**1.2.1 Canopy**

In many forests with closed canopies, only a small fraction (0.5-5%) of the solar radiation incident above the canopy reaches the understory. Understory plants in these forests experience a highly dynamic light environment, with brief, often unpredictable periods of direct solar irradiance (sunflecks), punctuating the dim and diffuse background irradiance.

The Enhancement of Photosynthesis by Fluctuating Light 117

In natural aquatic systems, there are several factors that cause variations in irradiance. Irregular variations are caused by surface wave movement (Walsh & Legendre, 1983), cloud cover (Marra & Heinemann, 1982), and the vertical movement of phytoplankton (Falkowski

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. 1a). Because flicker light potentially produces excessively strong light as well as dimmer light, such fluctuations may have profound effects on the photosynthesis of benthic algae, seagrasses, and zooxanthellate corals. We will limit our discussion to shallow-water sessile organisms that are particularly prone to be influenced by a recurrent light provision anomaly such as these flickers. The effect diminishes with depth due to the shape- and hence the focal length of waves and

Waves act as lenses because of the differences in refractive indices between air and water, focusing light below the wave for a brief period. In shallow water, this effect can be seen clearly by eye (Fig. 1), appearing against a dark background as flickering bands of focused light. The location of the focusing events depends upon the shape of the waves. Large, rounded waves focus into deeper regions than small, sharply curved ripple waves (Kirk,

Fig. 1. Underwater light patterns on a shallow sandy bottom due to surface waves

**1.2.2 In the aquatic environment** 

scattering by particles (Fig. 1).

1994; Schubert et al., 2001) (Fig. 1).

& Wirick, 1981).

These sunflecks are combined with diurnal fluctuations in irradiance, ranging from sunflecks lasting only a few seconds or less in heavily shaded sites to cloud-induced fluctuations ("shadeflecks") lasting up to an hour or longer in open sites (Knapp & Smith, 1987). The nature of sunflecks, their size, shape, duration, and peak photon flux density depends on the height and precise arrangement of vegetation within the forest canopy as well as the position of the sun in the sky. The occurrence of a sunfleck at a particular location and time in the forest understory depends on different, often interacting, factors: the coincidence of the solar path with a canopy opening; the movement of clouds that obscure or reveal the sun; and the wind-induced movement of foliage and branches (in the canopy or in the understory plants themselves). These factors interact to yield a highly dynamic light environment in which the photon flux reaching leaves can increase or decrease over orders of magnitude in a matter of seconds. The effects of sunflecks on understory photosynthesis were studied in several works and revealed conflicting results (Leakey et al., 2002, 2005). In some cases, photosynthesis as CO2 assimilation by understory seedlings was enhanced, whereas at elevated temperatures (38ºC), it decreased.


Table 1. Some historical observations of fluctuating light on the water surface and underwater
