GREAT BARRIER REEF
// Outlook Report 2014
3.4.3 Primary production
Most food webs are based on primary production — the production of food by photosynthesis using energy from the sun. It is closely linked to concentrations of available inorganic nutrients.124 In tropical marine ecosystems such as the Great Barrier Reef, primary production is undertaken by plants such as macroalgae, turf algae, seagrasses and mangroves, and, in large part, by phytoplankton and symbiotic algae in corals and some other animals (such as giant clams). The presence of elevated levels of chlorophyll a, together with extensive phytoplankton blooms following the discharge of nutrient-rich flood waters, suggests open water (pelagic) primary production in inshore areas of the southern two-thirds of the Region is significantly affected by elevated nutrient loads.46,124,125 This in turn affects zooplankton populations, such as larvae of the crown-of-thorns starfish (Section 3.6.2).125 Certain primary producers, such as seagrasses, have declined in some areas, resulting in a loss of primary production, especially in central and southern areas.83,126 However, there is evidence of increased macroalgae at some reefs127, indicating a possible increase in primary production.
Some seafloor primary producers, such as seagrass have declined; others such as macroalgae may be more abundant.
Consuming plants for food (herbivory) is a key process for the health and resilience of tropical marine ecosystems, including coral reefs96,128,129. Herbivores have a particularly important role in maintaining reef ecosystems — without their constant presence, many reefs would be rapidly overtaken by algae that compete with corals for space to establish and grow.96,128 Fish are important herbivores in the coral reef habitats of the Great Barrier Reef.129 Studies on the Great Barrier Reef suggest that populations and diversity of herbivorous fishes continue to be sufficient to control algal growth on most offshore reefs128,130, in part because there is minimal direct pressure on their populations. Dugongs and green turtles are important herbivores in seagrass meadow habitats.131,132,133,134 Dugongs forage mainly on seagrass, and green turtles on seagrass and macroalgae. The dugong population has declined significantly in the southern two-thirds of the Region135 but remains stable for the area north of Cooktown (see Section 2.4.17). Populations of green turtle in the Region are still affected by legacy impacts of commercial harvesting (see Figure 2.1). The southern population is now increasing while the northern population is showing early signs of decline after previous significant increases (see Section 2.4.11). Population changes affect levels of herbivory in the Region which can in turn affect seagrass community structure and productivity.136 Levels of herbivory are likely to have been affected by recent broadscale losses in seagrass abundance (see Section 2.3.4).33
Declines in dugongs are likely to have affected herbivory in the Region.