Ring width and vessel features of the mangrove Excoecaria agallocha L. depend on salinity in the Sundarbans,Bangladesh

The Bangladesh Sundarbans is the largest continuous mangrove in the world that providing crucial environmental services, particularly related to coastal protection and livelihoods of millions of people. However, anthropogenic disturbances, diseases infestation and environmental changes including sea level rise (SLR) and fresh-water flux into the delta are threatening the Sundarbans and other mangrove ecosystems worldwide. Protection of mangrove ecosystems requires knowledge on factors that mainly drive growth and vitality of tree species to evaluate which consequences can be expected from, mainly hydrology-related, environmental changes. In this study, we assessed the nature and periodicity of tree rings in Excoecaria agallocha, a wide spread mangrove species in the Bangladesh Sundarbans. We also analysed the influence of climatic factors, such as precipitation, temperature and vapor pressure deficit (VPD), and river discharge, as a proxy of salinity on ring width (RW) and vessel features, such as mean vessel area (MVA) and mean vessel density (MVD). E. agallocha forms distinct tree-ring boundary that characterized by a narrow (2–4 cells wide) band of radially flattened fibres. The RW as well as the MVA and MVD are crossdatable. The RW is mainly driven by salinity which is influenced by freshwater inputs through precipitation during monsoon along with river discharge January to April. The MVA and MVD responded to similar seasons and months as RW, but mostly with opposite signs in MVD. The results suggest that fresh water inputs through precipitation and river discharge positively influence the radial growth of E. agallocha in the Sundarbans. The RW and vessel features can be used as proxies to explore the growth dynamics of this species, especially in relation to global environmental changes.     

Climatic Signals in Tree Rings of Heritiera fomes Buch.-Ham. in the Sundarbans,Bangladesh

Mangroves occur along the coastlines throughout the tropics and sub-tropics, supporting a wide variety of resources and services. In order to understand the responses of future climate change on this ecosystem, we need to know how mangrove species have responded to climate changes in the recent past. This study aims at exploring the climatic influences on the radial growth of Heritiera fomes from a local to global scale. A total of 40 stem discs were collected at breast height position from two different zones with contrasting salinity in the Sundarbans, Bangladesh. All specimens showed distinct tree rings and most of the trees (70%) could be visually and statistically crossdated. Successful crossdating enabled the development of two zone-specific chronologies. The mean radial increment was significantly higher at low salinity (eastern) zone compared to higher salinity (western) zone. The two zone-specific chronologies synchronized significantly, allowing for the construction of a regional chronology. The annual and monsoon precipitation mainly influence the tree growth of H. fomes. The growth response to local precipitation is similar in both zones except June and November in the western zone, while the significant influence is lacking. The large-scale climatic drivers such as sea surface temperature (SST) of equatorial Pacific and Indian Ocean as well as the El Niño-Southern Oscillation (ENSO) revealed no teleconnection with tree growth. The tree rings of this species are thus an indicator for monsoon precipitation variations in Bangladesh. The wider distribution of this species from the South to South East Asian coast presents an outstanding opportunity for developing a large-scale tree-ring network of mangroves.     

Climate change impacts on Indian Sunderbans: a time series analysis (1924–2008)

Climate change induced sea level rise (SLR) added with anthropogenically altered environment leads to rapid land dynamics in terms of erosion and accretion; and alteration in species diversity and productivity, more pronouncedly in sensitive ecosystems such as river deltas. Here, we tried to analyze the historical records to understand the SLR with respect to hydrological conditions, sedimentation and morphological processes. We analyzed the land transformation of few islands in Indian Sunderbans using maps and satellite images in increasing order of temporal frequency between 1924 and 2008, which revealed that both the erosion and accretion processes go hand in hand. Increase of downstream salinity due obstruction in upstream has led to decrease in transparency of water causing decrease in phytoplankton and fish, density and diversity in the central sector of Indian Sunderbans. Analysis of the above ground biomass of three dominant mangrove species (Sonneratia apetala, Avicennia alba and Excoecaria agallocha) revealed better growth in the western sector compared to the central sector. The study reveals the cumulative effect of climate change and anthropogenic disturbance on the diversity and productivity in World’s largest ecosystem; and advocates mangrove plantation and effective management of freshwater resources for conservation of the most vulnerable and sensitive ecosystem.     

