RASHKB Journal 皇家亞洲學會香港分會學刊 | RAS-1984
https://digitalrepository.lib.hku.hk/catalog/5h73wh572
183

marsh, which has been enclosed by bunds to form kei wais, (iv) a shallow channel running parallel to the outer bund; (v) a zone of mangrove plants some 15-25 m. wide with Kandelia candel, Avicennia marina and Aegiceras corniculatum as the commonest species, (vi) Deep Bay, which forms part of the estuary of the Pearl River and also receives water from the Sham Chun River and smaller streams. At intervals channels run through the mangrove zone to join channel (iv) beside the outer bund to Deep Bay; water moves along these channels during the rise and fall of the tide and thus the kei wais are filled.

Within the kei wais are small, irregularly shaped, muddy islands. Essentially all of our results were obtained from Mai Po kei wai No. 7 (total area ca 9.3 ha, area of islands 5.6 ha, area of open water 3.7 ha). This kei wai is 820 × 120 metres with its shorter sides parallel to the shoreline. It is surrounded by a bund which, on the seaward side, is pierced by a sluicegate with a concrete frame. The frame is about 2 m. wide and is usually closed by a gate consisting of heavy wooden planks, placed horizontally, which can be raised or lowered in slots in the concrete frame (Plates 7-9). The sluicegate is essential to the operation of the kei wai because it enables the operator to control exchange of water between the kei wai and the adjoining estuary. The kei wai, like others in the neighbourhood, was held on lease renewable every seven years.

Source of Future Produce

As already mentioned, an important source of future produce is the adjoining estuary: fry and larvae are carried on the high tide through the open sluicegate into the kei wai where they are "trapped" when the gate is closed. Species entering in this way include marine fish, shrimps and crabs. In addition, the source of produce is supplemented and diversified by the operator, who may add fry of tilapia (Sarotherodon mossambicus, syn. Tilapia mossambica) and the brackish-water striped mullet (or grey mullet, Mugil cephalus). Small oysters (Crassostrea gigas) are bought from local fishermen and used to stock oyster beds set up in the vicinity of the sluicegate; their growth period is 1.5 to 3 years.

The basis of production of the kei wai is the usual two food
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RASHKB Journal 皇家亞洲學會香港分會學刊 | RAS-1984
https://digitalrepository.lib.hku.hk/catalog/5h73wh572
(ii) To estimate growth of periphyton, microscope slides were supported just below the water level in a specially constructed float. Slides were collected each month, the cells were scraped off, extracted with acetone, and the chlorophyll estimated as in (i).

c) Observations on plants

The bunds surrounding the kei wais and the islands carry a fairly dense growth of plants. In both cases, the main species are Kandelia candel (L.) Druce, Phragmites karka (Retz.) Trin., Acanthus ilicifolius L., and the fern Acrostichum aureum L. The first three species grow around the margins of the bunds and islands and thus overhang the water.

Throughout the experimental period, observations were made on the stages of growth of Kandelia and Phragmites. Litter fall from Kandelia was estimated by surrounding the lower half of two bushes with a funnel-shaped structure of fine nylon netting; the litter was collected from the net each month, and the quantity expressed on a dry weight basis. Probable litter production by Phragmites was estimated in August 1978 when the plants were becoming senescent: the stems and leaves within 4 × 1 m2 quadrats were harvested separately and oven-dried.

d) Decomposition of submerged Kandelia leaves

Matched sets of senescent leaves were immersed in the kei wai either in plastic mesh bags (1 × 1 mm mesh) or in plastic vials with 2 mm holes punched through them. Individual bags and vials were collected at weekly intervals. Leaves from the bags were used to study the progress of fragmentation and were analysed by the Kjeldahl method to determine their nitrogen content and thus their approximate protein content. Leaves from the vials were used to follow changes in dry weight and content of hot water.

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RASHKB Journal 皇家亞洲學會香港分會學刊 | RAS-1984
https://digitalrepository.lib.hku.hk/catalog/5h73wh572
194 Y.H. CHEUNG, K.Y. TAI, S.W. TSAO AND L.B. THROWER

kei wai, there would be an input of 146.5 kg. per year or 39.6 kg. per hectare of open water.

Thus, the total annual input of organic litter from these two species alone can be estimated as about 3,100 kg. dry weight per hectare or 850 per hectare of open water.

c) Colonization of Litter by Microorganisms

The occurrence of fungi in leaf litter after various periods of immersion is shown in Table 3; these fungi had been isolated from surface-sterilized tissue and therefore were present within the leaf itself. They may have infected the leaves while still on the parent bush or invaded them during immersion. Venturia sp. and Phyllosticta sp. probably represented the former group. Pythium spp. became an important component of the mycoflora after 3 days immersion and remained so until day 64. Chaetomium sp. and Trichoderma viride were also important by day 64. Representatives of an additional 9 genera of fungi were isolated from leaves after immersion but before surface sterilization, but these were considered as contaminants.

