Estuarine, Coastal and Shelf Science (1989) 29,75-87
Litter Production and Turnover of the Mangrove Kandelia candel (L.) Druce in a Hong Kong Tidal Shrimp Pond
S. Y. Lee Department of Zoology, The University of Hong Kong, Pokfulam Road, Hong Kong
Received 26July 1988 and in revisedform 22 March 1989
Keywords: litter production; turnover; tidal ponds; Kandelia candel; sesarmid crabs; management; Hong Kong
Production and turnover of Kandelia candel litter were studied for 2 years in a tidal shrimp pond (10 ha) at the Mai PO Marshes, northwest Hong Kong. Annual litter production averaged 11.070 t ha- (equivalent to 4.880 x 10 kcal ha-), with wood and leaf materials contributing, respectively, 6.15 and 53.94% of the total. The reproductive plus frass component contributed 40.69% of the total production. The mean standing litter biomass recorded during the same period was 9587 f 0.556 t ha- (4.968 x 10 kcal ha-), with respective contributions by the three components of 19.07,42.26 and 38.65%. Residence times (standing biomass/production) of the three components were extraordinarily long, esti- mated to be, respectively, 980,300 and 252 days. Stand characteristics of average tree height and dbh were good predictors of litter production while climatic variables, especially rainfall, could be used to predict the production of various litter components. Residence times were related to both inundation frequency and crab (Chiromanthes spp.) consumption. As the Kandelia candel stands were located largely above mean water level, there was little export of litter. The litter produced was predominantly decomposed or consumed by macrofauna in situ, creating a large energy sink which was not coupled to pelagic secondary produc- tion. The significance of these findings are discussed in relation to the fishery production and wildlife conservation value of the marshes.
The high productivity of mangals has long been utilized by man. Shrimp and fish farming in tidal ponds constructed from mangrove forests have long histories in southeast Asia. Ling (1977) and Macintosh (1982) both suggested a history for tidal pond (tambak) fish and shrimp farming of over 500 years in Indonesia. Fish and shrimp farming in tambaks are still important aquacultural methods in Indonesia (Knox & Miyabara, 1987). Tidal ponds were mainly constructed for shrimp farming along the Chinese coast (Macnae, 1968). In Hong Kong, the history of these ponds (gei wai> dates back to the 194Os, when a series of ponds were dug from the native mangrove forest fringing Deep Bay (Irving & Leung, 1988), on the eastern fringe of the Pearl River Estuary. Most of these ponds have
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76 S. Y. Lee
now been turned into freshwater fish ponds because of pollution threats. The whole area of the marsh (the Mai PO Marshes), covering just over 300 ha, has been preserved as a nature reserve for migrant birds since 1973. A total of over 250 bird species are recorded through- out the year in the Deep Bay area, including a number of endangered and threatened species.
Since tidal shrimp ponds receive no allochthonous nutrient input other than import through a small seaward sluice gate, they act as good enclosures for the study of ecologi- cal processes in a mangrove-dominated environment. The traditional fishing technique has undergone little change and the ponds still depend on natural recruitment for stocks. Fishery production rests upon energy derived from autochthonous detrital input from mangrove and other macrophytes. Management of these ponds is also important to the conservation of migrant birds which use these ponds and the Deep Bay area for feeding. The present study examines the production and turnover of the major detrital input, namely, litter from the dominant mangrove Kundelia candel and discusses the possible implications of these to the utilization of the ponds for fishery and conservation objectives.
The study pond
All data were collected from a tidal shrimp pond at the Mai PO Marshes fringing Deep Bay, northwest Hong Kong (Figure 1). The pond has an area of c. 100 x 1000 m2 (10 ha). The pond communicates with Deep Bay through a single sluice gate of 1.35(w) x 1.70(h) m2. Details of the operation of the ponds may be found in Macintosh (1982). Kundeliu cundel (L.) Druce is the dominant mangrove, occupying 25.99O, (2599 ha) of the pond area as isolated patches. Other species present are Avicenniu marina and Aegicerus corniculutum, in decreasing abundance. Water is usually changed during every spring tide and remains in the pond for 4-5 days during neap tide periods. Water depth is mostly < 1 m, and is nowhere greater than 1.8 m at most times.