Allometric growth and carbon storage in the mangrove Sonneratia apetala

Allometric growth reflects different allocation patterns and relationships of different components or traits of a plant and is closely related to ecosystem carbon storage. As an introduced species, the growth and carbon storage of Sonneratia apetala are still unclear. To derive allometric relationships of the mangrove S. apetala and to estimate carbon storage in mangrove ecosystems, we harvested 12 individual Sonneratia apetala trees from four different diameter classes in the Futian National Nature Reserve, Guangdong, China. Allometric growth models were fitted. The results showed that diameter at breast height (DBH) and wood density were better variables for predicting plant biomass (including above- and below-ground biomass) than plant height. There were significant power function relationships between biomass and DBH, with a mean allometric exponent of 2.22, and stem biomass accounted for 97% of the variation in S. apetala total biomass. Nearly isometric scaling relationships were developed between stem biomass and other biomass components. To better understand the carbon stocks of the S. apetala ecosystem, we categorized all trees into five age classes and quantified vegetation carbon storage. The S. apetala vegetation carbon storage ranged from 96.48 to 215.35 Mg C ha^?1, and the carbon storage significantly increased with stand age. The allometric equations developed in this study are useful to estimate biomass and carbon storage of S. apetala ecosystems.

     

Sonneratia apetala Buch.Ham in the mangrove ecosystems of China: An invasive species or restoration species?

By the end of 1990s when China initiated a 10-year mangrove reforestation project, the mangrove forest area had decreased from 250,000 to 15,000 ha. Over 80% of current Chinese mangroves are degraded secondary forests or plantations. As an initial restoration and reforestation effort, Sonneratia apetala, a native of India, Bengal and Sri Lanka, was introduced in 1985 to Dong Zhaigang Mangrove Nature Reserve in Hainan Island from Bengal. It has then been introduced into other places since 1991. However, the further use of the species is becoming increasingly controversial as there are emerging signs that it may become invasive in certain locations. A comprehensive evaluation of the species’ condition in China regarding benefits and risks is critically needed. Here, we map the introduction and dispersal routes and monitor the growth of S. apetala in China from 1985 to 2006. S. apetala grows fast and performs well in the introduced 2300 ha muddy beaches area. It greatly improves the soil fertility and shows a suite of suitable characteristics as a pioneer restoration species. Currently, no natural invasion of S. apetala has been observed in the northern mangrove area. However, invasion into natural forests does occur in southerly locations such as Shenzhen, Zhanjiang and Dong Zhaigang. In these locations, S. apetala exhibits invasive characteristics such as overgrowth and high spreading ability that evidently affects local mangrove ecosystem structure and function. While the species clearly offers some benefits at some locations where it cannot naturally invade, it appears harmful to other native mangrove species, posing a major practical problem to both ecologists and land managers. This situation will be similar to previously imported non-native and invasive intertidal wetland species, Spartina alterniflora (smooth cordgrass), with similar results and problems.     

