Abstract
Pollution, fishing, and outbreaks of predators can heavily impact coastal coral reef ecosystems, leading to decreased water quality and benthic community shifts. To determine the main environmental drivers of coral reef status in the Spermonde Archipelago, Indonesia, we monitored environmental variables and coral reef benthic community structure along an on-to-offshore gradient annually from 2012-2014. Findings revealed that concentrations of phosphate, chlorophyll a-like fluorescence, suspended particulate matter, and light attenuation significantly decreased from on-to-offshore, while concentrations of dissolved O2 and values of water pH significantly increased on-to-offshore. Nitrogen stable isotope signatures of sediment and an exemplary common brown alga were significantly enriched nearshore, identifying wastewater input as a primary N source from the city of Makassar. In contrast to the high temporal variability in water quality, coral reef benthic community cover did not show strong temporal, but rather, spatial patterns. Turf algae was the dominant group next to live coral, and was negatively correlated to live coral, crustose coralline algae (CCA), rubble and hard substrate. Variation in benthic cover along the gradient was explained by water quality variables linked to trophic status and physico-chemical variables. As an integrated measure of reef status and structural complexity, the benthic index, based on the ratio of relative cover of live coral and CCA to other coral reef organisms, and reef rugosity were determined. The benthic index was consistently low nearshore and increased offshore, with high variability in the midshelf sites across years. Reef rugosity was also lowest nearshore and increased further offshore. Both indices dropped in 2013, increasing again in 2014, indicating a period of acute disturbance and recovery within the study and suggesting that the mid-shelf reefs are more resilient to disturbance than nearshore reefs. We thus recommend using these two indices with a selected number of environmental variables as an integral part of future reef monitoring.
Highlights
Coastal coral reef systems close to populated urban areas are often highly impacted by increased nutrient inputs, sedimentation rates, and fishing pressure resulting from land based activities (Burke et al, 2011)
We investigated the following questions: (1) Which water quality and environmental variables drive benthic coral reef community structure across the Spermonde Archipelago, (2) is there temporal variability in these relationships, (3) which indices can be used to detect spatial and temporal change in coral reef status and health for future monitoring, and (4) can these indices help identify the contribution of localized disturbances? To answer these questions, we designed an annual survey on water quality, benthic reef cover, and fish communities along an on-to-offshore gradient in Spermonde Archipelago
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, 2011, 2013; Hoeksema, 2012; Plass-Johnson et al, 2015a, 2016a,b; Polonia et al, 2015)
Summary
Coastal coral reef systems close to populated urban areas are often highly impacted by increased nutrient inputs, sedimentation rates, and fishing pressure resulting from land based activities (Burke et al, 2011). The effects of terrestrial-derived inputs in coastal waters across coral reefs have led to noticeable spatial gradients in water quality in many parts of the world (Edinger et al, 1998; Mallela et al, 2004; Fabricius et al, 2005; Lirman and Fong, 2007). They generally lead to reduced scleractinian coral fitness and greater competition for benthic space between corals and other organisms (Fabricius, 2005). A more nuanced view that understands benthic community configuration as changing gradually along a continuum, notwithstanding the existence of thresholds and tipping points, is gaining prominence, necessitating a more holistic approach that considers benthic parameters beyond stony corals and macroalgae alone (Jouffray et al, 2014; Smith et al, 2016)
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