Subtidal erect calcareous algae

Long description

Subtidal erect calcareous macroalgae (i.e. those with an erect growth form and containing calcified segments[2]), including the genera Halimeda, the green Udotea and the red Amphiroa. Erect calcareous algae occurs on coral reefs on the reef flat and in the lagoon, and in shallow and deep nearshore areas, on unconsolidated substrates.

Macroalgae are marine plants that photosynthesise in all cells, and have a simple holdfast to anchor onto substrates such as rocks, shells or gravel. Halimeda spp. has developed special anchoring filaments (rhizoids), which bind sediment and sand into a lump that enables them to anchor onto unconsolidated substrates. Rather than having leaves like seagrasses, erect calcareous macroalgae have a blade-like growth form which is divided into segments. The calcified segments can form branching growths that form thickets which create benthic structure[1].

Special values

This algal type and type 58 (subtidal encrusting algae) contribute to calcium deposition on the sea floor. After death, eroded fragments add to the calcium content of unconsolidated sediments such as white sandy beaches and reef lagoons. The calcium carbonate is deposited as aragonite, calcite and high magnesium calcite in the algal tissues[1]. The Halimeda segments accumulate in bank-like, mound-like or reticular structures (up to 20 metres thick) over thousands of years[3]. Unique deep water banks of Halimeda spp. exist as very large, ancient structural ecosystems on the northern Great Barrier Reef, known as bioherms, where they are exposed to tidal currents and upwellings that transport nutrient-rich water over the continental shelf. These bioherms have ecosystem value in terms of biodiversity and nursery refugia and preserve high resolution records of palaeoclimate and palaeoceanography[3].

Diagnostic attributes

Inundation 'Subtidal'

Structural macrobiota 'Algae – Erect calcareous'

Qualifiers

The Naturalness qualifier may be relevant as Halimeda beds are vulnerable to trawling and dredging.

Distribution

Halimeda bioherms occur globally from Indonesia, to the Timor Sea and the Caribbean. On the Great Barrier Reef they occur between depths of 25 and 50 metres, often occupying hundreds, even thousands of square kilometres (e.g. off Cape York and in the Ribbon Reefs region, Lizard Island region, Cooktown region, and the Swains).

The following relates to distribution of this ecosystem type within the Central Queensland mapping area:

• Small areas of Halimeda spp. are found in the mapping area where algal inventory has occurred around Port Curtis, but areas comparable to those of the northern Great Barrier Reef have not been found. In the Port Curtis area, erect macrophytic algae co-occurred with erect calcareous algae.
• In Port Curtis, consolidated substrate, rubble and ‘live’ and dead rock formed attachment substrates for macroalgal ecosystems including both erect macrophytes and erect calcareous algae[2].

Comments

Other relevant attributes include Consolidation and Sediment texture. In Port Curtis, consolidated substrate, rubble and ‘live’ and dead rock formed attachment substrates for macro-algal ecosystems including both erect macrophytes and erect calcareous algae[2].

The massive Halimeda bioherms have distinct and complex Terrain morphology (mounds, reticulate shapes, ridges) which can be picked up on radar and Light Detection and Ranging (LiDAR), and are generally next to deep sea depressions[3]. Upwelling of cool, nutrient rich water from below the Coral Sea thermocline delivers nutrients to enable the bioherms to grow. Therefore, the relevant attributes are Energy source (Current – Upwelling) and Water temperature.

Additional Information

A Massive Reef Was Just Discovered Hiding Behind The Great Barrier Reef - ScienceAlert

Saltmarshes, seagrasses and algae - Queensland Department of Primary Industries

Algae and algae blooms - Queensland Government

Harmful algae - Queensland Government

Algae - WetlandInfo

References

1. ^ a b Diaz-Pulido, G & McCook, L (2008), State of the Great Barrier Reef report.. [online], Great Barrier Reef Marine Park Authority, Townsville, Qld.. Available at: http://www.gbrmpa.gov.au/__data/assets/pdf_file/0019/3970/SORR_Macroalgae.pdf.
2. ^ a b c McKenna SA, Bryant CV, Tol SJ & Rasheed MA (2013), 'Baseline Assessment of Benthic Communities (algae and macro‐invertebrates) in the Port Curtis Region. November 2013', JCU Publication, Centre for Tropical Water & Aquatic Ecosystem Research (TropWATER) Publication, Cairns, 26pp, vol. 14/54, JCU Publication, Centre for Tropical Water & Aquatic Ecosystem Research (TropWATER) Publication, Cairns.
3. ^ a b c McNeil, MA, Webster, JM, Beaman, RJ & Graham, TL (December 2016), 'New constraints on the spatial distribution and morphology of the Halimeda bioherms of the Great Barrier Reef, Australia', Coral Reefs. [online], vol. 35, no. 4, pp. 1343-1355. Available at: http://link.springer.com/10.1007/s00338-016-1492-2 [Accessed 15 March 2019].

Last updated: 11 July 2019