Restoration of mangrove plantations and colonisation by native species in Leizhou bay,South China

To examine the natural colonisation of native mangrove species into remediated exotic mangrove stands in Leizhou Bay, South China, we compared soil physical–chemical properties, community structure and recruitments of barren mangrove areas, native mangrove species plantations, and exotic mangrove species—Sonneratia apetala Buch.Ham—between plantations and natural forest. We found that severely degraded mangrove stands could not regenerate naturally without human intervention due to severely altered local environments, whereas some native species had been recruited into the 4–10 year S. apetala plantations. In the first 10 years, the exotic species S. apetala grew better than native species such as Rhizophora stylosa Griff and Kandelia candel (Linn.) Druce. The mangrove plantation gradually affected soil physical and chemical properties during its recovery. The exotic S. apetala was more competitive than native species and its plantation was able to restore soil organic matter in about 14 years. Thus, S. apetala can be considered as a pioneer species to improve degraded habitats to facilitate recolonisation by native mangrove species. However, removal to control proliferation may be needed at late stages to facilitate growth of native species. To ensure sustainability of mangroves in South China, the existing mangrove wetlands must be managed as an ecosystem, with long-term scientific monitoring program in place.     

Development of a salinity-secondary flow-approach model to predict mangrove spreading

This paper presents a new salinity-secondary flow-approach (SSA) model for identifying areas likely to be colonized by mangrove species based on two indices: water salinity concentration and river secondary flow intensity, R/W (the ratio of radius of curvature to river width). The mangrove Kandelia obovata is spreading rapidly in the Tanshui River system and therefore resulting in flooding impact to riparian wetlands; however, few studies have examined the dispersal characteristics of this species on the reach scale. According to the literature review and our observation, the salinity is the major factor in determining the spreading capabilities of mangroves. In addition, the effect of secondary flow can also facilitate mangrove invasion through the creation of bare mudflats.The case study of the Tanshui River system shows that the SSA model can be used for the determination of the habitat requirements and thus the dispersal capabilities of K. obovata. The results of the study show that the optimum conditions for the growth and dispersal of K. obovata exist in waters with mean annual salinity levels that are higher than 5 ppt (parts per thousand). Although K. obovata can survive in brackish wetlands with mean annual salinity levels lower than 5 ppt, its spreading capability is impaired. In tidal freshwater wetlands with mean annual salinity levels that are lower than 0.1 ppt, K. obovata cannot survive. The study also found that the R/W lower than 3 led to large mudflats, which provide an ideal environment for the growth of mangroves, due to the secondary flow.It shows that the SSA model can identify the potential habitat of mangrove in tidal region, so it might help with not only estuarine wetland management but also mangrove restoration project, especially for the sea level rise effect and its impact on potential mangrove invasion.     

Competitive Control of an Exotic Mangrove Species: Restoration of Native Mangrove Forests by Altering Light Availability

Recently, many studies have focused on the possibility of restoring mangrove ecosystems by introducing fast‐growing mangroves. However, methods for managing an exotic fast‐growing species to restore mangrove ecosystems and at the same time preventing invasion by introduced species remains unclear. Sonneratia apetala Buch‐Ham is one example of an exotic mangrove with both high ecological value and potential risk for invasion after introduction. To investigate the possibility of reducing the potential for invasion by altering light availability, we simulated different irradiances of S. apetala understory in the greenhouse. For each irradiance treatment, three levels of competition between S. apetala and native mangroves Aegiceras corniculatum (L.) were used: no competition, intraspecific competition and interspecific competition. Compared with A. corniculatum, S. apetala showed a significantly higher growth rate for both height and biomass accumulation under full irradiation. Compared to the full irradiation treatment, the shading treatment significantly reduced the height, total biomass and biomass allocation to leaves of S. apetala by 61.31, 71.0, and 76.2%, respectively, whereas the growth of A. corniculatum was not affected. The results suggested that lowering light availability could inhibit the growth of S. apetala and increase the competitiveness of A. corniculatum. Planting introduced fast‐growing mangroves at a density of approximately 2,000 plants/hm² is an effective strategy for preventing potential invasion and restoring wetland habitats. By taking advantage of the differences in shade tolerance between fast‐growing exotic mangroves and native mangroves, introduction of fast‐growing mangroves in coastal areas could have huge potential for reforesting mangrove ecosystems.     

