AO VON - A MANGROVE IN THE ANDAMAN SEA

A STUDY TOUR TO PHUKET MARINE BIOl::OGICAL CENTI;R

THAILAND

REPORl"

Editea by Jergen HyUeberg

Institute of Ecology and Genetics University of Aarhus, Denmark

REPORT ON A STUDY TOUR TO PHUKET MARINE BIOLOGICAL CENTER IN THAILAND FOR A GROUP OF BIOLOGY STUDENTS AND TEACHERS IN

JANUARY 1981

Edited by J~rgen Hylleberg

Institute of Ecology and Genetics, Ny Munkegade Universi ty of Aarhus

DK-8000 Aarhus C, Denmark

Published by Institute of Ecology and Genetics

Printed 1987

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VII. WATER EXCHANGE AND FLUXES OF PARTICULATE ORGANIC MATTER AT AO YON

by

Elin K. Svendsen in co-operation with Jorgen Hy1leberg

Description of the mangrove The mangrove is located on the southeast coast of Phuket Island (980

, 23' 40" E,7°, 48' 40"N), at Ao Yon. It is a relatively small mangrove (about 42,000 m2

), at the foot of low mountains (Fig. I-VII). A shallow tidal channel runs seaward through the mangrove. In the rainy season this channel drains freshwater from the nearby hills into the sea. (Boonruang, 1978).

Vegetation of the area (flooded at mean high tide) is primarily comprised of 4 species: Avicennia marina, ~ alba, Rhizophora maculata and ~ apiculata. The trees were cut 7 to 10 years ago; all of them were less than 10 m tall. Because of this it is an open mangrove forrest, in which most of the sediment is exposed to the sun. The vegetation around this area is comprised of Phoenix palms, coconut palms and other trees.

The substrate of the area flooded at high tide is firm mud along the banks of the tidal channel, soft muddy-sand in the middle of the area and more sandy (with small stones) near the edge.

The studies were carried out from January 2-14, 1981 (the dry season). In this period the mean tidal range at Ao Yon was measured to be 1.3 m. The tidal range at Phuket Island is over 3 m at spring tide and about 1 m at neap tide (Boonruang, 1978).

Export of particulate organic matter (POM) from the mangrove to the sea The purpose of this investigation was to find out, whether there was an export of POM from this mangrove to the sea, and if it would be possible to estimate the magnitude of this export.

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PHUKET ISLAND

Fig. I-VII Location of the Ao Yon mangrove on Phuket Island. (After Boonruang, 1978).

Initial observations: Both at incoming and outgoing tides the water flow varied considerably, at times being almost zero. Also the material in the outgoing water changed; to begin with mostly newly fallen leaves were transported seawards. At the end of the outgoing tide the water looked brown and turbid, because of small « 1 cm diameter) black pieces of leaves and wood. At times, bacterial films covered most of the surface. Because of these impressive variations in flow and material, it was decided to measure the water depth and flow and to take water samples at very short intervals (2 minutes).

Material and Methods All the water to and from the mangrove passed under a bridge at the mouth of the mangrove. By knowing the area of a cross section under the bridge and by measuring the water flow at particular water levels it was possible to calculate the water masses passing under the bridge at a known water level. The area of the mangrove was measured from an aerial photograph. The water flow was measured at the bridge over a distance of 10 m from the mangrove to the bridge. The time it took for a jar to pass 10 m in the middle of the stream was measured. The jar (25 cm high, 9 cm diam.) was filled with water, so that only the lid of the jar could be seen at the water surface. When the water level was lower than 30 cm, a smaller jar was used. A water sample was taken by putting a 600 ml glass half down into the water (Fig. 2-VII) quickly turning it to upright position and filtering the

content through a plankton net (20 pm). This was done 50 times by which we filtered 15 1 water under 0.54 m2 water surface through the net. The material was washed off the net (with pre-filtered water) into a plastic bag.

On January 8, 1981, the water flow and levels were measured, and 5 water samples were taken. The next day 11 water samples were taken, and for each sample the water level was noted. The water samples were taken 10 m from the bridge at the mangrove site.

l2.cm

Fig. 2-VII Technique for sampling of surface water.

