Shallow-water wave lensing in coral reefs: a physical and biological case study

Abstract

SUMMARYWave lensing produces the highest level of transient solar irradiances found in nature, ranging in intensity over several orders of magnitude in just a few tens of milliseconds. Shallow coral reefs can be exposed to wave lensing during light-wind, clear-sky conditions, which have been implicated as a secondary cause of mass coral bleaching through light stress. Management strategies to protect small areas of high-value reef from wave-lensed light stress were tested using seawater irrigation sprinklers to negate wave lensing by breaking up the water surface. A series of field and tank experiments investigated the physical and photophysiological response of the shallow-water species Stylophora pistillata and Favites abdita to wave lensing and sprinkler conditions. Results show that the sprinkler treatment only slightly reduces the total downwelling photosynthetically active and ultraviolet irradiance (~5.0%), whereas it dramatically reduces, by 460%, the irradiance variability caused by wave lensing. Despite this large reduction in variability and modest reduction in downwelling irradiance, there was no detectable difference in photophysiological response of the corals between control and sprinkler treatments under two thermal regimes of ambient (27°C) and heated treatment (31°C). This study suggests that shallow-water coral species are not negatively affected by the strong flashes that occur under wave-lensing conditions.

On the influence of wind and waves on underwater irradiance fluctuations

“…10b), but their occurrence frequency is low. Up to now, light flashes were recorded down to a depth of 21 m only (Veal et al, 2010;Hieronymi et al, 2012). Irradiance variability (CV) of around 10 % has been detected at the depth range of 30 to 35 m (Stramska and Dickey, 1998;Veal et al, 2010) which absolutely fits to our simulation results for moderate sea states (Fig.…”

“…9e). In rare events, irradiance values of more than 1.5 E d were measured at this depth level under moderate sea conditions (Veal et al, 2010;Hieronymi et al, 2012). According to Figs.…”

The Enhancement of Photosynthesis by Fluctuating Light

“…Yamasaki and Nakamura (2008); Veal et al (2010) (Walsh & Legendre, 1983), cloud cover (Marra & Heinemann, 1982), and the vertical movement of phytoplankton (Falkowski & Wirick, 1981). The high-frequency (less than 1 Hz) light fluctuations, known as 'flicker light', are produced by a lens effect of moving water surface, or waves, that simultaneously focuses and diffuses sunlight in the few upper meters (Hieronymi & Macke, 2010) (Fig.…”

“…4). The effect of small-scale roughness on preventing wave lensing was applied in the study by Veal et al (2010), who used water sprinklers to obtain non-lensing controls. …”

Screening by coral green fluorescent protein (GFP)-like chromoproteins supports a role in photoprotection of zooxanthellae

“…Consequently, corals and their symbionts need mechanisms for acclimation and adaptation to different irradiances, chromaticity and temporal variability in the light field. Even at the scale of a single shallow-water colony, there are large variations in light exposure of the coral's tissues controlled by factors such as seasonal and diel changes in surface irradiance, changes in the absorbing and scattering materials in the water column, orientation of the tissue surface relative to the incident irradiance and wave-focusing effects (Falkowski et al 1990;Veal et al 2010;Kaniewska et al 2011). The shallow-water environments where reefbuilding corals thrive are typified by irradiances often greater than photon fluxes of *300-750 lmol m -2 s -1 that saturate photosynthesis of the algal symbionts in hospite (Gorbunov et al 2001;Levy et al 2004;Hennige et al 2008).…”