4304
INTRODUCTION
Shallow-water marine ecosystems are one of the most biologically
productive regions on Earth (Ackelson, 2003), and light is the
predominant resource for which phototrophic organisms compete
by building complex three-dimensional structures, ultimately
designed for light acquisition. The spatial complexity resulting from
the structure of aquatic canopies, coupled with environmental
variability, causes significant seasonal, diurnal and spatial variability
in downwelling irradiance levels (Veal et al., 2009).
Downwelling irradiance is the most variable physical parameter
in the oceans of the world. It varies in intensity on the scales of
years to milliseconds (Stramski and Legendre, 1992). Over fractions
of seconds, fluctuations in downwelling irradiance are driven by
alternating focusing and defocusing of bundles of refracted sun rays
as they cross the curved surfaces of waves, reaching down to depths
of 35 m (Stramski and Legendre, 1992; Stramska and Dickey, 1998).
Convex sections of waves act as a converging lens, focusing the
light at various depths, dependent on the slope of the wave and the
incidence angle of the light (Kirk, 1994). These focused light fields,
also known as ‘underwater light flashes’ or light caustics, are defined
as a pulse of underwater downwelling irradiance that is at least 150%
of the average downwelling irradiance at that depth (Stramski, 1986).
The lensed component of light comprises solely unscattered or
forward-scatted radiation, with the intensity of the flash being
constant under varying sky conditions, provided that this directional
component of surface irradiance is >50% (Stramski, 1986; Stramski
and Dera, 1988). The flash duration lasts from 10 to 300 ms, with
intensity varying up to six times the mean downwelling irradiance
under ideal conditions in the visible spectrum (Stramski and Dera,
1988). Wave-lensing conditions occur under light winds (2–7 m s
–1
),
when small capillary or gravitational waves of up to 20 cm in
amplitude form sheets of waves extending in a direction
perpendicular to that of the wind (Weidemann et al., 1990; Stramski
and Legendre, 1992; Cepic, 2008). Waves occur along the phase
boundary of an aqueous medium where fluid dynamics are controlled
by surface tension. Under these wave conditions, it might be possible
to produce >350 light flashes every minute at a depth of 1 m
(Weidemann et al., 1990), with a single flash capable of exceeding
an intensity of 9000 mol quanta m
–2
s
–1
in extremely shallow waters
(Shubert et al., 2001).
Ecophysiological studies involving the effects of irradiance on
organisms cannot correctly characterise the response of an organism
without an accurate assessment of the irradiance climate (Shubert
et al., 2001). Despite extensive documentation of the optical
properties of shallow waters (Jerlov, 1976; Stramska and Dickey,
1998; Kirk, 1994) and knowledge of wave lensing for the past 50
years (Schenck, 1957), our understanding of organism function and
aquatic photosynthesis under fluctuating illumination, both
qualitatively (light spectrum) and quantitatively, is still very limited
(Rascher and Nedbal, 2002; Shubert et al., 2001).
It is well documented that the photosynthetic rate of an organism
depends not only on the amount of irradiance received but the
manner in which it is delivered (Stramska and Dickey, 1998). There
has been a handful of studies investigating the effect of flashing or
The Journal of Experimental Biology 213, 4304-4312
© 2010. Published by The Company of Biologists Ltd
doi:10.1242/jeb.044941
Shallow-water wave lensing in coral reefs: a physical and biological case study
Cameron James Veal
1,2,
*, Maya Carmi
2
, Gal Dishon
2
, Yoni Sharon
2
, Kelvin Michael
3
, Dan Tchernov
2,4,5
,
Ove Hoegh-Guldberg
1
and Maoz Fine
2,6
1
Global Change Institute, Coral Reef Ecosystem Laboratory, The University of Queensland, St Lucia, Brisbane, Queensland 4072,
Australia,
2
The Interuniversity Institute for Marine Science, Eilat 88103, Israel,
3
Institute for Marine and Antarctic Studies, University
of Tasmania, Sandy Bay, Tasmania 7001, Australia,
4
The Alexander Silberman Institute of Life Sciences, The Hebrew University of
Jerusalem, Jerusalem 91904, Israel,
5
Marine Biology Department, The Leon H. Charney School of Marine Sciences, University of
Haifa, Mount Carmel, Haifa 31905, Israel and
6
The Mina and Everard Goodman Faculty of Life Science, Bar-Ilan University, Ramat
Gan 52900, Israel
*Author for correspondence (c.veal@uq.edu.au)
Accepted 25 September 2010
SUMMARY
Wave 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.
Key words: coral, wave lensing, light.
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