According to Hong and San (1993), the growth rate of Rhizophora apiculata in Vietnam is highest between 10 and 15 years. Growth performance depends on a number of factors such as soil texture, tide amplitude, salinity and tree density. Growth was higher in areas with salinities 10‰ to 20‰, and the growth rate declines in sites with salinities 25‰ to 35‰. It was reported that in Ca Mau (southern Vietnam), R. apiculata younger than 10 years were replanted in the intertidal zones with silt-clay soil; the annual diameter and height increments were 0.74 cm and 0.81 m, respectively. We did not measure the annual growth of planted seedlings but it is clear that the growth rate of R. apiculata planted in 2004 in Dam Bay was lower, since they reached only 1.5 m in 4 years and about 3 m in 10 years.
Growth rate of Rhizophora stylosa and Kandelia candel planted on sandy-clay mudflats in Ha Thin Province (central Vietnam) was low (only 0.40 to 0.87 m in 3-year period) due to the unfavorable soil texture and climatic conditions (Hong and San 1993). So the low growth rate of seedlings planted in 2004 in Dam Bay may be caused by the same reasons as in Ha Thin.
Seedlings planted in 2007 in lower intertidal zone in Dam Bay have not grown sufficiently during 6 years. The reasons of their poor condition may be due to the almost permanent inundation by sea water, so this site is not suitable for planting this mangrove species.
Gastropod species composition
Fifty-two gastropod species were found in three mangrove associations. It is difficult to assess species richness of studied associations due to the lack of information about faunal richness in mangroves of central Vietnam. For other parts of Vietnam, the list is available only for a natural mangrove association in northern Vietnam, where 70 species were recorded (Hong and San 1993). Analysis of this list revealed that several included species that were collected in the lower intertidal zone, e.g., Conus striatus and C. coronatus, both predatory species in the subtidal zone, Turbinidae, that are typical to coral reefs, one species of Viviparidae, that inhabit only fresh water, and some trochids such as Angaria delphinus (=laciniata), which normally inhabit subtidal habitats, may not have been collected alive and that these empty shells were brought to the mangroves by hermit crabs or currents. If we exclude from this list the species that are unlikely to belong to the mangrove ecosystem, we have nearly the same species number (53 species) as in our study. Therefore, the observed diversity of molluscs in mangroves of central Vietnam appears similar to that of northern Vietnam.
The number of species recorded in this study is similar or even higher than in other areas of south-east Asia, for example, approximately 50 species were found in Malay peninsula (Ashton et al. 2003), 56 species were found in the Indian Sundarbans including areas adjacent to mangrove trees biotopes (Dey 2006), 30 gastropod species were found in natural and replanted mangroves on the west coast of Thailand (Macintosh et al. 2002), and 32 species were collected on the coast of south Thailand (Sri-aroon et al. 2005).
Probably the highest number, some 151 gastropod species in total, were found in mangroves of Abatan river on Bohol Island, Philippines (Lozouet and Plaziat 2008). The mentioned locality is characterized by a wide variety of habitats - from nearly freshwater, with Nypa palms, to typically full salinity marine areas. The authors restricted the number of species, which have been collected exclusively within mangroves to 65, although due to a large number of singletons, this number is definitely an underestimation.
Species richness in planted mangroves in Dam Bay alone (30 species) increased significantly during monitoring years, with appearance in 2008 and slow growth of number of predominantly mangrove-associated species (Littoraria). Nevertheless, most of predominantly mangrove-associated species are still absent in Dam Bay plantation (e.g., Potamididae - Cerithideopsilla djadjariensis, C. microptera, Cerithidea quadrata, Terebralia spp., some others) despite their presence in natural association in Nha Phu and pelagic development (Houbrick 1991) that should facilitate the species dispersal. We are unable to account for the absence of these species in Dam Bay at the moment. It should be mentioned that the relatively high number of recorded species is due to the continuous observations, and in any given year, the number of collected species was much lower and never exceeded 19 (in 2011), thus making it the least rich in gastropods among all studied sites in the Nha Trang area.
In the natural mangrove association in Dam Bay, we found 19 gastropod species. This was 11 species less than the total number observed in the mangrove plantation, but the sampling efforts in the latter were considerably higher. The main difference in species composition between mangrove plantations and the natural mangrove association in Dam Bay was that species associated with intertidal rocks were absent in mangrove plantations, such as Pictocolumbella ocellata, Tenguella musiva, Drupella margariticola, Planaxis sulcatus, and Nerita undata. On the contrary, some species that were common in the plantations were absent in the natural association, such as Clithon oualaniensis and Cerithideopsilla cingulata. This could not be explained only by differences in abiotic factors between the sites, and this difference in species composition may be possibly related to the ecology of these species. Amongst the predominantly mangrove-associated gastropods, only two Littoraria species were found here.