Litter production A total of 16 conical traps each of 0.20 m2 circular collection area were established at 0,8 m above the forest floor level under monospecific stands of K. cundel in December 1985. This height prevented wetting or loss of any litter collected. The traps were spaced in different parts of the pond with varying stand characteristics and inundation frequencies. Bi-weekly to monthly collections were made until January 1988. The litter collected was wrapped in paper bags and dried at 80 C for 48 h before the components of (a) leaf litter; (b) woody materials; and(c) reproductive materials plus frass (insect faeces, miscellaneous organic materials) were respectively sorted and weighed. The energy contents of the different components were estimated by a Parr semi-micro bomb calorimeter.
Standing litter biomass The standing litter biomass at each of the litter collection sites was estimated by collecting all surface litter in 0.25 m2 quadrats in close vicinity to the traps. A total of 12 sampling visits were made during the two-year period, covering different seasons. The litter was dried at 80 C for 48 h before sorting into the three components and weighed.
Litter dynamics of Kandelia candel in tidal ponds 77
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Figure 1. The geographical setting of the tidal shrimp ponds at Mai PO.
Stand characteristics and environmental measurements In order to correlate litter production and turnover with physical and environmental conditions of the stands, parameters of average tree height (Ht), average tree diameter at breast height (dbh), stand substrate elevation (E), crab density (Crabs), distance of the stand from the sluice (Dist) and inundation frequency of the stands (Exposure) were measured. Tree height was measured by a graduated telescoping rod or by a Range optical range finder. Average dbh was obtained by measuring the dbh of all trees taller than 1.3 m in a marked plot. A metric scale was established at the sluice gate to provide an arbitrary reference for recording water level fluctuations. By noting the time and water level at the gauge before and after each flooding or draining operation, an accurate record of water level variation was made. From this, the percentage time the substrate at a particular collection site would be exposed could be read from a sigmoid curve relating substrate elevation (on the metric scale) with exposure to air. Crab density was taken as the density of crab burrows, after eliminating the occurrence of burrows with dual openings. No actual crab density or biomass could be obtained, as the high densities of interlocking mangrove roots and the thick litter hampered effective digging. Distance of the stand from the sluice was estimated, on an arbitrary scale, from aerial photographs.
Rainfall (Rain) at Mai PO was recorded daily by a standard rain gauge positioned in the marsh. Other climatic parameters (monthly mean air temperature (Temp), mean daily global radiation (Globrad) and monthly total evaporation (Totevap)) were obtained from the Royal Observatory, Hong Kong.
78 S. Y. Lee
Stepwise multiple regression was used to investigate the relationship between litter production and turnover and the above stand characteristics and climatic variables. Production was regressed with the stand characteristics and again with the climatic variables in two respective groups. Litter residence times were regressed with the stand characteristics only. All calculations were performed by SPSS/PC + software (SPSS Inc., 1986). Violation of the assumption for the method was checked by residual analyses.
Litter production Annual production of total litter varied from 9.104 to 13.037 t ha- yrr and averaged 11.070 t ha- yr- for the sites, with respective contributions of 5.971 (53.949,), 0.681 (6.15%) and 4.418 (39.91%) t ha- yr- from the leaf, wood and reproductive plus frass components. After adjusting for the relative contributions from the three components, this figure is equivalent to an annual energy input of 4.880 x 1 O7 kcal ha- yr - .
The litter fall pattern was bimodal, with high inputs in spring (February to June) and in late summer (August to November) (Figure 2). This bimodal pattern is also evident for the reproductive plus frass and leaf components. The first peak in February to June corresponded to dropper production and the second in July to October to flowers for the reproductive plus frass component. The leaf component has a higher late summer peak, probably related to a high leafing rate in early summer. Pattern of wood litter input seemed less well defined, although a distinct mid-summer peak was consistent in July. Wood litter production was also highly variable among the sites, as reflected by the large standard errors.
The stand characteristics at the various stations and the climatic variations during the study period are summarized in Table 1.
