I)v LA
Wetlartds Ecologq and Manugemenr 9: 141-158,
2001
0 2001 Kluwer Academic Publishers Printed In the Netherlands
Spatial and temporal variability in benthic processes along a mangrove-seagrass transect near the Bangrong Mangrove, Thailand M. Holmer*,
[email protected], N. Holmboe, E. Kristensen & N. ~ h o n ~ t h a m ' Insrir~iteof Biology, Universirl) of Socitliern Dennzark. Main Campus: Otlense Universirl), Camp~isvej55, DK5230 Odense M, Dennzark; ~ h u k e Marine t Biological Center, I? 0. Box 60, 83000, Phuket, Thailand; *E-mail: holmer@biology sdu. dk
'
Received 6 April
2000: accepted
in revised form 17 November
2000
Key words: benthic primary production. mangrove, mineralization, nutrients, seagrass, tropical sediments
Abstract Benthic primary production and nutrient dynamics were examined along a transect in the Bangrong mangrove forest in Thailand. Six stations were established extending from a high-intertidal site within the mangrove forest to low-intertidal flats and seagrass beds in front of the mangrove forest. Benthic processes ( 0 2 and CO? fluxes) and nutrient dynamics (mineralization, sediment-water fluxes, pore water and sediment pools) were measured under light and dark conditions during wet and dry seasons over a 2-yr period. The sediments were mostly autotrophic, only the mangrove forest sites were net heterotrophic during the wet season. Maximum daily net primary production was found at the non-vegetated tidal flats (10-75 mmol02 mP2d-I), where light and nutrient availability were highest. The variation in benthic mineralization along the transect was minor (1.6-4.3 mmol C02 mP2h-') and did not reflect the large changes in organic matter content (organic carbon: 0 . 7 4 . 2 % DW) and quality (C:N ratio varied from 25 to loo), suggesting that the mineralizable pool of organic matter was of similar magnitude at all sites. There was only minor seasonal variation in rates of mineralization. The net primary production showed more variation with lower rates in the mangrove forest (reduced with 71%) and higher rates at the tidal flats (increased with 172%) and in the seagrass beds (increased with 228%) during the wet season. The nutrient pools and fluxes across the sediment-water interface were generally low along the transect, and the sediments were efficient in retaining nitrogen in the nutrient limited mangrovelseagrass environment. Pools and fluxes of phosphorus were generally very low suggesting that benthic primary production was phosphorus limited along the transect.
Introduction Mangrove forests and adjacent mudflats and seagrass beds are dominant intertidal ecosystems in the tropics. The ecological importance of these areas is considerable. and most mangrove forests and seagrass beds are very productive despite the generally low nutrient concentrations in tropical coastal waters (Clouph, 1992; Stapel, 1997; Tam et al., 1998). The lack of seasonal variation in day length and temperature allows a constant primary production throughout the year (Woodroffe, 1982: Lu and Lin, 1990; Tam et al., 1998), and mangrove forests, tidal flats and seagrass beds are efficient in retaining nutrients and sustain a high productivity (Kristensen et a].. 1995; Alongi.