In the laboratory the water samples were filtered through weighed glass microfibre papers under pressure. The papers plus material were dried 24 hours at 10SoC, weighed, and then burned in a muffle furnace at 550°C for 8 hours to determine the ash free dry weight of the material, i.e., the organic matter content.

Results Data on measured water flow, calculated cross section of the water mass, volume of water flowing out of the magrove every half an hour, and the amount of ash free organic matter per unit volume are presented in Table 1-VII.

It is seen that a left the mangrove of 11 kg ash free

total of 25 kg ash free organic matter during ebb on 9th January, while a total organic matter entered the

flood. As a result an estimated amount of 14 matter was exported from the mangrove to the

mangrove during kg organic bay.

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Table 1-VII. Parameters measured and estimated during ebb and flood at the Ao Yon mangrove . Data from 8th and 9th January are pooled in the Table. (n) = number of measurements.

TIDE GOES OUT

Water Ash Total Mean flow Area of Outflow free ash water Water rate cross of dry free dry level level (dm per section water weight weight

Time (m) (n) (m) sec) (n) (dm2 ) (1) (gil) (kg)

12.00-12·30 1.45 4 2.2 4 1553.5 6151860 12.30-13.00 1.43 2 2·5 5 1521. 5 6846750 - - 1.34 0.00029 - - 1.29 0.00089 13.00-13·30 1.26 2 2·3 9 1249 . 5 517~930 0.00103 5.794 - - 1.20 0.00227 13.30-14.00 1.14 4 2.7 8 1065.7 5179302 0.00160 - - 1.09 0 . 00418 14.970 14.00-14.30 0.93 3 2.1 6 700·5 2647890 0.00071 1. 880 14.30-15.00 0.80 2 1.7 3 559.5 1712070 - - 0.78 0.00048 0.822 - - 0.52 0.00074 - - 0·51 0.00253 15.00-15.30 0.42 4 2.2 4 135.0 534600 - - 0·35 0.00078 0.722 - - 0.30 0.00436 15.30-16.00 0.22 4 2.0 4 38.5 139680 0 .609 16.00-16.30 0.10 1 2.0 1 8.0 28800 0.01425 0 . 410 17.00-18.00 ca. 1000

Total 28423882 ca. 25

TIDE COMES IN 1.05 0.00053 1.38 28 xl07 0.00027 ca. 11

Discussion The method used for measuring water flow is coarse. It would have been better to use a flow meter at different depths, but such an instrument was not available. The flow is probably overestimated because the flow in the middle of the stream is stronger than near the banks. The water samples should have been taken at different depths; probably the concentration of POM is at times overestimated by taking samples at the surface only.

Generally, in marine waters the concentration of particulate organic carbon is in the range of 0.025-0.275 mg POC per liter. The measurement at Ao Yon at incoming tide averaged 0.40 mg POM per liter which corresponds to 0.23 mg particulate organic carbon per liter (multiplied by the conventional factor of 0.58).

The estimate of export of particulate organic matter is only based on few water samples collected over 2 days. The transport of material is probably not uniform throughout the year because of variation in litter fall, spring tide, storms etc., so it is not attempted to estimate the yearly export by simple multiplication.

Export of particuclate organic matter from mangroves was measured by Golley et ale (1962) (quoted in Odum & Heald, 1975). They found a net export of: 2.27 g POM per m2 per day from a red mangrove. It is not known whether this figure refers to g ash free dry weight or dry weight.

At the same time as these investigations were made, Dr. Hansa Chansang (PMBC) and Mr. Sombat Poovachiranon (PMBC) studied litter export from this mangrove. They caught the litter in a net placed under the bridge at the mouth of the mangrove. Data from that study will be published separately.

Unfortunately, none of us had the capacity to measure the possible export of dissolved organic matter. Odum and Heald (1975) think that this must be considerable since a mangrove leaf looses roughly one-fourth of its weight from the loss of soluble components during the first 2 weeks after falling from the tree.

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