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AN INVESTIGATION IN TO THE FACTORS AFFECTING RECRUITMENT DATES AND SURFACE PERCENTAGES OF REEF BUILDING MASSIVE CORALS ON THE SAMPELA WALL, HOME REEF AND KALEDUPA PRISTINE REEF.
Introduction
The three different sites have varying gradients of sedimentation that ultimately effect the coral composition and growth rate. The temperature remains at a 28° c throughout. No previous work has been conducted in the three areas under investigation especially in regard to sedimentation relationships with growth rate. The coral cover has to assessed by lying transects at different depths. Identifying the coral present to family level will produce the accuracy required for the study. The sedimentation present will be collected, weighed and particles assessed.
Objectives
The aim of this study is to determine the
relationship between the factors associated with a reef highly affected by
sedimentation, a bomb-damaged reef and a pristine reef and the coral cover and
recruitment dates, coral cover and diversity.
Recruitment dates of corals can be calculated from surface area and the growth rate calculation of 7.03mm/year. Using this data and storm data, correlations can be made between the average dates of recruitment and the number of weather disturbances in the local area. Recruitment of coral is ultimately affected by high intensities of bomb damage. Home reef both encountered high numbers of bombs for fishing which can be associated with the amount of corals recruited after this time.
The coral cover varies between depths and sites, by calculating the percentage cover of coral varieties illustrates the different types of coral reef growth.
Methods
On each of the three sites three or four transects were laid. Each transect was 10m long with a marker at each metre and 2m wide. One was laid horizontally on the reef wall on the crest (4-5m) at 5m, 10m and 15m. The transects were made out of bright blue toughened string and tied down to the reef with ties. The points of attachment proved to be difficult to find. It was important not to damage the reef by attaching the transect. Aiming for points of dead hard coral especially hooked are a large hole to feed the string through was ideal. Also when laying down the transect it was important to keep the transect line all along the same depth and not to drift upwards or downwards. The transect needed to be close to the reef wall so that measuring is made easier which also posed the problem of touching and rubbing against live coral next to the line. Therefore during and after the transect was laid care was given to move the transect aside from live coral and ‘tucking’ it into the coral wall to reduce damage.
Once the transects were laid, two divers moved along the transect identifying massive coral 1m either side of the transect line. The name was recorded and the surface area measured. This was done using a metric measuring tape and recording the longest distance across the coral and then the length at 90° to that length. This gives two lengths to plug into the surface area equation.
Kaledupa and Home reef all have the total four transects set at the depths required. However Sampela Wall only goes to depths of 13m and therefore the final 15m depth transect was omitted. Each transect was measured as stated and the totally detached from the reef and relocated at the same depth further along the reef. It was suggested to simply ‘flip over’ the transects to save time, but this would lead to a bias sample as the corals found on the second transect are affected by the corals found on the first. To gain reproducible and reliable results the transect were relocated to completely different areas.
Once the transects had been measured and prior to their relocation they were videoed and stored for computer analysis. No damage was caused to the corals during their filming. These videos are then used for percentage cover analysis and more accurate surface area analysis. In order for calibration on the video analysis and tape measure of a known width was held over the top of the transects. The markers first attached to the transect also serves as a guide to the position on the transect whilst filming.
To record the sedimentation at each site sediment traps were laid. These were made using small 250ml water bottles. One whole one as attached to the top section turned upside down to produce an hourglass effect. These were placed at each depth where a transect was laid and allowed to collect any sediment present for 3-4 days. The content was then weighed and analysed for particle size and content.
Analysis of results
Data collected will be used to produce percentage cover of massive, branching and soft coral. Statistical significance tests will be used to assess the sedimentation data between each site. Due to limitations in scientific equipment available the sediment could not be analysed accurately until taken back to the UK for analysis.
The methodology has been tested to verify it’s accuracy. The physical data when compared to video photography shows a ± 8% and can therefore be deemed as reliable. From this calculation it is then possible to measure other types of coral via the video photography method.
Expected results
The greater amount of sedimentation present in the environment surrounding the reef will decrease the growth rates of corals within that environment. Sedimentation smothers coral leaving the photosynthetic zooxanthellae contained within it unable to trap light. This leads to nutrient deficiencies that will slow the growth rate down. The greatest amount of sedimentation can be found at Sampela, not through the traps but by assessing the visibility when diving. The coral is covered with a layer of sediment which includes small rock, waste and general land debris. The visibility when diving can be 7m at maximum whereas at Kaledupa or Home reef can go up to 15 – 20m. This qualitative assessment alone will give an idea of the expected results from the growth rate data, sedimentation data and percentage cover of different corals. Kaledupa is the pristine reef displaying an enormous array of different corals and other benthic species. This leads to the preliminary thought of the decrease the abundance in reef building due to increase in competing benthic species.
Diversity of coral species would also be an interesting calculation to relate to sedimentation.
The wave action on the top of the reef walls will mostly affect the coral diversity on the crest transects. These will probably show a greater amount of the steadier species such as porites and show less species of earlier recruitment dates. The recruitment dates will be the earliest for corals found deeper on the coral wall. Bombs mainly hit reefs in areas close to the surface and easily accessible by young free-divers so they can catch the fish killed or stunned by the blast.
Further experiments embarking on lower depths on the coral wall may truly prove this point with a greater significance.
Output:
A report describing the above work was produced by Louise May, Kings College London in May 2002. A copy of the dissertation is available at the Hoga field centre and from the Op Wall UK offices.
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