Fish herbivory as key ecological function in a heavily degraded coral reef system

Abstract

Localized impacts, such as eutrophication and overfishing, have been identified as major threats to reefs worldwide, resulting in changes in coral reef community composition, from coral‐dominance to algal‐dominance. Herbivory by fishes is frequently cited as a key process structuring benthic community response to stressors. Here, we experimentally tested the capacity of herbivorous fishes to remove macroalgae along an onshore–offshore spatial gradient in the Spermonde Archipelago, Indonesia; a coral reef system exposed to long‐term anthropogenic disturbances. Bioassays of Sargassum and Padina spp. were collected from the reef flat and transplanted onto the reef slope at five islands varying in distance (1–55 km) from the coastline and city of Makassar, South Sulawesi. Supplementary underwater video deployment and fish surveys were used to identify species‐specific differences in herbivore communities and rates of algal removal. Over a 24 h period, Padina was almost completely removed from the two outermost islands, but removal declined toward shore. With the exception of the site closest to Makassar, more than 50% of all transplanted macroalgae were removed. Species diversity of algal‐consuming fishes and total bites increased toward off‐shore, with only one species, Siganus virgatus (Siganidae), feeding at all sites. These changes in herbivore communities underline the capacity for functional resilience under varying levels of reef degradation. Our results exemplify the complexities of ecological functioning along environmental gradients.

Contrasting Responses in the Niches of Two Coral Reef Herbivores Along a Gradient of Habitat Disturbance in the Spermonde Archipelago, Indonesia

“…Our results agree with previous studies in the Spermonde Archipelago (Edinger et al, 1998(Edinger et al, , 2000Sawall et al, 2012;Plass-Johnson et al, 2015a, 2016a, where the three sites closer to shore were characterized by low coral cover, higher rubble and higher abundances of turf algae. Cross-shelf variation in these benthic components are related to isotopic metrics of the parrotfish, Chlorurus bleekeri.…”

“…Coral reefs closer to shore are generally characterized by less hard coral abundance, an increase in fleshy algae, a general loss of habitat complexity, and by a decrease in coral and fish species diversity or even complete shifts in the sets of species (Fabricius et al, 2005;Plass-Johnson et al, 2015a. Anthropogenic impacts can alter the resource availability and predator-prey relationships on a coral reef thus affecting habitat use and the trophic niche of the biotic constituents (Jones and Syms, 1998;Syms and Jones, 2000).…”

Spatio-Temporal Patterns in Coral Reef Communities of the Spermonde Archipelago, 2012–2014, I: Comprehensive Reef Monitoring of Water and Benthic Indicators Reflect Changes in Reef Health

“…The coral reef system of the Spermonde Archipelago has been the focus of extensive studies in the past two decades, serving as a model system to examine spatial differences in biodiversity, ecological and geological processes, and anthropogenic influences along a cross-shelf gradient within the Coral Triangle (Edinger et al, 1998;Renema and Troelstra, 2001;Cleary et al, 2005;Becking et al, 2006;Sawall et al, 2011Sawall et al, , 2013Hoeksema, 2012;Plass-Johnson et al, 2015a, 2016aPolonia et al, 2015). In this study, we go beyond previous research by identifying several important indicators of water quality and the main environmental drivers of coral reef community composition in the Spermonde Archipelago, including an assessment of temporal trends.…”

“…While macroalgae and other invertebrates were relatively low in % cover compared to live coral and turf algae throughout the system, their increased presence and the low live coral cover at <10% in the nearshore sites was linked to poor water quality in this area. Although macroalgae can become abundant where nutrient inputs are high, a recent study (Plass-Johnson et al, 2015a) found nearly no top-down control at the site nearest shore suggesting that water conditions can also be detrimental to macroalgal proliferation. This includes light limitation as well as high DOC, which both have led to negative impacts on corals and marine algae, causing mortality and reduced net photosynthesis (Kline et al, 2006;Meyer et al, 2015).…”

Small Scale Genetic Population Structure of Coral Reef Organisms in Spermonde Archipelago, Indonesia

“…Islands found within the archipelago differ in size, are uninhabited to densely populated, and surrounding reefs are exploited to varying degrees and with various techniques. Reef systems are therefore differentially impacted and the health of the reefs differs quite dramatically as a result of these impacts (e.g., bomb fishing, cyanide fishing, sediments, sewage; Plass-Johnson et al, 2015). Located on the southern end of the archipelago's coastline is the city of Makassar with about 1.34 million inhabitants and an industrial harbor.…”