Species diversity of Nha Phu (28 species in single collecting year) is much higher than in other sites and is comparable with the total number of species in mangrove plantation (collected in 2005 to 2013), though species composition is different here. The most diverse gastropod family in Nha Phu was the Potamididae, while in the mangrove plantation, only one species in this family was found. Most of the species of this family except Cerithideopsilla cingulata are considered as predominantly mangrove associated (Reid et al. 2008). Some species of Ellobiidae are considered as one of the main components of mangrove-associated gastropods (Reid et al. 2008, Reid et al. 2013), and they were present only in Nha Phu. These predominantly mangrove-associated species are mostly arboreal, and this can explain the fact that NP mangroves have lower average species richness, as shown by quantitative samples taken on the surface of sediments (only 2.8 species per sample) comparing with DB (six species per sample in year 2013).
A possible explanation of the absence of key groups of predominantly mangrove-associated gastropod species in both study areas in Dam Bay may be due to differences in the types of vegetative associations in Dam Bay and Nha Phu. In Dam Bay, the mangrove trees comprise small monospecific associations, while at Nha Phu the mangrove association has more complex vegetation structure, which is also much larger than both associations in Dam Bay and experiences a wider range of hydrological conditions. This confirms the statement of Macintosh et al. (2002) about the correlation between the diversity of mangrove vegetation and mangal-associated fauna.
Biomass and abundance of Gastropoda
Only gastropods that inhabit the surface of the substratum were used for analysis of biomass and abundance and therefore the following discussion has some limitations, since most of predominantly mangrove-associated species (mostly living on the roots and branches of the mangrove trees) were not quantitatively sampled.
The mangrove plantation in Dam Bay is characterized by high level of micro heterogeneity. This is confirmed by very high values of standard deviation in biomass and abundance indexes. The abundance of gastropods strongly depends on proximity of the frame to the nearest tree. In general, the samples, collected in close proximity, were poorer than samples taken at some distance from the trees (pers. obs.). This heterogeneity obscures the general tendencies. In the discussion below, we used the average values of biomass and abundance.
In the upper intertidal zone, biomass and abundance varied from year to year and both parameters significantly increased during monitoring period (Figure 7). In later years, biomass and abundance of Batillaria zonalis and Cerithideopsilla cingulata were relatively high, while in the early years (2005 to 2006), the two species were absent. One might expect that the parameters would remain stable, since no changes in vegetation happened in the upper intertidal zone over the period of observation. Thus, the changes in gastropod population parameters in the upper intertidal zone may have been caused by changes in the mid-intertidal zone after planting the mangrove seedlings, and where biomass and abundance of gastropods, especially of B. zonalis and C. cingulata, increased even more dramatically in the later period of monitoring. Low biomass and abundance or even absence of gastropod molluscs in mid-intertidal zone in 2005 to 2006 can be explained by the fact that molluscs just started to populate the young plantation.
Both in upper and mid-intertidal zones in late period (2008 to 2013), biomass and abundance of gastropods increased due to Batillaria zonalis and Clithon oualaniensis. Batillaria zonalis, Cerithideopsilla cingulata, and C. oualaniensis occur in different habitats in addition to the mangal - in fish ponds, estuarine systems, intertidal muddy, and sand banks sometimes in high densities (Vohra 1970; Grüneberg and Nugaliyadde 1976; Kamimura and Tsuchiya 2004; Fujioka et al. 2007; Tan and Clements 2008). In the mangrove plantation, these species can be considered as opportunistic. They penetrated the mid-intertidal zone in Dam Bay with the appearance of mangrove vegetation providing enrichment of detritus and conditions suitable for the growth of micro-algae on which these species feed (Kamimura and Tsuchiya 2004; Vohra 1970).
The facts that the mangrove plantation in Dam Bay is highly dominated by opportunistic species together with paucity of predominantly mangrove-associated gastropods suggest that the whole ecosystem is not balanced yet. Macintosh et al. (2002) obtained a similar result - the species diversity indexes were lower, and abundance was higher in planted mangroves than in mature mangroves. Therefore, we can expect that biomass and abundance of these species eventually will reach a plateau and may even decrease as the whole system reaches an equilibrium.
In the natural mangrove association in Dam Bay, the dominant species was the same as in the plantation - Batillaria zonalis. Although this species occurred in comparatively lower density in the natural mangrove, its biomass was comparable with that of the mangrove plantation, due to larger sizes of individuals in DBn. Two other most abundant species in the plantation, namely Cerithideopsilla cingulata and Clithon oualaniensis were absent in the natural mangrove. This difference can probably be explained by unequal amount of organic matter in the substratum of these two associations. Since the natural association in Dam Bay is represented only by a narrow belt of trees, large amounts of organic matter are washed away by tides, while in thicker mangrove plantations, it is reposed, thus allowing opportunistic species to attain high densities. Since this natural association corresponds to the upper intertidal zone of the mangrove plantation area with mature Rhizophora trees, the low abundance of opportunistic species confirms the fact that mid-intertidal zone is strongly affecting the fauna of upper intertidal zone. Cerithideopsilla cingulata and Clithon oualaniensis also dominated quantitative samples collected in Nha Phu. This suggests that the natural mangrove association despite its richness in gastropods is unbalanced too. One of the reasons may be the inputs from eutrophicated ponds through the channels in the natural mangrove vegetation.