Abstract
Coral health indices are important components of the management assessments of coral reefs, providing insight into local variation in reef condition, as well as tools for comparisons between reefs and across various time scales. Understanding how such health indices vary in space and time is critical to their successful implementation as management tools. Here we compare autotrophic and heterotrophic coral health indices, examining specifically the temporal variation driven by the local environmental variation, at three scales (diel, daily and seasonal). We compared metabolic indices (RNA/DNA ratio, protein concentration) and autotrophic indices (Chlorophyll a (Chl a), zooxanthellae density, effective quantum yield (yield) and relative electron transport rate (rETR)) for two dominant Acropora species, A. digitifera and A. spicifera at Ningaloo Reef (north-western Australia) in August 2010 (austral winter) and February 2011 (austral summer). Clear seasonal patterns were documented for metabolic indices, zooxanthellae density and rETR, while cyclic diel patterns only occurred for yield and rETR, and RNA/DNA ratio. Significant daily variation was observed for RNA/DNA ratio, Chl a concentration, yield and rETR. Results suggest that zooxanthellae density and protein concentrations are good long-term indicators of coral health whose variance is largely seasonal, while RNA/DNA ratio and rETR can be used for both long-term (seasonal) and short-term (diel) coral monitoring. Chl a can be used to describe changes between days and yield for both diel and daily variations. Correlations between health indices and light history showed that short-term changes in irradiance had the strongest impact on all health indices except zooxanthellae density for A. digitifera; for A. spicifera no correlation was observed at all. However, cumulative irradiance over the several days before sampling showed significant correlations with most health indices suggesting that a time-lag effect has to be taken into account when interpreting diel variations in coral condition.
Highlights
Reef-building corals live in symbiosis with zooxanthellae, which enables the coral to obtain energy through autotrophy as well as through heterotrophy
Pair-wise tests for protein concentration revealed that only A. spicifera showed seasonal differences (p,0.01) with higher protein concentrations in summer than in winter, while for zooxanthellae density significant differences were only found for A. digitifera (p,0.001) based on higher values during winter (Fig. 1) compared with summer
Previous work [30] reviewed the suitability of different health indices for projects monitoring coral health based on a variety of studies, but no study so far investigated the full range of metabolic indices and autotrophic indices
Summary
Reef-building corals live in symbiosis with zooxanthellae (endosymbiotic algae), which enables the coral to obtain energy through autotrophy (light-derived) as well as through heterotrophy (active uptake of particles). Photosynthesis in the symbionts of corals provide a carbon source for the coral host that is energy-rich but nitrogen-poor, while the zooxanthellae benefit from high nitrogen and phosphorus metabolic waste products of the corals [1,2,3]. The presence of these photoautotrophic symbionts within the coral tissue suggests that corals should experience large daily fluctuations in O2, CO2 and NH4 tension and pH driven by algal photosynthesis and coral metabolism (respiration rates) over a normal light/dark cycle [4,5]. There are diel fluctuations in relative electron transport rate, an indicator for photosynthetic activity [12]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
