Light-use efficiency for coral reef communities and benthic functional types

Coral reef metabolism is dominated by benthic photoautotrophic communities that comprise varying combinations of algae, coral, and sand. Rates of daily gross primary production (GPP) for these benthic functional types (BFTs) are remarkably consistent across biogeographical regions, supporting the idea that reefs exhibit modal metabolism. Most variability in reported rates likely arises from differences in light availability. In fact, GPP is a linear function of incident photosynthetically active radiation (PAR), the fraction of PAR absorbed (fAPAR) by photoautotrophic organisms or communities, and light-use efficiency (epsilon), which parameterizes photosynthesizers' biochemical capacity for CO2 fixation: GPP = epsilon x fAPAR x PAR. On time scales of days to weeks, fAPAR and epsilon are far more stable than PAR. epsilon is a critical parameter, because it represents productive response integrated across all environmental conditions, other than light. If BFTs exhibit consistent GPP across wide geographic ranges, then their epsilon s must also be consistent. The aim of this study was to estimate epsilon for algae, coral, and sand. Using data collected during NASA's CORAL mission in 2016-2017, epsilon was calculated for 32 mixed communities at Lizard Island, Australia (10); Kane'ohe Bay, Hawai'i (8); Guam (6); and Palau (8). Nonnegative least squares was used to solve for epsilon of each BFT, producing values of 0.038, 0.060, and 0.016 C photon-1 for algae, coral, and sand, respectively. These values can be used in light-driven models of reef metabolism. Further work is necessary to refine these estimates and, importantly, to explore how epsilon is affected by environmental conditions.