GREAT BARRIER REEF
// Outlook Report 2014
overexploitation.159 There is limited information about the status of spawning aggregations in the Region. However, declines in some fish species (see Section 2.4.8) and the changes in fish abundance between zones open and closed to fishing139,160,161,162 indicates that recruitment is likely to have been affected in areas open to fishing. Broadscale losses of seagrass meadows are likely to have affected both the recruitment of seagrass, and a range of other species that rely on the habitat as nursery grounds163,164 or for food — for example the availability of seagrass is a key factor in the reproductive rate and successful recruitment of dugongs and green turtles (see Chapter 2).165 For slow-breeding species that are in low abundance, such as dugongs166,167, recruitment of juveniles into the adult population is a key part of their recovery. For green and loggerhead turtles there is reduced recruitment of juveniles into the foraging stock168 and reduced recruitment rates of first-time nesting females into the nesting stock.169,170 Deteriorating recruitment has been evident in some seabird populations. Some years have seen almost complete reproductive failure of the wedge-tailed shearwaters in the Capricorn–Bunker group of islands. This is likely due to a decreased growth rate of chicks as a result of a reduced ability for adults to supply food, linked to higher than normal sea surface temperatures.171 This directly affects the level of recruitment of juveniles into the adult population.
3.4.8 Reef building
Only a small proportion of a coral reef is living coral — the remainder is coral-based pavement, boulders, fragments, beach-rock accretions and sediment.172 Reef building is the net result of processes that form calcium carbonate (calcification) and the physical, chemical or biological erosion that removes it. The formation of calcium carbonate skeletons by living coral is the primary source of calcification, however corals are only one of a number of groups that contribute to reef construction.172 Others include molluscs, crustaceans, foraminifera and red and green algae.172 Many of the organisms that calcify at high rates benefit from photosynthesis by symbiotic algae (Section 3.4.6).173 The rate of deposition of calcium carbonate is dependent on light (Section 3.2.7), temperature (Section 3.2.6) and the availability of carbonate ions in the water column.174 Increasing sea temperature and ocean acidification are likely to be contributing to reduced calcification rates of corals throughout the Region.101,175 Skeletal records of massive corals from the inshore Great Barrier Reef indicate that between 1990 and 2005 there was an 11 per cent decline in calcification.101,176 This is the fastest and most severe decline in at least 400 years.101 There is no information on more recent trends. The impact of future changes in temperature and ocean acidification on the process of calcification is uncertain. Decreasing ocean pH has an increasing negative effect on the calcification process and thus progressively slows the process of reef building.101,174,177 However, the impact varies between coral species as well as between organisms.178 The predicted concurrent warming of the oceans speeds up the calcification process — potentially counteracting to some extent the negative effects of decreasing ocean pH at some reefs.179 In addition, ocean chemistry fluctuates greatly at small scales across a reef, and corals are capable of modifying their seawater carbon chemistry, thus potentially negating some of the possible large-scale impacts of climate change on this reef building.180 The contribution of coral to the reef building process is likely to be higher in the northern areas of the Region as coral cover remains relatively high14. The reduced amount of living coral14 in the southern twothirds of the Region14 is likely to have affected its contribution to reef building processes.