Tonga - Ha'apai (TOHA23)

Setting

A lagoonal pinnacle reef (massive coral dominated) in Tonga's Ha'apai group (Living Oceans Foundation habitat map; see Notes). Anderson dived this station on 18 September 2013 during the Khaled bin Sultan Living Oceans Foundation's Global Reef Expedition to Tonga, where five Pocillopora acuta colonies were sampled for coral-health work (sampling sheet).

General benthic features

Acropora was the most common coral genus on this station's transects (24% of the benthos), followed by Montipora (2.2%), Isopora and the fire coral Millepora (1.5% each), Pocillopora (1.4%), and Stylophora (1.3%). Alongside the hard coral already reported for this site, the same transects recorded 22% crustose coralline algae, 13% turf, 3.3% macroalgae, 1.4% erect algae, and 8.2% cyanobacteria (48% algae in all). Non-living substrate came to 44% pavement, 13% dead coral, 3.4% rubble, and 2.4% sand.

Coral cover

• Reference: Khaled bin Sultan Living Oceans Foundation, Global Reef Expedition (in situ transect)
• The expedition's benthic transects recorded 37% hard coral cover (LOF-GRE in situ transects; mean of all of this station's own transects) — a diver transect measurement, not a photographic or computer vision estimate.
• Transect mean 37 ± 5.4% SE (n = 8 transects; Living Oceans Foundation per-transect benthic data); the headline above is this all-transect mean.

Stressors

The 2013 survey notes for this site record 12 crown-of-thorns starfish, with about two dozen patches of small corals they had recently eaten, and one Culcita cushion star (Khaled bin Sultan Living Oceans Foundation expedition site notes, 2013; regional summary in Bruckner 2014). Anderson's own sampling sheet for this dive records three of the five Pocillopora acuta colonies sampled here as severely bleached (more than 90%, about 100% in a colony Joao collected, and 90%); a fourth was healthy, and the fifth pale but possibly healthy. The Living Oceans Foundation transects here recorded 8.2% cyanobacteria and no recently dead coral.

Fish and other fauna

Paired fish transects recorded about 13 fish species per transect and about 3.1 kg per 100 m2 of biomass.

Comparisons with other data

No Reef Check station lies within 5 km of this station. The closest Reef Check site, Kito si'I, is 11.6 km away; its only survey date, 15 August 2002, recorded 18% hard coral (mean of 2 surveys, 3 m and 10 m; Reef Check data, a different reef, not a survey of this station).

Dive record

Anderson's dive: in at 14:52 for 45 minutes, to a maximum depth of 25.3 m on the dive computer (the team dive log gives 54 ft, 16.5 m, for the same dive), in calm, murky water with about 20 m visibility; the five colonies were sampled at 19.5-24 m between 15:00 and 15:25. The camera was left behind (photo folder: 'forgot to take camera'), so Anderson has no photographs from this dive; the sheet notes that Joao photographed the sampled colonies.

Other notes

Field terms: the survey team's field report opens 'Fringing reef'; Anderson's sampling sheet (LOF fellowship 2016 workbook) gives 'lagoonal', 'patch reef', with 'leeward' on some rows and 'windward' on others; the 933-site table lists a 'protected' patch reef inside the lagoon. The Living Oceans Foundation habitat map places the pin inside 'lagoonal pinnacle reefs - massive coral dominated', with a lagoonal fringing reef 146 m to the east; the reef type printed above is the map's, and no aspect is printed because the sheet rows disagree.

Survey team's site notes: Fringing reef. Landward side has mounds that slope gradually from just below the water’s surface to a sand flat at 10 m depth. A second reef system extends seaward, sloping up from the sand to 5-7 m, then dropping steeply to 40+ m. The back side of this reef has large stands of thick-branched staghorn mixed with branched Isopora, small table acroporids, and bottlebrush acropora. The slope is near vertical in hardground areas but this is intersected with sand/rubble channels. The top of the reef is dominated by dense stands of bottlebrush/short-branched staghorn acroporids intermixed with other corals. Within holes and crevices between the acroporids are small Galaxea, Pocillopora, Coscinaraea, Caulastraea, Astreopora, Leptoseris, and other corals as well as patches of false corals. The acroporids extend to about 15 m depth, being most dense at the top of the reef and dropping off to 5.0-10% cover by 15 m; these corals also have considerable old mortality on the slope, less so on the top of the reef. Below this is a band of branching Millepora from 15-20 m, then lower coral cover (5.0-10%) dominated by plating Montipora, Echinophyllia, Echinopora, Goniastrea, Pachyseris, submassive Porites, Stylophora, small table acroporids. Deep areas have few leather corals but higher numbers of gorgonian sea fans. There are many areas on the reef slope with dead or partially dead staghorn and table acroporids with dense turf algae and interspersed patches of cyanobacteria. Fused rubble on the slope has good cover of CCA but this is covered in cyanobacteria, some macroalgae (Tydemania), and turf in places. The first reef with Lobophora brown algae. The reef had a number of larger giant clams.