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Species composition, habitat structure and sedimentation in a Sonneratia caseolaris stand at the Lam River estuary, Vietnam

This study aims to provide some information for development of silvofishery systems with a mangrove (S. caseolaris stand) for shrimp-culture ponds at the Lam River estuary, Vietnam. To achieve this objective, species composition, habitat structure and sedimentation effect in the S. caseolaris stand distributing along the canal for shrimp culture were surveyed. The study reveals that the stand contains four species or more under the canopy; dominant understory species are Aegiceras corniculatum, Cyperus malaccensis and Acanthus spp. (A. ilicifolius and A. ebracteatus). A. corniculatum was observed expanding its distribution area toward the low tidal zone. As a result, the habitat for seedlings/saplings of this species becomes significantly lower in land height than that of Acanthus spp. (ANOVA; p<0.05), but not different from that of S. caseolaris. Sedimentation rates of A. corniculatum and Acanthus sp. recorded were 13.2, 2.9 g m−2 h−1 and 13.0 4.4 g m−2 h−1, respectively. S. caseolaris sparsely grows with less developing of pneumatophore, which effectively traps sediment, at the early stage of succession. Considering reinforcement of sedimentation effect, especially right after restoration of the mangrove in the silvofishery systems, A. corniculatum and Acanthus spp. with S. caseolaris are recommendable to be introduced to the systems.


Introduction
Loss of mangroves due to mainly conversion to shrimp-culture ponds has been continued in Vietnam despite of increasing awareness of the value of mangrove ecosystems in the world (De Graaf and Xuan, 1998;Thu and Populus, 2007;FAO, 2008). Hung Hoa commune Vinh city Nghe An province located at the Lam River estuary is no exception. Right after the dike road was constructed, providing convenient transport infrastructure to this region in 2005, shrimpculture pond area had been drastically increased according to Hung Hoa Commune People's Committee. Approximately 1,000 ha of shrimp-culture ponds exist in Nghe An province, accounting for 0.4% of total shrimp-culture ponds in Vietnam (Phuong, 2014a). Considering the situation of mangroves mentioned above, efforts for mangrove restoration had been implemented in Nghe An province, however, it had been conducted on the coastal line, but not in the inland area due to aiming of disaster prevention from high flood tide damages (IFRRCS, 2011). Thus, Takahashi et al. (2019) recommended introducing silvofishery systems with mangrove forests in this region, especially inland area. Silvofishery systems i.e. mixture of silviculture and aquaculture ponds nowadays have been widely adopted in Asian tropical countries (Fitzgerald, 2002;Clough et al., 2002;Ahmed et al., 2018). It was firstly developed in Myanmar and was introduced to Indonesia in 1978 (Takashima, 2000). Benefits from this system is not only re-existence of mangrove forests but also feeding naturally shrimps by organic matters from mangroves (Ashton, 2008), and prevention from pond water contamination (Hastuti and Budihastuti, 2017). Therefore, it contributes to keep good water condition of pond water for longer time comparing to ordinary shrimp-culture ponds abandoned for three to four years in general after operation due to infected by diseases (Khoon et al., 2004).
Mangroves for silvofishery systems, in many cases, consist of only the species forming canopy such as Rhizophora apiculata, Rhizophora mucronata, Avicennia marina (Clough et al., 2002;Hastuti and Budihastuti, 2017), but not contain species under canopies. Sonneratia caseolaris forest, which is a typical mangrove in the study area Phuong (2014b), however, involves some species under canopies, Aegiceras corniculatum (shrub) and Acanthus spp.; A. ilicifolius and A. ebracteatus (herb) (Takahashi et al., 2019). Mangrove forest accompanied with under canopy species could totally provide ecological services (Chen et al., 2016;Wei et al., 2020). In order to introduce under canopy species such as A. corniculatum and Acanthus spp. to silvofishery systems, their habitats especially relation to land height and their ecological functions are required to be understood for designing and determination of water operation framework. In terms of the habitats there have been a lot of qualitative descriptions, for A. corniculatum habitat is lower edge of the river estuary (Clarke, 1995), for Acanthus spp. distributes along the tidal creek or higher edge of the mangrove forest (Duke, 2006), however, not so many quantitative descriptions of habitats (Takahashi et al., 2019).
Sedimentation effect is one of the important functions for water purification to be considered for designing silvofishery systems. There have been many previous studies of sedimentation effect in different mangrove stands, trapped in Ceriops sp. stand and Rhizophora sp. stand (Furukawa and Wolanski, 1996), in Avicennia sp. stand and mixture stand of Avicennia sp. and Rhizophora sp. (Kathiresan, 2003). Bird and Barson (1977) reported Avicennia sp. with pneumatophore is considered to be more effective in trapping sediments than stilt rooted mangroves such as Rhizophora spp. or buttressed mangroves such as Bruguiera spp. Focusing on A. corniculatum and Acanthus spp. Santen et al. (2007) reported sedimentation rate of mixture of A. corniculatum (75%) and Acanthus ilicifolius (20%), but not separately evaluated. With background mentioned above, this study aims to elucidate species composition, habitat structure and sedimentation rate in the S. caseolaris stands, to provide some information for development of silvofishery systems with multi-layered mangrove stands for shrimp culture.