Results of the stepwise multiple regressions between litter production and turnover, the stand characteristics and climatic variables are presented in Tables 2 and 3. In all equations generated, except that for leaf litter production with environmental variables, only one of the six parameters was included, indicating that production could be effec- tively predicted by one of the variables. In terms of stand characteristics, average tree height and average dbh were important in predicting leaf, wood and total litter produc- tion. The positive slopes indicate that production increased with increasing tree height and dbh. Production of reproductive plus frass, however, could not be predicted with accuracy from any of the stand characters. Table 2 shows the models of environmental control on litter production. Rainfall was the only variable included in the predictive equations for both the reproductive plus frass and total litter, indicating increasing inputs during the rainy seasons. This is in agreement with the generalization of Day et al. (1987) that peak litter production usually occurs in the wet season. Wood production was only related positively with mean air temperature. Leaf litter production was, however, related to both monthly total evaporation and mean daily global radiation. As the entry of the second independent variable (Globrad) brought a significant increase in the multiple r from 0.4898 to 0.6981, the second variable is also of importance.
Standing litter biomass and turnover High standing litter biomasses were recorded under K. candel stands in the pond. The mean value (n = 162) was 9587 f O-556 t ha-. An approximate energy equivalent would be 4.968 x lo7 kcal ha-. Of this, respective contributions from the leaf, reproductive
Litter dynamics of Kandelia candel in tidal ponds 79
plus frass and wood components were 4,051 (42.27%), 3.705 (38.66%) and 1.828 (19.07%) t ha-.
Table 4 shows the results of a Mann-Whitney U test for yearly differences in standing biomasses of the various litter components. As no significant difference can be detected for any of the components nor for total litter, residence times (standing biomass/production) for the various components were calculated according to the model of Olson (1963), assuming steady state conditions and monomolecular accumulation (Chapman, 1986; Vogt et al., 1986). The overall values for the entire ponds were, respectively, 252,300 and 980 days. The residence time for total litter was 317 days.
Relationships between site specific residence times for the various components and total litter were further analysed by multiple regression (Table 3). Exposure (percentage time exposed) seemed to influence residence times for both leaf litter and total litter, with increasing residence times for longer exposure times. Biotic factors also contributed to the turnover of the reproductive plus frass component, as the equation indicates an inverse relationship between residence time and the density of Chiromanthes spp. The production and turnover of litter components in the tidal pond is summarized in Table 5.
Although K. candel is widespread along the south coast of China and has a northern limit at Japan (Hosokawa et al., 1977; Chen et al., 1985; Lin, 1987), it is a relatively little known species. Both Watson (1928) and Tomlinson (1986) regarded it as being nowhere abundant and only occurring along river banks in small, isolated patches. Notwithstanding, this species is the dominant pioneer species in Hong Kong, grows to about 6-7 m and is a major source of detrital energy input into tidal shrimp ponds at Mai PO.
Litter production by K. candel at Mai PO falls within the documented range of c. 1-15 t ha- yr- for mangroves (Christensen, 1979; Odum et al., 1982; Bunt, 1982; Twilley et al., 1986), but is considerably higher than the value of 8.52 t ha- yr- recorded for the same species by Lin et al. (1985). According to the physiognomic classification of Lugo and Snedaker (1974), the K. candel stands in the shrimp pond may be regarded as belonging to the fringing forest type, with only rare inundation by water and a fairly high litter production. Since the stands are only covered for < 10% of the time, they may also be classified as the equivalent to inundation classes 4 or 5 of Watson (1928). This charac- teristic seems to be important in the production and turnover of litter at the stands.
With this annual rate of litter production, the overall litter input into the entire pond is estimated to be 28.77 t yr- (equivalent to 1.268 x lO*kcal yr-). The great majority (24.92 t, 86.6%) of this, unlike most other mangroves, is retained on the forest floor. The energy equivalent of this resident litter is about 1.291 x lo* kcal. This high standing litter biomass also indicates that the residence time for the litter at these stands is extra- ordinarily long. Poovachiranon (in press) recorded standing litter biomass of only 9.93 g dry wt. mP2 (0.0993 t ha-) under dwarf K. candel normally inundated by tides at Three Fathoms Cove, Hong Kong. The residence time at Mai PO is also much higher than all values reported upon by Christensen (1978), Gong et al. (1984) and Twilley et al. (1986), where most records were obtained from normal tidally inundat...