Effects of turbid runoff on the coral communities of Gebe and Gag Islands, Eastern Indonesia

Full description

Surveys of the reefs around Gebe and Gag Islands were undertaken from 27th January to 7th February 1997, using SCUBA. Sites selected for surveys coincided as much as practicable with monitoring sites previously selected for placement of sediment tubes. Fifteen sites were surveyed on Gebe Island and 11 sites were surveyed on Gag Island. The position of each site was recorded using a portable GPS unit.Each survey took between 30 and 60 minutes to complete and covered distances of between 100m and 300m. The sites were not equivalent in size, rather they varied in respect to the amount of reef or coral community present and time available at a given location. Each site was classified in terms of the amount of reef development observed. To determine coral community composition, taxa were identified in situ to genus and species for most hard corals, genus for most soft corals and macro-algae and to higher taxonomic rank for sponges, zoanthids and other macro-benthos. It was impossible to differentiate all species of hard coral underwater and accurate identifications of certain taxa to species required genetic methods beyond the scope of this survey. Thus some hard corals were grouped at the level of genus and colony morphology (e.g. Porites spp. of massive growth-form). At the end of each survey, each taxon was ranked subjectively in terms of its relative abundance in the area surveyed. The abundance ranks do not relate directly to benthic cover, rather they represent the relative numbers of taxa in the sites. For each species, its size frequency distribution was estimated as the relative proportion of colonies in each of 3 size categories.Sites were described using visual estimates of the approximate percentage cover occupied by the major benthic attributes, of the overall substrate at each site. Field notes were recorded on the presence of any unique or outstanding biological features, such as particularly large corals or unusual community compositions. Each site was also classified in terms of the amount of reef development observed. At each site, a video 'profile' was filmed across the reef zonation from the base of the reef slope to the inner reef flat, using a SONY Hi-8 video camera in a waterproof housing. Profiles were filmed perpendicular to the reef surface, from a distance of approximately 40cm, giving a video record of the reef of width of approximately 40cm. The resulting profiles are stored at AIMS as a permanent visual record of the reef communities. To examine the extent of mine influence, the profiles of sites in the vicinity of the Gebe Island mine were analysed for percent cover of the major benthic attributes (including silt) using a SONY Hi-8 video editor. The elapsed time of each profile was determined and the tape was stopped regularly at 4 second intervals along the profile. At each stop, the benthic attribute located under each of 5 fixed points (one near each corner; one in the center of the video monitor) was identified and recorded into a computer spreadsheet, as one of 9 benthic categories. The resultant data were expressed as five point running means of percent cover of the major benthos along the profile from the base of the reef slope to the inner reef flat.
The objectives of this survey were:1) to reconnoitre the coral reefs around Gag Island and Gebe Island (both disturbed and undisturbed), provide a brief description of community structure and provide an opinion as to whether the reefs are comparable for impact assessment purposes.2) to examine a series of reef sites around Gebe Island at various distances from the active mine site and assess and rate the severity of damage at each location.3) to compare the ratings of observed damage with other sites that have been disturbed by high sedimentation and/or turbidity.4) To comment on the potential for settlement, recruitment and recovery of damaged reefs after cessation of mining.
Large deposits of nickel and other minerals have been discovered on the eastern Indonesian islands of Gebe (northern Maluku Province) and Gag (Irian Jaya Province). PT Aneka Tambang has mined the deposit on Gebe Island since 1979, with infrastructure development occurring since the late 1960s. The nickel deposit on Gag Island has not been mined to date, but a mining camp has been established and exploratory studies are currently underway. Experience at Gebe Island suggests that total control of highly coloured and turbid runoff will be very difficult to achieve at Gag Island (especially due to its more mountainous terrain).

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Maintenance and Update Frequency: notPlanned

Statement: Statement: The methods used in this survey are described in:Done, TJ (1982) Patterns in the distribution of coral communities across the central Great Barrier Reef. Coral Reefs 1: 95-107.DeVantier LM and Turak E (1995) Surveys of coral community structure on fringing reefs in the Whitsunday region, central Great Barrier Reef. Unpublished report to the Great Barrier Reef Marine Park Authority, 13 pp. plus Appendices.DeVantier LM, De'ath AG, Done TJ and Turak EI (1998) Ecological assessment of a complex natural system: a case study from the Great Barrier Reef. Ecological Applications 8: 480-496. Relative abundance categories used in surveys:0 - absent1 - rare2 - uncommon3 - common4 - abundant5 - dominantSize categories (colony diameter) used in surveys:1 - 10 cm11 - 50 cm> 50 cmDamage categories used in surveys:The level of damage (dead colony surfaces) exhibited by each species was also estimated visually, as an approximate proportion (0 - 1.0, increments of 0.1) of the total surface area of each species present.Benthic attributes used in site description percent cover estimates:Biotic (ecological) attributes:Hard coralDead coralSoft coralSpongeAlgal assemblageMacro-algaeAbiotic (physical) attributes:Hard substrateContinuous pavementLarge blocks (diam. > 1 m)Small blocks (diam. RubbleSandSiltReef Development: Each site was classified in terms of the amount of reef development as described in:Hopley, D (1982) The Geomorphology of the Great Barrier Reef: Quaternary Development of Coral Reefs. John Wiley-Interscience, New York, 453p.Hopley, D., Parnell, K.E. and PJ. Isdale (1989) The Great Barrier Reef Marine Park: Dimensions and regional patterns. Aust. Geog. Studies 27: 47-66.Reef Development categories:1 - reefs with extensive reef flats (> 50m wide)2 - reefs with moderate flats « 50m wide)3 - reefs with no flats but with some carbonate accretion (incipient reefs)4 - coral communities developed on rock, sand or rubble.Benthic categories used in analysis of video profiles:1 - Silt2 - Sand3 - Dead coral covered in silt4 - Soft corals and gorgonions5 - Coral rubble6 - Acropora hard corals7 - Other hard corals8 - Algae and seagrasses9 - Sponges and other benthos

Notes

Credit
Done, Terence J (Terry), Dr (Principal Investigator)