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

Highlights

• Benthic composition differed by atoll, level of disturbance and exposure.

• Degraded sites were dominated by turf algae, sponges, and cyanobacteria.

• Different genera of macroalgae were associated with high and low human disturbance.

• Macroalgal genera may be better indicators of human-caused reef degradation than overall quantity of macroalgae.

Abstract

Climate change and human disturbance threatens coral reefs across the Pacific, yet there is little consensus on what characterizes a “healthy” reef. Benthic cover, particularly low coral cover and high macroalgae cover, are often used as an indicator of reef degradation, despite uncertainty about the typical algal community compositions associated with either near-pristine or damaged reefs. In this study, we examine differences in coral and algal community compositions and their response to human disturbance and past heat stress, by analysing 25 sites along a gradient of human disturbance in Majuro and Arno Atolls of the Republic of the Marshall Islands. Our results show that total macroalgae cover indicators of reef degradation may mask the influence of local human disturbance, with different taxa responding to disturbance differently. Identifying macroalgae to a lower taxonomic level (e.g. the genus level) is critical for a more accurate measure of Pacific coral reef health.

Introduction

The health of coral reef ecosystems is declining worldwide (Aswani et al., 2015; Fenner, 2012; Hoegh-Guldberg et al., 2018; Hughes et al., 2019; Williams et al., 2015). Threats to coral reefs include both global pressures (such as climate change and ocean acidification) and local pressures (such as overfishing, sedimentation, and eutrophication). In addition to threatening the livelihoods and source of food for millions of people (Cinner, 2014), many of these threats can kill coral and reduce habitat complexity, influencing the ability of reefs to protect shorelines (Alvarez-Filip et al., 2009; Ferrario et al., 2014). Additionally, the potentially additive properties of these impacts may make corals and coral reef ecosystems less resilient to natural disturbances like storms (Scheffer et al., 2015, but see Côté and Darling, 2010).

Global sea temperatures have been rising for the past century and are projected to continue to rise (IPCC, 2014). This is leading to more frequent and severe heat waves and heat stress (Easterling et al., 2000; Donner et al., 2005; Maynard et al., 2015a) which can cause coral bleaching, coral mortality, and a reduced resilience to other stressors (Carpenter et al., 2008; Hughes et al., 2007; Magris et al., 2015; Toth et al., 2015). As a result, ongoing research is aiming to identify management actions that could confer coral reef resilience to climate change (e.g.; Bruno et al., 2019; Guest et al., 2018; Maynard et al., 2015b; McClanahan et al., 2012; Van Oppen and Gates, 2006). Applying these actions, however, depends on robust estimators of reef status. There has been disagreement in the literature about the relative role of local human activities in coral reef degradation with some articles reporting that local impacts are important (e.g. Fabricius et al., 2005; Scheffer et al., 2015; Williams et al., 2015; Crane et al., 2017), and other articles finding that local impacts are negligible in comparison to the impacts of climate change (e.g. Aronson and Precht, 2006; Carpenter et al., 2008; Bruno and Valdivia, 2016).

The debate about the relative role of local and global threats may in part stem from disagreements over the metrics of reef health, as well as the very definition of a healthy reef. Total macroalgae cover is commonly used to indicate that reefs are ‘unhealthy’ or have a different benthic composition than what was found prior to disturbance (e.g. McCook et al., 1997; Mumby et al., 2005; Hughes et al., 2007; Bruno and Valdivia, 2016). However, the broad category of “macroalgae cover” used in many field studies includes a variety of alga with different sensitivities to disturbance and life history strategies (Diaz-Pulido et al., 2010). The use of this coarse category could lead to the mischaracterization of reefs as unhealthy versus healthy. It neglects the differences within algae communities, overlooks other key reef organisms such as cyanobacteria and turf algae which are common on degraded reefs, and disregards the important role some algal assemblages may play in reef ecosystem function (e.g. Perry et al., 2016). Some reefs with high macroalgae cover are not necessarily degraded; in fact, some reefs far from human disturbance may have high percentages of macroalgae cover (Vroom et al., 2006; Vroom and Braun, 2010; Williams et al., 2015).

The difficulties in defining ‘healthy’ reefs are also influenced by the debate about what reefs may have looked like before the Industrial Revolution. Some studies argue that some reefs may have naturally high macroalgae cover (Howe, 1912; Setchell, 1928; Vroom, 2011; Finnegan et al., 2015), while others argue that macroalgae has increased at the expense of live coral even on the most remote reefs due to recent climate change (Aronson and Precht, 2006; Carpenter et al., 2008; Bruno and Valdivia, 2016). However, while phase shifts from hard coral- to macroalgae-dominated reefs after disturbances are well documented in the Caribbean (Hughes, 1994; Hughes et al., 2007; Suchley et al., 2016), these may be uncommon in the Indo-Pacific (Bruno et al., 2009; Smith et al., 2016). It is possible that disturbances may instead cause coral reefs to shift towards other dominant taxa, such as sponges (Bell et al., 2013; Powell et al., 2014), corallimorphs (Work et al., 2008), or corals with ‘weedy’ life history strategies (Darling et al., 2012; Davenport and Haner, 2015; Crane et al., 2016).

The existence of remote Pacific atolls with negligible local impacts has allowed researchers to contrast coral and fish communities between pristine and more disturbed Pacific reefs (Houk and Musburger, 2013; Martin et al., 2017; Pecl et al., 2017; Sandin et al., 2008; Williams et al., 2013). The atolls of Majuro and Arno in the Republic of the Marshall Islands (RMI) provide a unique opportunity for targeted analysis of possible indicators of reef health and the relative threats posed by climate change and human disturbance. Only 19 km apart, the two atolls have comparable climatic and oceanographic experiences but a diversity of local human disturbance histories. In 2014, the most extensive thermal stress event in recorded history impacted the RMI. Abnormally high sea surface temperatures from July through December 2014 led to Bleaching Alert Level II warnings from NOAA Coral Reef Watch for September through November for the entire country, although heat stress was not as high in the southern atolls, including Majuro and Arno (Coral Reef Watch, 2017). There were some reports of possible shallow water bleaching-related coral mortality based on limited observations in Majuro and Arno (Fellenius, 2014), but the overall effect of the heat stress event on coral and algal communities remains unclear.

In this study, we examined how human disturbance impacts the composition of coral and algae communities across the more populated Majuro Atoll and more pristine Arno Atoll to explore the local impact of these disturbances and their effect on the recovery of reefs after the 2014 thermal stress event. We first investigated patterns in percent cover of key coral and alga taxa and their relationship with metrics of human influence across 25 sites in Majuro and Arno. We then examined how size structure of the coral communities varies across sites within each atoll and between the two atolls. Finally, we assessed the recovery from the recent (2014) thermal stress event by comparing percent cover surveys of key benthic life forms from 2007 in Arno and 2014 in Majuro to our data from 2016, at a subset of the original sites. The results provide insight into the limitations of macroalgae as an indicator of reef health and the impacts of human activities on coral communities in the central Pacific.