Arbuscular mycorrhizal fungi in two mangroves in South China

The symbiosis between arbuscular mycorrhizal fungi (AMF) and mangrove plant species was investigated in two mangrove swamps in south China. AMF were mostly found in the form of hyphae and were commonly associated with all the mangrove species we investigated. Six AMF species belonging to the genera Glomus or Acaulospora were identified. Multiple step-wise linear regression analyses showed that hydrological conditions and phosphorus levels in the rhizosphere were the main abiotic factors affecting the colonization of mangrove species by AMF. A greenhouse experiment was conducted to evaluate the effects of AMF inoculation on the growth and nutrient uptake of a true mangrove plant species, Sonneratia apetala B. Ham. The inoculated AMF significantly improved growth, resulting in greater plant height, diameter at ground level and plant biomass, as well as increased absorption of N, P and K. These findings suggest that AMF play important roles in mangrove ecosystems.     

Reasons for the limitation of mangrove along the west coast of northern Peru

Along the west coast of South America mangroves are found only outside the area influenced by the cold Peruvian Current. At 6° S (near ‘Cerro Illescas’) the current turns west to the open sea in the direction of the Galapagos Islands. Dense mangrove vegetation with a tree height up to 15 m occurs only north of 3° 35′ S from the delta of the river Tumbes (Peru). At 3° 44′ S some small individuals of Rhizophora and at 5° 30′ S a small stand of Avicennia can be found. In the transition zone between 3° 35′ and 6° S no mangrove forest occurs. The reasons for the lack of mangal in the transition zone are:

1. Evapotranspiration and atmospheric humidity show significant differences between the mangrove region and the transition zone. In this zone soil conditions like salinity, water and organic matter content and the geological structure can also be considered as inhibiting mangrove growth.
2. Topographic conditions in this zone are not suitable for mangal and the lack of a regular annual flow from rivers provides a sharp limit for the existence of mangal in the delta of the river Tumbes. Nevertheless, cultivation of mangrove species south of the mangrove region is possible and seems promising.

     

Inter-proxy evidence for the development of the Amazonian mangroves during the Holocene

The dynamics of mangrove forest on the island of Marajó (Ilha de Marajó) at the mouth of the river Amazon during the past ~7,500 cal. b.p. were studied using multiple proxies, including sedimentary facies, pollen, δ¹³C, δ¹⁵N and C/N ratio, related to 15 sediment samples by ¹⁴C dating. The results allow us to propose a scheme of palaeogeographical development, with changes in vegetation, hydrology and organic matter dynamics. Today, the interior of the island is occupied by várzea freshwater herbaceous vegetation, but during the early to middle Holocene, mangroves with accumulations of estuarine organic matter colonized the tidal mud flats. This spread of mangroves was caused by post-glacial sea-level rise, which combined with tectonic subsidence, produced a marine transgression. It is likely that the relatively greater marine influence at the studied area was favoured by reduced discharge from the river Amazon, which was itself caused by a dry period that occurred during the early and mid Holocene. During the late Holocene, there was a reduction of mangrove vegetation and the contribution of freshwater organic matter to the area was higher than during the early and mid Holocene. This suggests a decrease in marine influence during the late Holocene which led to a gradual migration of mangrove vegetation from the central region to the northeastern littoral zone of the island, and, consequently, its isolation since at least ~1,150 cal. b.p. This was probably a result of lower tidal water salinity caused by a wet period that resulted in greater river discharge during the late Holocene. This work details the contraction of mangrove forest from the northeastern part of the island of Marajó under the influence of Amazon climatic changes, chronologically and spatially. This allows us to propose a model of successive phases of sediment accumulation and vegetation change, according to the marine-freshwater influence gradient. As demonstrated by this work, the use of a combination of proxies is efficient for establishing a relationship between the changes in estuarine salinity gradient and depositional environment/vegetation.     

