3.2.7 Light

Light is an important environmental variable in shallow marine environments. It plays a critical role in determining the depth range, growth rate, physiology and distribution of photosynthetic species, such as seagrasses, algal symbionts of corals and phytoplankton. 

Shallow inshore waters are generally more turbid (have more suspended particles which reflect and scatter light) than deeper offshore areas, which makes them subject to lower and more variable light conditions.672 Increased sediment and nutrient loads contribute to higher turbidity and prolonged reductions in light availability along certain sections of the inner and middle shelves of the Reef lagoon.586 Although larger sediments tend to settle in the vicinity of river mouths (Section 3.2.4), fine particles that remain in the water column can greatly attenuate light reaching the seafloor.586 Artificial light is discussed in Section 6.4.

Patterns in the availability and intensity of benthic light affect the maximum depths that coral can grow,673 which can be as shallow as 5 metres in inshore areas and exceed 50 metres in clear offshore waters.674 Some photosymbiotic corals even occur at depths below 100 metres.675,676 Although different corals survive under a wide range of light regimes across the Region, and inshore corals naturally experience periods of low light,677 significant reductions in light can lead to coral bleaching, decreases in growth and lipid content,677,678,679,680 and diminished recruitment success 681 and juvenile survival.680 Although light is important for maintaining productivity in corals,682 high levels of light can aggravate the effects of thermal stress, such as bleaching.683 While reduced light (for example, due to cloudy skies) appears to reduce these impacts,683 extremely low levels of light also exacerbate heat stress, highlighting the different ways light stress can affect corals.684 Potential acclimatisation to changes in light conditions is likely to be species-specific.685,686

Seagrass growth and abundance are also highly correlated with availability of benthic light. In combination with other environmental and regional variables, benthic light has been used to help define seagrass habitat assessment zones 687 and to predict seagrass distribution and community types within the Region.688 Seagrass species vary in their sensitivities to light limitation,689 and various physiological and morphological effects are observed within days to months of low light conditions.690,691 Deepwater seagrasses living near their functional limits may be particularly susceptible to decreasing light conditions.692

Seagrass growth and abundance are highly correlated with availability of benthic light

At inshore reefs within the Region, water clarity (measured by Secchi depth) has generally remained stable or improved from 2017 to 2023, although it still does not meet annual guideline values at most locations.579 Satellite remote sensing data show a clear spatial pattern in the optical properties of reef waters: inshore areas are more frequently exposed to sediment and phytoplankton-enriched water (which cause light attenuation) during the wet season, while mid-shelf and offshore reefs typically experience water qualities that facilitate high-light conditions.363 Analysis of satellite imagery (2002 to 2019) and information on optimum light thresholds for sensitive species suggest that light limitation is experienced by corals and seagrasses at several nearshore locations across the Region, and this varies significantly between years and seasons 597 (Figure 3.10). A broad pattern of increasing nearshore light stress from north to south in the Region may be the result of sediments transported northwards along the coast from rivers in the south.597 While offshore reefs typically experience less variable light conditions, resuspension of benthic sediments during strong winds can result in light limitation for sensitive species in some locations.597

The availability of light at the seabed is below optimum levels in several nearshore locations across the Region, although light levels across most inshore areas have generally remained stable or improved over the past 5 years.

Figure 3.10
Benthic photosynthetic available radiation, daily average, 2002 to 2019

Benthic photosynthetic available radiation (bPAR) is the amount of light available for photosynthesis reaching the sea floor. Sufficient light is required for corals, algae and seagrasses to thrive, and poor water quality can restrict the amount available (asterisk on scale indicates the long-term minimum light requirement for seagrasses).693 bPAR data from remote sensing has been used in developing a benthic light index for use as a water quality indicator.597 Source: eReefs and Magno-Canto et al.(2019).694,695

Map of the Great Barrier Reef Region and World Heritage boundary showing amount of light available for photosynthesis that reaches the sea floor.
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