2. Biodiversity

Marine benthic algae are a diverse group of photosynthetic organisms incorporating several evolutionary lineages that diverged deep in Earth’s history. Benthic algae include multicellular macroalgae and single-celled microalgae. The macroalgae are highly diverse and occupy most of the Region’s habitats. There are 3 main types, green, brown and red, and a range of forms, from encrusting species to those making up algal turfs, to large fleshy species (seaweeds).²⁶⁴ More than 600 species of macroalgae have been found on the Reef, based on limited collections; that figure represents around 30 per cent of the species in Australia.²⁶⁴ Genetic studies have revealed this number may be considerably higher.²⁶⁵ 

Benthic algae perform a range of ecological roles, including contributing a large proportion of the primary production on reefs and in other habitats (Section 3.4.3).²⁶⁴ Algal turfs stabilise sediment, produce oxygen, cycle nutrients, and provide food to grazing herbivores. Filamentous cyanobacteria (blue-green algae) living in intensely grazed algal turf communities fix atmospheric nitrogen, contributing to the supply of this key nutrient (Section 3.3.1).²⁶⁶ Crustose calcareous algae contribute to framework cementation, accumulating calcium carbonate and helping bind substrate together into durable surfaces suitable for juvenile corals to settle on.^267,268 The green calcareous alga Halimeda and other upright calcareous algae are important producers of carbonate sediment on reef flats and elsewhere (Section 2.3.8).¹⁰⁰ Canopy-forming macroalgae contribute structural complexity and other key functions across the seascape, roles that may become increasingly important in a changing climate (Section 6.3.2).²⁶⁹ Nevertheless, how different algal groups will respond to a changing climate remain major knowledge gaps.²⁷⁰

Excessive growth of turf and fleshy macroalgae can occur when there is a shift in the balance of processes affecting algal biomass — for example, a reduction in herbivory or increased availability of limiting nutrients (Section 6.5.2). Once established, some macroalgal species, especially those that grow densely across a range of habitats and depths, can inhibit the recovery of corals following disturbance by suppressing recruitment.^271,272 Palaeoecological records show long-term fluctuations in dominance between macroalgae and hard coral of the genus Acropora in the inshore Reef.²⁷³

Variation in cover of macroalgae along natural spatial gradients, as well as their functional diversity, seasonality, and variable responses to physical and chemical drivers, precludes any simple interpretation of macroalgal cover as a metric of human disturbance,²⁷⁴ and links to availability of land-sourced nutrients are not straightforward.²⁷⁵ The risks of nutrient enrichment promoting macroalgal overgrowth of corals are greatest in areas where herbivores are naturally scarce, such as inshore reefs.²⁷⁶ Dominance of macroalgae is primarily limited to wave-exposed shallow inshore reef flats, crests and upper slopes where hard coral cover is often low.²⁷⁵ Reef-wide monitoring from 1995 to 2021 shows that cover of upright fleshy and calcareous macroalgae increases steeply towards the coast, with increasing latitude and towards shallow depths (less than 3 metres).^42,275 However, no clear trends in macroalgal percentage cover were detected over that time, particularly for mid- and outer-shelf reefs.²⁷⁵

There is no dedicated monitoring of benthic algal species on the Reef, and most observations are limited to percentage cover of major algal groups in shallow water as part of coral monitoring programs. On inshore reefs overall, the relative proportion of large fleshy macroalgae has increased or remained stable over the past 5 years in most regions, although variation exists between individual reefs.¹⁹⁰ This increase was likely in response to major events, such as cyclone Debbie in the Mackay Whitsunday region. However, local variability is high in the Wet Tropics and Fitzroy regions, where average percentage of macroalgae can reach up to 35 per cent in some locations.¹⁸⁵ In offshore areas, macroalgal cover is generally low (less than 5 per cent) but with fluctuations of around 2 to 3 per cent in the Fitzroy region.²⁷⁷ Potential changes in macroalgal cover in response to recent 2023–24 cyclones have not been assessed.

Turf algae is a consortium of microalgae and macroalgae, including cyanobacteria, which dominates benthic communities on many reefs worldwide.²⁷⁸ In the Region, turf algae are one of the most common benthic algae in assemblages in shallow (less than 5 metres) areas of the Reef, and this has been relatively stable over the past 5 years.²⁷⁷ Turf algae cover ranges from around 50 to 65 per cent in the inshore and mid-shelf reefs of Cape York, but drops to around 25 per cent in outer-shelf reefs.²⁷⁷ In other regions, the distribution pattern is similar between inshore and offshore reefs at around 25 to 50 per cent, except at some sites where turf algae reach average cover of up to 70 per cent.²⁷⁷

Crustose coralline algae (CCA) are calcifying red macroalgae important for reef building ²⁷⁹ (Section 3.4.8). Certain species induce metamorphosis and settlement of corals, while others can inhibit coral settlement.²⁸⁰ Some species support recruitment of crown-of-thorns starfish.²⁸¹ Cover of CCA has remained stable over the past 5 years across the Region, usually ranging from one to 40 per cent.²⁷⁷ Highest cover (up to 50 per cent of total benthic algae) occurs in some of the offshore Ribbon Reefs in Cape York.²⁷⁷ Recently, 3 new species of CCA belonging to the genus Porolithon have been described in the Region.²⁸² The increasing prominence of a group of rapidly spreading encrusting algae known as peyssonnelid algal crusts is an emerging issue reported from the Caribbean and wider Indo-Pacific.²⁸³ It is unclear if this is occurring in the Region. 

Among the benthic microalgae, Symbiodiniaceae is the most common group to establish symbiotic relationships with corals (as zooxanthellae) and other organisms like benthic foraminifera, giant clams and sea anemones.^284,285,286 Macroalgal surfaces support high abundances of these organisms, suggesting a key role for macroalgae in ensuring the ecological success of corals, particularly after bleaching.²⁸⁷ While no monitoring program currently exists to assess diversity of benthic microalgae, several studies suggest that benthic populations of free-living coral symbionts have shifted to more thermally tolerant variants following the 2016 mass coral bleaching.²⁸⁸

Benthic algal cover in shallow reef areas has been relatively stable over the past 5 years. Significant variation exists across this diverse group. Future ocean warming and acidification may adversely affect coralline algae while providing increased opportunities for turf algae and fleshy macroalgae. Populations of microalgae important for coral symbioses may be shifting to more thermally tolerant variants.