The availability of light is central to the health and productivity of seagrasses and other plants as well as the symbiotic relationship between some animals (for example corals and clams) and algae. Levels of available light control the depth range of marine plants (for example, seagrasses83 and algae) as well as animals which rely on photosynthesis through symbiosis with plants.84 The rate at which light decreases in the water column is determined by both depth and water turbidity.85 As a result, light becomes limiting at shallower depths in inshore, more turbid areas compared to offshore habitats which have less turbid water. Turbidity is affected by a number of external factors, such as sediment becoming resuspended by wind21, currents and tides86; nutrients from land-based run-off85; as well as activities within the Region such as anchoring87, vessel wash87, dredging and the resuspension of dredge material88. Nutrients from land-based run-off can increase the growth of phytoplankton resulting in a decrease in the ambient light levels.85 Extended periods of cloud cover also reduce light availability for the ecosystem.33 Turbidity is very variable from year to year (Figure 3.7) and week to week (Figure 3.8). In recent years turbidity is likely to have increased due to extreme flooding and the resuspension of sediment associated with storms and cyclones.89 A comparison of secchi disc readings from the 1928–29 British Museum Expedition to Low Isles with more recent readings from nearby sites offshore from Cairns suggest a 50 per cent decline in mean water clarity,90 although there was less data in the 1928–29 sample. Land-based run-off strongly affects light availability, not only in inshore areas but can extend up to 80 kilometres from the coast.91 Given the increased input of sediments since European settlement (Section 3.2.4), it can be assumed that light availability has decreased substantially in inshore areas in the southern two-thirds of the Region.85
It is likely that light availability has decreased substantially in the inshore areas of the southern two-thirds of the Region.
Figure 3.7 regional trends in turbidity of inshore areas, 2008–2013
The solid line curves represent regional trends bounded by dashed lines depicting 95 per cent confidence intervals. (Data to October 2013). Data presented in graphs relate to turbidity levels in inshore areas of mapped regions above. Source: Thompson et
Figure 3.8 Water clarity at two tourism sites, 2007–2013
Clear water is a major motivation for people to visit the Reef. Secchi disc depth data collected voluntarily by tourism operators as part of the Great Barrier Reef Marine Park Authority’s Eye on the Reef program from Moore Reef near Cairns (two operators), and Hardy Reef in the Whitsundays (one operator) provides an indicator of water clarity. The horizontal lines indicate the mean annual water quality trigger level for water clarity relevant to that site, based on the Water Quality Guidelines for the Great Barrier Reef Marine Park.93 Source: Great Barrier Reef Marine Park Authority 201494