1996; Stapel, 1997). Mangrove soils are rich in organic matter, but the detritus is relatively nutrient-poor and refractory resulting in low net mineralization rates (Kristensen et al.. 1992, 1995). The low net rates of mineralization and pools of dissolved nutrients suggest a tight coupling between nutrient assimilation and mineralization. Therefore, most mangroves appear to be limited by the availability of nitrogen and phosphorus (Boto and Wellington, 1983; Alongi, 1996). Benthic microalgae dominates primary production at the tidal flats (Kristensen et al.. 1995). The nutrient concentrations are, however, even lower than in the mangrove forests. and the primary production is considered to be nutrient limited. Nutrient pools are also low in tropical seagrass beds and uptake of nutrients
by leaves and roots of seagmsses are often equally important (Stapel et al.. 1996). which is in contrast to temperate seagrass beds. In temperate seagrass systems, root uptake from nutrient-rich sediments supports high productivity when nutrient concentrations are low in the overlying water column (Pedersen and Borum, 1992: Risgaard-Petersen et al., 1998). Carbon. nitrogen and phosphorus budgets have indicated that mangrove forests export organic matter mainly as mangrove forest litter and import dissolved inorganic nutrients and particulate seagrass detritus (Wiebe, 1989; Kristensen et al.. 1995; Alongi, 1996; Ayakai et al.: 1998). The burial of C and N in mangrove sediments has been estimated as -2% and -8% of the input, respectively (from litterfall, benthic algal biomass and nitrogen fixation; Kristensen et al., 1995). Also, phosphorus is considered to be retained within the mangrove forest (Alongi et al., 1992), but no budgets have been published. Exported mangrove litter is an important source of detritus in seagrass ecosystems (Alongi et al., 1989; Hemminga et al., 1994), but it is not clear whether seagrass beds are net sinks or sources of dissolved inorganic and organic matter. Enhanced mineralization rates have generally been found (Blackburn et al., 1994), but the low nutrient concentrations in the sediments suggest that uptake by seagrasses exceeds mineralization and that nutrients are efficiently retained (Stapel, 1997). Some studies have observed seasonal variations in primary production and nutrient dynamics in the tropics, but the variation is often low due to the constant environment and the intense recycling of nutrients (Boto and Wellington, 1988; Alongi, 1994). Enhanced outwelling during rainy seasons may affect nutrient concentrations in mangrove forests (Boto and Wellington, 1988) stimulating primary production in the water column (Rivera-Monroy et a]., 1998). Enhanced mineralization has also been found in seagrass beds during rainy season (Blackburn et a].. 1994). but there is a serious need for more information on the seasonal variations in the tropics. The aim of this project was to examine the seasonal and spatial variability in benthic processes including nutrient cycling along a transect extending from high-intertidal sites in a mangrove, to low-intertidal mangrove sites and to adjacent mudflats and seagrass beds outside a mangrove. This transect provides a widespread variation in soils and sediment characteristics with a mix of bare and vegetated sediments. This allows for a cross examination of the seasonal variations in an important tropical coastal ecosystem. The paper describes a two-year study
of seasonal variations in benthic primary production (measured as O2 and CO, flux) and nutrient dynamics (sediment-water fluxes of inorganic nitrogen, phosphorus, organic nitrogen and organic carbon, and pore water pools) along a mangrove forest-seagrass bed transect.
Materials and methods Study site The study was conducted in the Bangronp mangrove forest (8" 03' N, 98" 25' E). Thailand, at 3 stations within the mangrove forest (2 vegetated. 1 nonvegetated) and 3 stations outside the mangrove forest (2 seagrass beds, 1 non-vegetated: Figure 1). The climate in the area is monsoonal with an annual precipitation of about 2300 mm. The dry season extends from December to April and the wet season from May to November. The annual average temperature and salinity are 28 "C and 35 PSU, respectively. The mangrove forest is tidally influenced with no major freshwater discharges. Tidal exchange occurs through one main channel system connected to smaller creeks along the length axis of the forest (Figure 1j. The tidal range is about 3 m during spring tides and I n~ during neap tides (Table I). The vegetation within the forest is dominated by Rhi,-oplzorcr crpiclrlcrrcl and R. n~ucronrrtrr.The forest has suffered from extensive cuttings during the last decades and is now surrounded by shrimp farms (Figure I). Tidal flats and seagrass beds are located along the coast just south of the mangrove forest (Figure 1). The study was conducted during 1996-97 with a total of 4 sampling periods, 2 during the dry season (January 1996 and 1997) and 2 during the wet season (August 1996 and 1997). Station MA (high intertidal) was situated close to the landward edge of the mangrove forest and was inundated only a few hours per day during spring tides (Table 1). This station was vegetated with R. apiculata. R. mucronata and Ceriops tagal and the soils were intensively bioturbated by sesarmid and ocypodid crabs (72-1 17 m-2). Station MB was situated in the mid-intertidal zone between stilt roots of R.apiculate and R. mucronata about 10 m from the main channel. This station was flooded for 7 hours during spring tides, but remained air-exposed during neap tides (Table 1). Benthic fauna were almost absent except for small infaunal sipunculids. Station MC was established adjacent to MB. but at a nonvegetated mudflat in the main channel and was flooded
Figure 1 The Bangrong mangrove forest along the northeast coast of Phuket Island in Southern Thailand (8'03' N. 98' 25' E). Sampl~ng
stations are indicated. Hatched area represents shrimp farm locations.
