The Spermonde Archipelago along the west coast o f South Sulawesi, Indonesia, consists o f a large group o f coral islands and submerged reefs, distributed over the Spermonde continental shelf (Fig 1.1; De Klerk, 1982). The reef flats o f most o f the islands and shallow areas along the coast are covered with seagrass beds (Erftemeijer et al., 1993a). Barang Lompo island, located approximately 14 km off the coast, is surrounded by a large intertidal reef flat. The sediment consists of relative coarse carbonate sand and coral rubble (93-100% CaCO3; Erftemeijer, 1994). Approximately 50 ha o f the reef flat is covered with a dense mixed-species seagrass vegetation (Erftemeijer and Herman, 1994). A site (approximately 50 x 50 m) at the south-west part of the reef flat, with a homogenous distribution o f seagrasses and alpheid 34
Chapter 4: Leaf harvesting and sediment reworking
shrimps, was selected for the present study. A well developed Thalassia hemprichii vegetation (approximately 770 shoots m-2), mixed which patches o f Enhalus acoroides (L.f.) is present at this site.
Seagrass biomass, density and leaf litter
Above-ground seagrass biomass was determined by harvesting the shoot material from 25 x 25 cm plots (n = 6). The samples were washed free from sediment. The shoots in the samples were counted, dried (24 hours, 80°C) and weighed. The amount o f organic litter lying at the sediment surface o f the seagrass bed was determined by sampling litter from randomly chosen 50 x 50 cm plots (n = 10). After sorting the litter samples in a seagrass and a non-seagrass fraction, these fractions were dried (24 hours, 80°C), weighed and stored for later carbon (C), nitrogen (N) and phosphorus (P) analysis. Total C and N content was determined with a Fisons NA 1500 CN-analyzer. Total P was analysed colorimetrically (Allen, 1974) after strong acid microwave destruction o f the organic material (Nieuwenhuize ef al., 1991).
Alpheid shrimp observations
Shrimp abundance was assessed in randomly selected 50 x 50 cm quadrates (n = 23) by counting the burrow openings inside the quadrate. The diameter o f 20 randomly selected burrow openings was measured. A few specimens o f the alpheid shrimps were collected for species identification.
The behaviour o f alpheid shrimps was observed in the morning (9h), at noon (12h) and in the afternoon (15h). Each time, six different inhabited burrows were carefully approached and observed while snorkelling. During four successive 5-min. intervals, we counted how many times sediment was expelled from a burrow, or living (still attached) or dead seagrass leaves (lying on the sediment surface) were harvested. The four intervals were treated as one 20-min. sample.
Our estimation o f sediment expulsion is based on the specific weight o f dry sediment: 1.32 g dry weight ml-1. The shrimp uses its large chela to push sediment out o f its burrow. Sediment expulsion was mimicked in the laboratory, based on visual observations, using a spatula o f the same dimensions as the large chela o f the shrimp (reported by Banner and Banner, 1966; 1982). Twenty portions o f ‘expelled’
sediment were thus produced, dried and weighed. On the basis o f these data, we estimated that a sediment load o f approximately 21 ± 8 mg dry weight was translocated with each expulsion movement.
Quantification o f the amount o f leaf material that was harvested by the shrimps, was based on estimations o f the lengths o f the harvested pieces. These lengths were estimated during observation and classified in five size categories: 0-5, 5-10, 10-20, 20-30 and 30-50 mm. The number o f harvested leaf fragments (living and dead) in each category was counted. Some shoots o f Thalassia hemprichii were harvested and in the laboratory cut in pieces with random length. From this pool o f living leaf fragments we randomly picked pieces that were classified in the same five categories as mentioned above. This was done until the quantity o f leaf fragments in each size category corresponded with the number o f fragments from the same category, counted in the field. This was repeated five times, after which the fragments were dried and weighed. With these results we estimated an average dry weight o f 5.1 ± 0.6 mg per harvested leaf fragment. This figure was used to calculate the total biomass harvested by the shrimps (living leaves and leaf litter).
