated with sites that had high NDVI values (lower disturbance), while the genera Hypnea was closely associated with sites that had low NDVI (high disturbance).

Our sites in Arno were all unexposed to wind and waves (the most exposed side of the atoll is the northern rim), while we had both unexposed and exposed sites in Majuro. This made exposure to wind and waves a confounding factor, but we were able to address it in the paper and we successfully ruled out the role of exposure in the distribution of different macroalgae genera at our sites. Still, other studies have found that the distribution of macroalgae genera is influenced by exposure to wind and waves. This sensitivity to environmental factors is yet another reason why macroalgae as a group is not a reliable way to measure human disturbance on coral reefs. Instead, we suggest using other benthic taxa, such as cyanobacteria, turf algae, sponges, or identifying macroalgae to at least the genera level, to quantify human disturbance.

I hope that this work will be useful for both researchers investigating human influences on reefs and local organizations like MIMRA who are doing reef monitoring work. After fieldwork for this project, I had the opportunity to continue to with MIMRA as an intern, which allowed me to learn more about the work they do.  MIMRA was involved in the creation of the world-renowned Reimaanlok Plan, a national framework for conservation planning in the Marshall Islands, and have been identifying algae to the genera level for some time now. Their methods for reef monitoring and marine resource management are a great example for other local communities doing similar work throughout the Micronesia region and the Pacific more broadly.

References:

Brown, Kristen T., Dorothea Bender-Champ, Dominic E. P. Bryant, Sophie Dove, and Ove Hoegh-Guldberg. 2017. “Human Activities Influence Benthic Community Structure and the Composition of the Coral-Algal Interactions in the Central Maldives.” Journal of Experimental Marine Biology and Ecology 497(December):33–40. http://dx.doi.org/10.1016/j.jembe.2017.09.006

Cannon, S.E., S.D. Donner, D. Fenner, M. Beger. (2019). The relationship between macroalgae taxa and human disturbance on central Pacific coral reefs. Marine Pollution Bulletin, 145, 161-173. https://doi.org/10.1016/j.marpolbul.2019.05.024

Crane, Nicole L., Michelle Paddack, Peter Nelson, Avigdor Abelson, Kristin Precoda, John Jr Rulmal, and Giacomo Bernardi. 2017. “Atoll-Scale Patterns in Coral Reef Community Structure: Human Signatures on Ulithi Atoll, Micronesia.” PloS One 12(5):1–19.

Hughes, Terence P. 1994. “Catastrophes, Phase Shifts, and Large-Scale Degradation of a Caribbean Coral Reef.” Science 265(5178):1547–51.

Vroom, Peter S. 2011. “‘Coral Dominance’: A Dangerous Ecosystem Misnomer?” Journal of Marine Biology 2011:1–8.

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