Table I. Elevation Im) and time (h) of inundation per 24 hours at the stations in the Bangrong mangrove estimated from the tide table for KO Taphao Noi (Phuket), January 1997. The tidal range was 2.9 and 1.0 tn for spring and neap tides, respectively. Mean sea level was 1.5-1.6 m above the lowest water level. Station
MA
MB
Height (m) above lowest water level Inundation (h) Spring tide Neap tide
2.5
2.2
5 0
7 0
MC
1.8 11 13
MD 1.4 12 17
SA 0.2 23 24
SB 0.3 23 24
for a minimum of 1 1 hours per day (Table 1). The sediment at MC was bioturbated by burrowing crabs (Ucu sp. 55 m-?) and mudskippers (Periophthamus sp. and Scnrtelao~sp.) were quite abundant. Three additional stations were established outside the mangrove forest. One station (MD) was situated about 50 m outside the forest at a bare low-intertidal mud- and sandflat. Station MD was only exposed to air during low tide (Table 1). There were no deep dwelling fauna at this station, but an abundant epibenthic fauna consisting of cerithidic snails and hermit crabs were present. Two seagrass stations were situated about 500 m (SA) and 800 m (SB) from the mangrove forest in low-intertidal sandy sediments vegetated with Enhal~isacoro~des. Station SA and SB were only air-exposed during low spring tides (Table 1). There was almost no bioturbation at these stations. only a few worm burrows and brittle stars were observed among the seagrasses. Sediment collectior~ Three sediment cores ( 8 cm i d . , length 20 cm) were sampled per station during low tide for determination of sediment-water fluxes of 0 2 . T C 0 2 (total inorganic carbon). dissolved organic carbon (DOC), dissolved organic nitrogen (DON), dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP). These cores were sampled for pore water solutes (TCO?, DIN, DIP) after flux incubations. At the same time 3 sediment cores (5 cm i.d. and 20 cm length) were sampled for measurement of particulate organic carbon (POC), nitrogen (PON) and phosphorus (TP). Cores were collected close to the vegetation, and. if possible. without cutting roots and rhizomes to prevent leakage of root exudates. Only cores with a minimum of benthic animals were chosen to limit the effect of fauna on the sediment processes. Solid phase nieasurenients Measurements on the solid phase were made on separate 5 cm i.d. cores in 1 cm (0-4 cm) and 2 cm sections (6-8. 10-12, 14-16 and 18-20 cm) depth intervals. Bulk density was determined as the weight of a known sediment volume. Water content was determined after drying at 105 "C for 12 h, and POC and PON were analyzed on a Carlo Erba 1 1 OOEA elemental analyzer according to the method of Kristensen and Andersen (1987). Total phosphorus content of the solid phase was determined by combustion at 520 "C followed by extraction with 1 M HC1 and analysed
as DIP (Andersen, 1976). Chlorophyll-a (Chl-a) content was measured by extraction with acetone. The Chl-a content was determined by spectrophotometric absorbance at 665 and 750 nm according to Strickland and Parson ( 1972). Grain size distribution was determined by wet sieving through a Wentworth series of sieves (me
