Introduction

During this project I will examine the physical structure (both in terms of reef morphology and the physical structure of the coral communities) of three stretches of coral reef in the Wakatobi Marine National Park. This is located in the Tusang-Besi archipelago in the Wallacea region of Indonesia. The first of these three sites, Sampela, is thought to be under threat due to a reduction in the amount of light reaching the coral here. This is thought to be caused by the cutting down of mangrove and sea grasses around the Bajo sea-gipsy village of Sampela and the harvesting of the hard corals in this area for use in the construction of the village, which is about 500m to the landward side of the Sampela Reef. This usage of the corals, mangroves and sea grasses and the emission of all the villages waste into the sea close to the reef at Sampela is thought to be causing the sedimentation of the reef by the sub-littoral sands from the surrounding coastline. It is feared that if this reef is degraded any further then many important habitats around the village of Sampela will be lost, to the detriment of both the environment and the people of the village of Sampela (as they rely upon the reef and the habitats that it protects not only for construction but also for their livelihoods (sea cucumber harvesting, fishing etc.)).

The second study site will be the Hoga home reef, which forms a fringing reef around the island of Hoga, which neighbours the main island of Kaledupa (on which both Sampela and Kaledupa are located). This section of reef is thought to be an intermediary between the Sampela and Kaledupa sites as it is thought that this reef has been mildly affected by sedimentation and is also fairly heavily dived.

The Sampela and Hoga sites will be compared to a pristine section of reef at Kaledupa in order to assess the impact that light limitation (due to sedimentation) has had upon the reef at Sampela and at Hoga. Also net accumulation and loss (degradation) rates will be calculated at each site in order to assess whether there is net accumulation or loss at each of the sites studied.

Aims

Ø To understand the physical structure of a light limited and pristine coral reef

Ø To discover what effects light limitation has upon the physical structure of a coral reef.

Ø To discover what relationships exist between the physical and biological characteristics of a coral reef.

Ø To discover what changes occur in the physical/biological structure of a coral reef with increasing depth.

Ø To discover whether or not there is net erosion of the coral reef at Sampela, Hoga

Objectives

(1) To measure the rugosity, aspect and slope of each reef system at 2 metre intervals at each of the reef sites

(2) To record the biological structure of the reef (type/abundance of corals) at 2 metre intervals at both reef sites.

(3) To measure the light intensity at each reef site

(4) To measure accumulation rates at each of the reef sites

(5) To measure degradation rates at each of the reef sites

(6) To measure the sediment load of the waters at each reef site

(7) To measure the sedimentation rates at each of the reef sites

Methodology

1) At each reef site 5 random measurements of all the variables being studied will be made at 2 metre intervals up the reef wall from 14m up to 4m at Hoga and Kaledupa and from 12m up to 2m at Sampela.

To measure the rugosity of the reef a metal chain of known length will be laid across the reef horizontally at each 2 metre interval. Then the distance that the chain takes up once it has been pushed into all indentations and crevices in the reef wall will be measured and the rugosity worked out using the following formula; Rugosity (%) = (length of chain in situ/total length of chain) x 100

To measure the slope of the reef wall a tape will be held against the reef wall at 10m in depth whilst the other end is taken up 5m vertically (measured using a depth gauge) and held against the reef wall. The length of tape therefore will represent the second side of a right angle triangle (with the first being 5m in length) and this measurement can be used to work out the gradient of the slope at a later date. Aspect will be measured using a compass at each of the sites.

(2) In order to measure the biological structure of each reef system, quadrats (1m x 1m) will be laid down at two metre intervals along the reef wall on all reefs. Then the percentage cover and type of coral present (massive, sub-massive, encrusting etc.) will be recorded. Using this method will allow changes in the biological structure with depth and light limitation to be seen.

(3) In order to measure the light intensity a secci disk will be used to record the turbidity at each reef site. This will be done by immersing the disk beneath the surface of the water until it is no longer visible from the surface. The distance taken for the disk to become invisible from the surface can then be used to work out the light intensity at each site. The turbidity measurements will be taken 10 times at each site allowing an average to be calculated and thereby discounting the effects that daily changes in currents and other environmental factors may have upon the results achieved.

(4) To measure the accumulation rates at each of the sites the growth rates of the corals within three permanent quadrats that were placed at each of the study sites last year by Dr Dave Smith will be measured in order to assess the amount that the corals have grown in the last year. Also the growth rates of some specific branching corals that were tagged and recorded last year at each of the sites will be recorded. These two measurements will enable a rough estimate of the amount of coral accumulating on the reef per square metre per year and will allow the amount of calcium carbonate accumulating on the reef to be calculated.

(5) In order to measure loss rates from the reef the contents of several sediment traps (Bamboo baskets placed at the base of the reef wall) which were placed at the base of each reef in 2001 will be examined both in terms of weight of material lost and particle size (sieving) in order to establish an average loss rate for each reef (by dividing the catchment area of the sediment trap (i.e. the width of the trap x the height of the reef above that point) by the weight of sediment caught in the trap). Also, using the same method the loss rates from the numerous coral rubble slips that are present, particularly on the Sampela reef (where the reef wall is eroding faster) will be calculated. Then in order to work out an average loss rate for each reef system as a whole the number and size of coral slips at each study site will be recorded and an estimate made of the cover of rubble slips and normal reef wall for each site (this may be done by selecting areas (e.g. between two marker buoys) and working out the area of the wall and area of coral rubble slips within this area, this method will then be repeated three times at each site and an average calculated). Form this information the average loss rate can be calculated using the following equation;

Average loss rate = (% average coral slip cover/100 x average coral slip loss rate) + ( % average normal reef wall cover/100 x average wall loss rate)

(6) The sediment load of the water at each of the sites will be measured by taking three 500ml samples at 1 metre of depth at each of the sites studied. These samples will then be analysed back in the UK by filtering the water and measuring the weight of the filter paper before an after the water has been passed through it, the difference being the amount of sediment present within the sample. This result when doubled will give a result in mg per litre.

(7) The rate of sedimentation at each of the study sites will be measured using sediment traps. Six of these traps, which are constructed of a 500ml bottle and a funnel, will be placed on each of the sites being studied (3 landward/3 seaward) and replaced every 7 days during the 4-week period of study. The contents of the traps will then be filtered, using pre-weighed filter paper whilst in Indonesia and the paper + contents brought back to the UK for drying in an oven and re-weighing in order to work out the sedimentation rates.

Data Analysis

The data collected will then be analysed using appropriate statistical techniques including; descriptive statistics, correlation analysis (rugosity / depth etc) and T-test analysis. Also the data will be displayed using histograms and scatter grams. Finally accumulation and loss rates for each site studied will be worked out and a net loss/accumulation rate calculated for both the Sampela and Kaledupa study sites.

Report

A dissertation entitled The effects of sedimentation on reef structure will be produced by Chris Payne, University of Durham by May 2003.