New publication: The relationship between macroalgae taxa and human disturbance on central Pacific coral reefs

I am delighted to share my first first-authored peer-reviewed journal article, The relationship between macroalgae taxa and human disturbance on central Pacific coral reefs, now out in Marine Pollution Bulletin! This paper is a result of the research I did for my Master of Science degree, which I finished in 2017, so it feels great to have the results of all that work out in the world.

For this paper, 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 (Cannon et al. 2019)

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, Maj2 (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. These 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.

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 am excited to share this work with you and I hope that it will be useful for both researchers investigating human influences on reefs and local organizations like MIMRA who are doing reef monitoring work. I was so fortunate to get to work with MIMRA, both through this project but also as an intern for the summer following my fieldwork, to get to learn from the work that they do. MIMRA was also involved in the creation of the world-renowned Reimaanlok Plan, a national framework for conservation planning in the Marshall Islands. MIMRA has 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.

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