Study site
The present study was conducted at the mangrove stands of which the highest layer is formed by S. caseolaris in the canal network system connected to the estuary of the Lam River ( Fig. 1(a)). The mangroves in the study site, thus, are inundated regularly by brackish water except for the water gate for irrigation to the shrimp-culture ponds being operated. The Lam River originates from Nam Can area in Laos, flowing for 556 km mainly through Nghe An province, discharging into Tonkin Bay (WVS, 2011;WB, 2012). Nghe An province is affected by South West monsoon from May to October; it is rainy and temperature reaches 30 -35°C, and affected by North East monsoon from November to April; it is dry and temperature lows of 14 -16°C. Annual average rainfall is 1,968 mm yr −1 (Vinh city; 18°40 N, 105°40 E) (Averyanov et al., 2003;Giang et al., 2014). The present study consists of two components; one is a vegetation survey (R1 -R4 on Fig. 1(b); 18°41 24 N, 105°45 38 E nearby shrimp-culture ponds ( Fig. 1(c)), and another one is an evaluation of sedimentation effect of A. corniculatum and Acanthus sp. populations (St.1 and St.2 on Fig. 1

Habitat structure 2.2.1 Cross section of the lands
Transects were set up, extending from the shoreline (at 7:00 am, on July 15th, 2019; Fig. 2) to the landward end of the vegetation. Their width is four meters; two meters for both sides from the center line. Above the center line the tape measure was set up horizontally and perpendicular to the shoreline kept by a level and a compass, and height from the land surface to the tape measure was recorded every meter. After cross section survey in the field, the measurements were converted into land height from datum line (LH) based on the tide fluctuation chart in Cua Hoi; 18°48 N, 105°46 E (OC, 2019). Cross section survey was conducted in four transects (R1 -R4) on July 15th and 16th, 2019.

Vegetation
Every plant individual occurring in the transects was recorded with species name, its position and LH, growth stage, plant height and diameter. For woody species, position was described with two parameters; distance from the shoreline and distance from the center line. For herbaceous species, distribution range in the direction of the center line was recorded. LH of each individual plant was represented by that on the center line. For woody species (S. caseolaris and A. corniculatum) it was divided into three age classifications; seedlings (height ≤ 30 cm), saplings (height > 30 cm, diameter < 10.2 cm) and trees (diameter ≥ 10.2 cm) (Good and Good, 1972). Both S. caseolaris and A. corniculatum at tree growth stage have reproductive ability based on observation in the study site. Diameter was measured at intermediate point of tree height (tree height < 2.6 m), except for two S. caseolaris individuals of which breast height diameter (at 1.3 m, tree height ≥ 2.6 m) can be measured (Batcheler and Craib, 1985). If trees are branched, diameter of all the branches were measured. Tree height and diameter were not measured for the tree individuals of which height is less than 30 cm and diameter is less than 1 cm. Vegetation survey was conducted at the same time as cross sections were measured.

Sedimentation effect
Sediment traps were set up at two different places covered by A. corniculatum at 1.20 m in LH (St. 1) and Acanthus sp. at 1.30 m in LH (St. 2) in the same transect (S1) dominated by S. caseolaris at the highest layer (Takahashi et al., 2019) (Fig. 1(c), Fig. 2(a, b). Plants cover the land by 80% for both St. 1 and St. 2 ( Fig. 2(c)), they are not completely submerged even at high tide.

Sediment traps
The sediment trap is made of seven cylindrical plastic bottles gathered (3 cm in diameter × 9.5 cm in height for each bottle), one bottle is surrounded by six bottles ( Fig. 2(c)). It is anchored (connected to the weight with strings which can stretch) and is floated up and down corresponding to water level fluctuation, i.e. if the land is not submerged by brackish water it is on the land, but if it is being submerged it will be floated up gradually and suspended in the water. Maximum height from the land to the top end of the trap is 0.2 m, thus water with sediments enters the traps at the 1.40 m ≤ water level for St. 1 and at the 1.50 m ≤ water level for St. 2. Cross-sectional area of the trap was 49.455 cm 2 [π(3/2)2 × 7].

