, , , , and Linking reef ecology to island building: Parrotfish identified as major producers of island-building sediment in the MaldivesGeology, June 2015, v. 43, p. 503–506, first published on April 27, 2015, doi:10.1130/G36623.1
Islands are vulnerable to nature’s forces. They can be small and are constantly affected by variable ocean conditions. They get exposed to seawater, waves, winds, currents, tides, storms, and are threated by rising sea level. Furthermore, islands are influenced by their unique surroundings, making it difficult to assume all islands will be affected the same under similar conditions.
Reef islands, for example, have a special and dependent relationship on the local biology. Reef islands are made of sediment from coral reefs in the vicinity that is supplied by fish. A group of scientists were trying to better understand and quantify this relationship. They focus their research on the production, transport, and deposition of the sediment between the reef and reef islands. The specifically wanted to determine which processes were necessary to supply sediment for island building. Researchers were curious about where the fish hang out to generate the most sediment, how many and what species are doing all the work, how quickly they generate sediment, and what size fraction sediment is generated.
Scientists investigated their questions in a reef lagoon habitat called the Maldives Archipelago (figure 1). The area has 22 atolls, which are circular reefs surrounding a lagoon and sometimes a small island, and around 1,200 reef islands. The research area was on Vakkaru Island, interior of the Northern Maalhosmadulu Atoll. The island occupies 42% of the platform covering an area of .19 km2. Census and sedimentation data from Vakkaru Island indicate surficial content as medium grained sand, coral, and Halimeda (a macroaglea).
Modified Reef Budget methodology was used to determine community composition, minor changes in surface elevation, and the abundance of sediment generators. Rates of new carbonate sediment generation and relative proportions of sediment size fractions produced by different species were estimated from published data. Reef and island sediment composition was determined via thin sections. More information on the techniques used in this research are available from the GSA data repository- follow the doi link to the paper for more details on access. A profile of the area and information on sediment composition and grain size distribution are provided in figure 2.
Researchers witnessed a diverse world around Vakkaru island. On the outer flat they noted little sediment and abundant Halimeda (90.4 thalli/m2), gastropods, and parrotfish (.02 fish/m2). For parrotfish they observed both excavator species: Chlorurus sordidus and Chlorurus strongylocephalus, and scraper species Scarus niger, Scarus frenatus, and Scarus rubroviolaceous. On the inner reef they noted live coral outcrops (19.7 +/- 8%) with sandy bottom between them. The interior lagoon and sand moat they noted as roughly 95% sands with no live coral, parrotfish, or sediment producing biota observed.
On the inner and outer reef just under half of new sediment is fine to medium sand (125-500 microns) and is primary coral (~50%). The silt size fraction (<63 microns) comprises 22% of the new sediment, 10% of the new sediment is gravel (2-4mm); it is all poorly sorted. Towards the lagoon/moat the sediment is less poorly sorted and there is a decrease in coral to 41.7% coupled with an increase in Halimeda to 35.9% Surficial sediment on the island is moderately well sorted medium to fine sands with 58.5% coral.
Researchers determined that 380 m2 (or 685,000 kg) of sediment is produced annually. Three quarters is generated on the narrow outer reef flat. Parrotfish contribute 5.7 kg/m2 of sand sized sediment, accounting for a bit over 85% of that produced annually. Halimeda, contribute roughly 10% of material, mostly gravel sized. Estimates of sediment production rates are listed in table 1 and a summary of grain sizes on the inner and outer reefs by species are listed in table 2.
The outer reef is where there is the most active sediment generation annually with 5. 71 kg of calcium carbonate per square meter. Corals contribute primarily to the coarse to fine sand fractions while and Halimeda to the coarse sand and gravel fractions. The observation of intact Halimeda plates in the lagoon area is suggestive of rapid transport over the reef flats, agreeing with previous research related to sediment transport modeling. The high abundance of coral sediment on the reef island and the flats suggests it is robust and suggests high sediment supply. Bioerosion by parrotfish is the reason for the sediment supply. Parrotfish produce the sediment as a byproduct of grazing— they are just messy eaters; they build mountains with their hunger. Not all of their mess contributes to the island though, some settles out before reaching the island and some is carried offshore (figure 3).
A correlation between sedimentation of the island and estimated sediment production from the reef suggest a link between island building and reef ecology. The point is, the sustainability and formation of the islands is dependent on the reef ecology on a fairly small section of the reef. There is a need for healthy parrotfish to maintain the reef islands.