Diaz-Pulido, 2008:

Diversity

"There are c. 630 species and varieties of macroalgae recorded for the GBR according to the Australian Marine Algal Name Index. However, this list is preliminary and it is likely that the number will increase with future field exploration. In fact, the macroalgal flora from the GBR,together with the northern Australian coast, is one of the lesser known floras on the Australian continent."

Functional groups

 

Spatial and temporal distribution 

"Macroalgal communities of the GBR are highly variable, showing latitudinal, cross shelf and within reef variation in composition and abundance. Many GBR macroalgae are also highly seasonal.

Cross shelf distributions – offshore and inshore reefs. Offshore reefs usually have low abundance of fleshy macroalgae and high cover of algal turfs and crustose calcareous algae (CCA) compared to inshore reefs. Some fleshy macroalgae, such as the green fleshy macroalgae Caulerpa, Chlorodesmis, Halimeda, and the reds Laurencia, Galaxaura and Liagora are common in offshore reefs but in low abundance. Crustose calcareous algae are abundant and diverse on offshore reefs and play significant roles in reef construction. Common taxa on offshore reefs include the CCA Porolithon onkodes, and species of Neogoniolithon and Lithophyllum. The cross shelf distribution of the algal functional groups is predominantly affected by fish grazing and water quality (nutrient availability and sedimentation).

Inshore reefs usually have abundant and conspicuous stands of fleshy macroalgae. In particular, brown macroalgae of the Order Fucales such as Sargassum, Hormophysa, Turbinaria and Cystoseira, form dense and highly productive beds of c. 2 m height. Other fleshy brown macroalgae such as Lobophora variegata, Dictyota, Colpomenia, Chnoospora and Padina and the red Asparagopsis taxiformis, may also be abundant in shallow inshore reefs. Lobophora variegata can be particularly abundant in inshore reefs, especially between branches of corals and after coral disturbance such as bleaching. Crustose calcareous algae are common but are not abundant.

Within reef distribution. Algal zonation is quite clear in rocky intertidal coasts but is normally diffuse in subtidal reefs, where algal communities are distributed as a continuum along environmental gradients (e.g. depth). A number of reef zones can be recognised in a cross-section of an offshore reef from shallow to deep areas (Table 15.3):

Intertidal and beach rock diverse fleshy macroalgal communities, reduced grazing by large animals, intense solar radiation

• Reef lagoon limited macroalgal growth due to sandy bottom; however, the microphytobenthos community that grows on sand can be highly productive
• Reef flat (back-reef ) diverse fleshy macroalgal communities, low grazing
• Reef crest abundant crustose coralline algae and algal turfs, intense grazing and wave action
• Reef front and upper reef slope abundance and diversity of macroalgae decreases with increasing depth, algal communities dominated by turfs and CCA, poorly developed fleshy macroalgal populations
• Walls low algal cover and high coral cover, some upright calcareous macroalgae like Halimeda can be locally abundant

Inter-reef areas and Halimeda beds. Macroalgae associated with seagrass beds, particularly in deep, softbottom areas of the northern GBR are quite abundant. In contrast to seagrasses, most algae do not attach to sand, although a number of green macroalgae have adapted to such environments by developing special anchoring features. This is the case for green algae such as Halimeda, Caulerpa and Udotea, commonly found intermixed with seagrasses. Macroalgae growing on leaves of seagrasses are called epiphytes and may play
important roles as food for invertebrates and vertebrates in seagrass meadows. Halimeda mounds grow in nutrient rich upwelling water in between reefs that make up the outer barrier reef (see Box 15.1)."

Seasonality

"The abundance, growth and reproduction of many GBR macroalgae are highly variable in time. Large seaweeds like Sargassum are strongly seasonal, with peaks in biomass and reproduction generally during the summer and lowest biomass during the winter. Extensive blooms of fleshy brown macroalgae like Chnoospora and Hydroclathrus are common on shallow reef flats during winter. Due to these strong seasonal changes, some authors argue that the seasonality of the GBR flora may be as strong as that from temperate zones."

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Citation and/or URL

Diaz-Pulido, G., 2008, Macroalgae, In The Great Barrier Reef: biology, environment and management, eds P.A., Hutchings, M.J., Kingsford and Hoegh-Guldberg, O., CSIRO Publishing, pp. 145-155. © ACRS 2008. http://www.publish.csiro.au/pid/5921.htm

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Spatial Coverage

Not applicable - a review chapter   

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Temporal Coverage

 Not applicable - a review chapter 

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Update Frequency

 Not applicable - a review chapter 

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Other Information

 None