Sediment reworking
To investigate the rate of sediment reworking, carbonate sediment was collected from the reef flat of Barang Lompo. Approximately 50 kg o f this moist sediment was sieved (mesh size 2 mm) and mixed for two hours in a concrete mixer with 1 kg fluorescence red paint (Visprox Fluorescent Flame Red) and 120
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Chapter 4: Leaf harvesting and sediment reworking
ml ethanol, according to a method described by Nieuwenhuize and Sips (1977) and Van Noort and Kraay (1992) and slightly modified by us. Hereafter, the sand was dried for 6 days at approximately 50°C and subsequently washed. In November 1992, an approximately 3 mm thick tracer sediment layer was spread out at three randomly selected plots o f 1 x 1 m in the study area. The plots were levelled by hand first, to create a flat area with a tracer sediment layer o f approximately uniform depth. After 1, 2, 3 and 4 weeks, three replicate cores (diameter = 2 cm) were randomly taken from each plot to a depth o f 10 cm. The resulting holes were filled with sediment from outside the plot to prevent transport o f tracer sediment merely as a result o f caving in o f the holes. One blank core was taken outside the experimental plots. Each core was separated in 1 cm sections. The outer layer o f each section was removed to get rid o f tracer sediment dragged down to deeper sections during coring. Sections were dried at 80°C for 24 hours and weighed. Hereafter, the sections were extracted for 24 hours in opaque 100 ml bottles filled with a solution consisting o f a mixture o f 80% acetone and 20% di-methyl sulphate (1 ml g-1 dry weight). Every 8 hours, the bottles were thoroughly shaken for 10 seconds. Subsequently, the content o f the bottles was centrifuged and the intensity o f the red colour o f the supernatant was measured colorimetrically at 540 nm, using a Nanocolor 100 D-MN filter photometer. Calibration series were made weekly with coloured and non-coloured sediment mixed in proportions o f 0, 1, 5, 10, 50, 90 and 100% tracer sediment. The tracer sediment used for the calibration series was kept in sea water, which was renewed weekly. The relationship between colour intensity and percentage tracer was linear after logarithmic transformation of both scales. The percentage tracer in a sediment section was divided by the sum o f percentages o f tracer in the various sections o f the core in question and subsequently multiplied by 100%. This standardisation was carried through to correct for the patchy distribution o f tracer sediment, as deployment o f tracer in a layer with exactly the same thickness within a plot was not practically feasible.
Statistics
Differences in shrimp activity in the morning, at noon and in the afternoon were tested using one-way ANOVA. Differences in tracer content in the different (1 to 9 cm) sediment sections after t = 1, 2, 3 or 4 weeks were tested a posteriori using the Bonferroni post-hoc test after applying one-way ANOVA. The increase in the variance o f tracer distribution over total core depth and an increase in the weighted average tracer depth, were tested using one-way ANOVA.
An attempt was made to calculate the 95% confidence limits for sediment expulsion. The average daily sediment expulsion (g m-2 d-1) is the product o f the average number o f daily sediment expulsion events (events m-2 d-1) and the average sediment dry weight per expulsion movement (mg event-1). The variance o f a product is not directly computable. The variance o f a sum, however, is the sum o f the variance o f the observations, in case there is no co-variance between the two observations (Sokal and Rohlf, 1995).
Therefore, the data on expulsion events and the estimated dry weight per expulsion movement were log transformed. The inverse log o f the sum o f the log transformed observations is equal to the product o f the not transformed observations. Now, the variance o f the log transformed data on expulsion events can be summed with the variance o f the log transformed data on estimated expelled sediment dry weight. The square root o f this sum was multiplied with the value from the ¿-distribution at 0.05 probability and n-1 degrees o f freedom, where n is the total o f observations o f events (18) summed with the total of observations o f dry weight estimation (20). Hereafter, the inverse log o f this figure was calculated. The lower 95% confidence limit is calculated by dividing the average daily sediment expulsion by the outcome o f the above described calculation; the upper limit is calculated by multiplication. The resulting 95%
confidence interval is asymmetric around the average and is an approximation o f the actual range.
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Chapter 4: Leaf harvesting and sediment reworking
RESULTS AND DISCUSSION