3. Ecosystem health

Nutrients are fundamental for the growth and survival of organisms and are a vital component of ecosystem function.⁶⁹⁶ Nutrients transfer from the physical environment to organisms through food chains, and back to the environment via decay or death.³¹⁷ In marine ecosystems, the major nutrient cycles are carbon, nitrogen and phosphorus.^697,698 

Carbon cycling includes carbon transformation and use by photosynthesising primary producers (Section 3.4.3), microbes,^698,699 and calcifying corals ⁷⁰⁰ and algae such as Halimeda.²³⁴ Primary producers take up dissolved forms of nitrogen and phosphorus (phosphate), while animals gain these nutrients by consuming other organisms. These nutrients are also found in particulate forms, often attached to fine sediments, that are not immediately available for biological uptake but can be mobilised by remineralisation.⁷⁰¹

Nutrient cycling occurs on widely varying spatial and temporal scales.^702,703 Nutrients are delivered to the Reef lagoon from the Catchment via land-based runoff and groundwater ⁷⁰⁴ (Section 6.5), through upwelling (which brings nutrients in from the deep ocean), via the air from dust ^705,706 and fires, and through nitrogen fixation (Figure 3.11). Seabird-derived nutrient inputs are also important for island reefs.^74,707 Nutrients are exported from the Reef lagoon through burial ⁷⁰⁸ and denitrification.^702,709 

Land-derived nutrient influx to the Region largely occurs during the wet season. Most dissolved nutrients are quickly taken up biologically or bound chemically to particles in the ocean. In years of large river flow, land-derived nutrients can be transported vast distances. Uptake of dissolved nutrients creates a pool of nutrients held within the water column in the form of phytoplankton and other types of suspended particulate matter.^702,711

Some studies show that the frequent resuspension of sediments within estuaries and the coastal zone helps promote rapid cycling of particulate, largely mineralised nutrients by microbial communities.^710,712 Catchment wildfires are known to influence the transport of nutrients to the marine environment, through effects on runoff water quality (Section 6.3.2) and the atmospheric transport of limiting nutrients and trace metals.⁷¹³ Deposition of fire-derived nutrients via rainfall has the potential to cause phytoplankton blooms,⁷¹⁴ as has been recorded in recent years in southern Australia.^713,715 The extent to which increasing fire frequency may be influencing nutrient cycling in the Region is a knowledge gap.

Upwelling is a key source of nutrients in the southern Great Barrier Reef, for example, in the Swain Reefs and Capricorn Channel.⁷¹⁶ In the north, the narrow gaps between outer barrier reefs funnel nutrient-rich water into fast-flowing currents that promote the growth of Halimeda bioherms (Section 2.3.8). There is no systematic in situ monitoring of nutrient concentrations in offshore areas, and current understanding of the spatial extent of upwelling influence relies on modelled data (Figure 3.11).

Nutrient limitation constrains growth and productivity.⁶⁹⁷ Nutrient concentrations are naturally low in the open ocean, and nitrogen is often the main limiting nutrient.^697,717 Scarcity of nitrogen can curtail the growth potential of organisms, making the nitrogen cycle central to the productivity of marine ecosystems ^718,719 Phosphorus, iron,^720,721 sulfate,⁷²² vitamins, and various micronutrients can also limit or co-limit particular organisms.⁶⁹⁷ Changes in the relative availability of different nutrients can exacerbate impacts of nutrient limitation.

Some marine microbes, notably cyanobacteria in the genus Trichodesmium, can fix gaseous nitrogen directly from the atmosphere.⁷²³ Trichodesmium is a major contributor of nitrogen to marine ecosystems ^724,725 and is increasing in abundance on the Reef in response to increasing sea temperatures and phosphate availability.³⁵⁶  The extent to which the increase has been driven by increased anthropogenic inputs of phosphorus and other nutrients is not yet known.^356,724

Nitrogen fixation occurs on coral reefs, by cyanobacterial mats, bacteria and reef-associated seagrasses, and in other habitats, such as seagrass meadows.^726,727 Coral reefs thrive under low-nutrient conditions ^266,728,729 by fixing nitrogen from the atmosphere and efficient recycling of nitrogen and carbon.³¹⁷ The productivity of zooxanthellae is highly dependent on available nitrogen, so efficient nitrogen cycling within the coral holobiont (Section 3.4.6) may be crucial for sustaining primary productivity in coral ecosystems.⁷³⁰ Elevated temperatures stress corals by limiting the efficiency of their internal nutrient cycling.⁷³¹ The presence of excess nutrients in coral ecosystems can favour competitors like macroalgae over corals.⁷³²

On a regional to global scale, coral reefs are important hotspots of marine sulfate emissions, which play a role in cloud formation and affects regional climates.⁷²² These processes could be disrupted under future climate change ⁷²² (Section 6.3.2).

Compared to historical conditions, the inshore Reef has a chronically elevated nutrient status.^276,710 Additional information on nutrient loads entering the Region since 2019 is provided in Section 6.5.1. Elevated nutrient availability is correlated with macroalgal abundance, low coral cover and low coral recruitment.²⁷⁶ Changes in nutrient ratios, particularly increases in nitrogen relative to phosphorus, can also exacerbate coral stress and decline.^733,734 Phytoplankton productivity, aided by nutrient availability in the inshore and mid shelf, is linked to improved survival of crown-of-thorns starfish larvae (Section 3.6.2).^276,735 High nutrient levels contribute to reduced light availability for seagrasses and corals through the proliferation of phytoplankton in the water column, and may increase the susceptibility of corals to disease ²⁷⁶ (Section 6.5.2). 

Microbial communities (bacteria) are promising candidates for potential indicators of environmental nutrient status.⁷¹² Recognition of the centrality of microbes in nutrient cycling has led to important advances over the past 5 years in analytical capabilities, including genomics and ‘big data’. This has led to the discovery of new microbial functional groups and emerging conceptual perspectives on their role in nutrient cycling.⁷¹⁹ 

The available suite of nested physical and biogeochemical models for the Reef and Catchment are instrumental in integrating knowledge of nutrient cycling and other key processes with in situ observations from field research and monitoring.^{248,703,736,737} Notably, they have been applied to investigate the impact of rivers on marine water quality ⁷³⁸  and to establish targets for land-based runoff that are more ecologically relevant (Section 6.5.1). 

Although substantial and ongoing changes to nutrient cycling since European settlement continue to affect inshore habitats, this process is understood to have been generally stable since 2019. There have been important advances in our understanding of nutrient cycling and in its integration into decision-making through improved models since 2019.