Sediment and water collection
Sediment collection by the sediment traps was conducted at rising tide and ebb tide, separately for eight times at rising tide and seven times at ebb tide from August 18th to November 30th, 2019, mainly in rainy season (May to October). Sedimentation rate (SR) was calculated by the equation as follows: Water samples were collected as well when sediment traps were set up at high tide. Sediments and water samples were immediately brought to the laboratory and measured total quantity of sediments trapped and analyzed total suspended sediment (TSS) of water samples with protocol of ISO 11923: 1997. Water salinity was measured in the study site immediately after samples were collected by 'TS-391 Thermo Salinity Meter' (As One Co., Ltd, Japan). Fig. 3 displays the cross sections; R1 -R4. LH of the transects are gradually upward to inland with average gradients; 3.6% -4.1%. S. caseolaris stand grows intertidal zone (between mean high-water level (MHWL) and mean low-water level (MLWL)) and LHs are close to mean water level (MWL) or slightly higher than MWL.  thus spp. and Cyperus malaccensis. Beside S. caseolaris woody species are A. corniculatum and Derris trifoliata growing in the transects. The S. caseolaris stand in the study site has two stories in structure, S. caseolaris is occupied at high layer (1.62 ± 0.13 m ∼ 3.88 ± 1.03 m in tree height), and low layer involves A. corniculatum (0.86 ± 0.06 m ∼ 1.41 ± 0.09 m in tree height), D. trifoliata, Acanthus spp. and C. malaccensis.  Fig. 4(c)). Fig. 4(b) displays distribution range of herbaceous species; C. malaccensis and Acanthus spp. Former is distributed widely in LH; from 1.52 m to 1.81m, while latter are distributed narrowly in LH; from 1.70 m to 1.84 m. Focusing on average LH, Acanthus spp. distribute on significantly higher LH than A. corniculatum (seedlings/saplings) (ANOVA; p < 0.05). Average LH for A. corniculatum (seedlings/saplings) is below MWL (1.71 m in LH), but that for Acanthus spp. is above MWL and difference in LH of these two is 0.11 m in average. However, seedlings/saplings and trees for A. corniculatum are not significantly different in LH. For other species, S. caseolaris and C. malaccensis distribute as the same LH as A. corniculatum (seedlings/saplings and trees) and Acanthus spp. (Fig. 5).

Habitats of S. caseolaris stand
S. caselorais stand in the study site involves four species or more at the low layer, of which A. corniculatum and Acanthus spp. are mangrove species (Phuong, 2014b). Those component species are distributed on the same LH in the stand, but focusing on the growth stage A. corniculatum (seedlings/saplings) distributes on the lower LH than Acanthus spp. Since S. caseolaris (trees) accounts for 13% to the total number (seedlings/saplings; n = 13, tress; n = 2), the stand is on the earlier stage of secondary succession. A. corniclatum (seedlings/saplings) grows on the lower land, implying A. corniculatum expands to the lower edge of the stand (Clarke, 1995) considering LH is descending toward shoreline. As a result, distribution zone of Acanthus spp. becomes relatively higher than that of A. corniculatum (seedlings/saplings); p < 0.05 (Duke, 2006). A. corniculatum propagates by crypto-viviparous seedlings whereas others are nonviviparous (Tomlinson, 1986), it could be an advantage for A. corniculatum to remain on the lower edge affected more by tide fluctuation (Jiang et al., 2019). In the case that S. caseolaris stand with A. corniculatum and Acanthus spp. is introduced to silvofishery systems, a mound for mangroves to grow should be a slope, gradually descending to the shrimp-culture pond with gradient of 3 -4%. Since S. caseolaris stand ranges from approximately 1.55 m to 1.85 m in LH ( Fig. 4(a)), relative height from the regular water surface to the top of mound is recommended around 0.2 -0.3 m and it should be submerged regularly. This geomorphological feature of the mound and water operation systems would enable A. corniculatum to propagate and expand its distribution area by itself and Acanthus spp. to be recruited on the higher land. Therefore, mixture of plantation approach and ecological restoration approach; planting minimum number of S. caseolaris and A. corniculatum and ecological restoration to form mangrove habitat is proposed (Lewis, 1999).

Sedimentation effects
SRs in A. corniculatum and Acanthus sp. populations per one tide fluctuation are 13.2 ± 2.9 g m −2 h −1 and 13.0 ± 4.4 g m −2 h −1 , respectively. The previous study (Santen et al., 2007) at Ba Lat estuary of the Red River, Vietnam reported the amount of sediments collected by the traps in the A. corniculatum pioneer       (Santen et al., 2007). Difference in these two figures might attribute to consideration without flushing out of sediments after sedimentation in the former case. SRs in this study are gross amount of sediments trapped, not considered resuspension and flushing out of sediments. Therefore, further study on actual sediment retention rates is needed to evaluate reduction of pollutant loads in the shrimp-culture ponds. Previous studies reported that herbaceous species such as salt marsh grasses tend to trap more sediments than woody mangrove species (Willemsen et al., 2016), but SRs of Acanthus sp. are not different from those of A. corniculatum in the study site. Sediment trapping is caused by hydraulic mechanism; changing current velocity and directly caught by vegetation (Willemsen et al., 2016). Acanthus sp. have a hard stem, thus both Acanthus sp. and A. corniculatum probably affect hydraulic conditions in the same way when they are submerged. S. caseolaris is tall tree up to 15 m with pneumatophore (Tomlinson, 1986). It is a canopy forming species, thus, tree density is much smaller than that of A. corniculatum (S. caseolaris; 0.04 indv. m −2 , A. corniculatum; 0.07 indv. m −2 in the study site; Table 1). When it is matured it develops pneumatophores, which are effective to trap sediments (Bird and Barson, 1977), but it does not have many pneumatophores emerged at younger growth stage. Therefore, considering sedimentation effect under canopy species; A. corniculatum and Acanthus sp. are required at early stage of restoration.