Shifting roles of heterotrophy and autotrophy in coral energetics under varying turbidity

Suspended particulate matter (SPM) strongly alters the trophic environment of photosymbiotic aquatic organisms. At high particles loads, phototrophic energy gains can be diminished due to light absorption by suspended particles, and stress from particle abrasion or deposition on tissues. However, energy gains are enhanced if organisms are able to use SPM as a food source. For photosymbiotic benthic suspension feeders, increases in SPM concentrations may require both phototrophic and heterotrophic acclimation to sustain a positive energy balance. This study provides an experimental analysis of the effects of contrasting light and SPM regimes on the energy budget (scope for growth) of two zooxanthellate corals (Goniastrea retiformis and Porites cylindrica). Using a factorial design in a flow-through tank system, corals were exposed for 2 months to shaded and unshaded conditions (equivalent to 3–4 m depth at 4 and 16 mg dry weight SPM l−1, respectively) and a range of controlled SPM loads with a natural organic content (∼3% w/w). In G. retiformis, rates of particle ingestion were a linear function of SPM concentration within a broad range (1–30 mg dry weight l−1). After 2 months of shading, photosynthetic acclimation was significant in G. retiformis, but did not compensate for the reduced light level, as daily respiration exceeded daily photosynthesis. However, in response to the prolonged shading, G. retiformis more than doubled its rate of particle feeding. At high SPM treatments (16 mg dw l−1), sediment feeding by this species compensated fully for the 35–47% lower phototrophy in the shaded treatment. Due to both photo- and heterotrophic plasticity, G. retiformis gained tissue and skeletal mass at all experimental levels of light and SPM. In contrast, rates of particle intake by P. cylindrica contributed <10% to the energy budget in shaded and