Abstract
Despite the traditional view of coral reefs occurring in oligotrophic tropical conditions, water optical properties over coral reefs differ substantially from nearby clear oceanic waters. Through an extensive set of optical measurements across the tropical Pacific, our results suggest that coral reefs themselves exert a high degree of influence over water column optics, primarily through release of colored dissolved organic matter (CDOM). The relative contributions of phytoplankton, non-algal particles, and CDOM were estimated from measurements of absorption and scattering across different geomorphic shallow-water reef zones (<10 m) in Hawaii, the Great Barrier Reef, Guam, and Palau (n = 172). Absorption was dominated at the majority of stations by CDOM, with mixtures of phytoplankton and CDOM more prevalent at the protected back reef and lagoon zones. Absorption could be dominated by sediments and phytoplankton at fringing reefs and terrestrially impacted sites where particulate backscattering was significantly higher than in the other zones. Scattering at three angles in the backward direction followed recent measurements of the particulate phase function. Optical properties derived from satellite imagery indicate that offshore waters are consistently lower in absorption and backscattering than reef waters. Therefore, the use of satellite-derived offshore parameters in modeling reef optics could lead to significant underestimation of absorption and scattering, and overestimation of benthic light availability. If local measurements are not available, average optical properties based on the general reef zone could provide a more accurate means of assessing light conditions on coral reefs than using offshore water as a proxy.
Highlights
While there is a view that tropical coral reefs inhabit clear, oligotrophic waters [1,2,3,4,5], these systems can exhibit a range of water clarities, from relatively clear oceanic-type to high-attenuation regimes with complex optical properties [6,7,8]
We provide a snapshot of the optical properties of coral reefs measured across diverse regions in the Pacific to provide insight into the light environment found in these dynamic ecosystems
We show that a range of Inherent Optical Properties (IOPs) exist across Pacific coral reef waters, from nearly oceanic to heavily attenuating coastal regimes
Summary
While there is a view that tropical coral reefs inhabit clear, oligotrophic waters [1,2,3,4,5], these systems can exhibit a range of water clarities, from relatively clear oceanic-type to high-attenuation regimes with complex optical properties [6,7,8]. The intensity and spectral quality of light available at the benthos is determined by the absorption and scattering of photons as they travel downwards through the water column These processes are defined by the Inherent Optical Properties (IOPs) of the water, which vary according to the composition, concentration, and size of suspended particles and dissolved material [16]. Particulate organic matter, i.e., detritus or marine snow, may remain suspended for long periods of time and can be produced on reefs from a variety of sources including coral mucus [51]. Sourced particulates such as river input or run-off can increase particle load, limiting PAR and smothering or physically burying corals [37,38]. As well as associated instrument calibration files, are available freely [60] from NASA (SeaBASS)
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