Table 3. Frequency (%) of fungal species isolated from leaves of Kandelia candel after various periods of immersion in kei wei

  
    Period of immersion (days)
    
    0
    3-8
    21-32
    64
  
  
    Isolates*
    
    
    
    
    
  
  
    Venturia sp. (conidial form)
    
    31
    
    
    
  
  
    Colletotrichum sp.
    
    8
    
    
    
  
  
    Phyllosticta sp.
    
    11
    
    
    
  
  
    Nigrospora sp.
    
    4
    
    
    
  
  
    Pythium spp.
    
    
    90
    33
    33
  
  
    Fusarium spp.
    
    
    
    2
    
  
  
    Chaetomium globosum
    
    
    
    
    33
  
  
    Trichoderma viride
    
    
    
    
    44
  
  
    Hormiscium sp.
    
    
    
    11
    
  
  
    O. Melanconiales
    
    
    
    
    11
  
  
    * from surface-sterilized leaf tissue.
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RASHKB Journal 皇家亞洲學會香港分會學刊 | RAS-1984
https://digitalrepository.lib.hku.hk/catalog/5h73wh572
196

Y.H. CHEUNG, K.Y. TAI, S.W. TSAO AND L.B. THROWER

items of diet for the higher trophic levels include phytoplankton, periphyton, polychaetes (eg. Nereis) and miscellaneous other invertebrates such as chironomid larvae, crustacean larvae, copepods (eg. Tigriopus japonicus), amphipods and isopods. The percentage composition of the gut contents of a range of animals is given in Table 5.

Table 4. Characteristics of the bacterial colonies isolated from surface of mangrove leaves (Kandelia candel) after various periods of immersion in kei wai

  
    Period of immersion (days)
    8
    14
    21
    43
  
  
    Character
    
    
    
    
  
  
    Gram stain (+)
    40*
    30
    67
    60
  
  
    Gram stain (-)
    60
    70
    33
    40
  
  
    Rod
    80
    90
    100
    100
  
  
    Coccus
    20
    10
    0
    0
  
  
    Pigment
    
    
    
    
  
  
    White
    25
    67
    40
    75
  
  
    Yellow
    22
    Orange
    0
    0
  
  
    Pink
    11
    Grey
    0
    0
  
  
    Transparent
    0
    MooMoo
    0
    20
  
  
    
    
    0
    40
    
  
  
    Biochemical character
    
    
    
    
  
  
    Glucose fermentation
    25
    
    
    
  
  
    Starch hydrolysis
    50
    
    
    
  
  
    Proteolysis
    25
    
    
    
  
  
    Lipid hydrolysis
    
    
    
    
  
  
    Cellulolysis
    KAKKA
    64
    55
    36
  
  
    
    20
    75
    63
    50
  
  
    
    73
    80
    82
    88
  
  
    
    30
    30
    70
    22
  
  
    
    075
    44
    
    
  

* Percentage of isolates possessing each character

The results show the importance of detritus of plant origin (plant fragments) in the diet of these animals. It was most important in the case of invertebrates where it accounted for 22-40% of the gut contents, but made up 30% of the gut content of striped mullet (Mugil cephalus). Moreover, other undefined detrital material (sediments and inorganic particles) also made up a large
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RASHKB Journal 皇家亞洲學會香港分會學刊 | RAS-1995
https://digitalrepository.lib.hku.hk/catalog/95941j25g
162

Table 1. Mangrove species in Hong Kong

Acanthus ilicifolius L.

(Acanthaceae)

Aegiceras corniculatum (L.) Blanco

(Aegicerataceae)

Avicennia marina (Forsk.) Vierh.

(Verbenaceae)

Bruguiera conjugata (L.) Merr.

(Rhizophoraceae)

Excoecaria agallocha L.

(Euphorbiaceae)

Heritiera littoralis Dryand. ex. W. Ait.

(Sterculiaceae)

Kandelia candel (L.) Druce

(Rhizophoraceae)

Lumnitzera racemosa Willd.

(Combretaceae)

Table 2. Salt concentration and salt exclusion/secretion/deposition in the various mangrove species.

  
    Species
    Salt cone
    Salt excluders
    Salt glands
    Salt crystals
    g/L in leaf
  
  
    Kandelia
    
    X
    
    X
    0.08-0.9
  
  
    Excoecaria
    
    
    X
    X
    0.8-3.4
  
  
    Aegiceras
    
    
    X
    X
    0.9-1.8
  
  
    Bruguiera
    
    X
    
    X
    4-11.4
  
  
    Avicennia
    
    
    X
    X
    6.7-11
  
  
    Acanthus
    no data
    X
    
    
    
  
  
    Lumnitzera
    no data
    X
    
    X
    
  
  
    Heritiera
    no data
    X
    
    X
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