Modeling Productivity in Mangrove Forests as Impacted by Effective Soil Water Availability and Its Sensitivity to Climate Change Using Biome-BGC

Ecosystem dynamics and the responses to climate change in mangrove forests are poorly understood. We applied the biogeochemical process model Biome-BGC to simulate the dynamics of net primary productivity (NPP) and leaf area index (LAI) under the present and future climate conditions in mangrove forests in Shenzhen, Zhanjiang, and Qiongshan across the southern coast of China, and in three monocultural mangrove stands of two native species, Avicennia marina and Kandelia obovata, and one exotic species, Sonneratia apetala, in Shenzhen. The soil hydrological process of the model was modified by incorporating a soil water (SW) stress index to account for the impact of the effective SW availability in the coastal wetland. Our modified Biome-BGC well predicted the dynamics of NPP and LAI in the mangrove forests at the study sites. We found that the six mangrove systems differed in sensitivity to variations in the effective SW availability. At the ecosystem level, however, soil salinity alone could not entirely explain the limitation of the effective SW availability on the productivity of mangrove forests. Increasing atmospheric CO₂ concentration differentially affected growth of different mangrove species but only had a small impact on NPP (<7%); whereas a doubling of atmospheric CO₂ concentration associated with a 2°C temperature rise would increase NPP by 14–19% across the three geographically separate mangrove forests and by 12% to as much as 68% across the three monocultural mangrove stands. Our simulation analysis indicates that temperature change is more important than increasing CO₂ concentration in affecting productivity of mangroves at the ecosystem level, and that different mangrove species differ in sensitivity to increases in temperature and CO₂ concentration.     

Cambial dormancy induced growth rings in <Emphasis Type="Italic">Heritiera fomes</Emphasis> Buch.- Ham.: a proxy for exploring the dynamics of Sundarbans,Bangladesh

Key message

Cambial marking experiment and cambial activity analysis offer strong evidence on existence of annual growth rings in Heritiera fomes and revealing the potential of dendrochronological applications in Bangladesh mangroves.

Abstract

Despite enormous significance in coastal protection, biodiversity conservation and livelihood support to the local communities, mangrove ecosystems have been continuously degrading mainly due to anthropogenic disturbances and climate change. Time series based on dated tree ring is an option to identify the causes of forest dilapidation. In this study, we investigated the structure and periodicity of the growth ring in Heritiera fomes, the flagship tree species of the Bangladesh Sundarbans, combining cambial marking experiment and cambial activity analysis. Distinct growth rings were found which are delineated by a band of marginal parenchyma, predominantly one cell wide but up to three and occasionally interrupted with fiber. Of the 13 trees with cambium marking experiment, one growth ring was found in each tree during a year. The dormant cambium was characterized by the abrupt boundary between xylem and cambial zone, absence of enlarging or differentiating cambial derivatives, lower number of cambial cells and thicker radial walls in cambial cells. Growth ring anomalies, i.e., wedging and partially missing rings were also found. In most of the cases, the lower part of the eccentric discs had low radial increment (<0.75 mm) and therefore the growth ring in that area merged with previous one and produced wedging or partially missing ring. However, the existence of annual rings suggests its great potential for future dendrochronological applications to reveal the dynamics of vegetation and climate in Sundarbans.

     

A comparative ecology of estuaries in Nigeria

A bar-built estuary, a drowned river valley and two river delta estuaries were compared and contrasted to elucidate the impact of some abiotic factors, notably climate, salinity, and oxygen on the distribution of the aquatic fauna and flora.Salinity was recognizable as the key factor responsible for the population dynamics in these habitats. The mangrove community is characteristically zoned, but development and distribution of the trees is restricted in view of coastal geomorphic and hydrological processes to the western banks of the estuaries.The preponderance of marine fish species in estuaries was confirmed, although the wide salinity tolerance of some prolific-breeding Cichlidae from freshwater was noted. Forty-eight species of bony fishes were recorded in Lagos Lagoon, the Escravos, and Qua Ibo estuaries and classified by salinity tolerance into stenohaline and euryhaline marine species (66%), and freshwater species (34%). The same estuarine ichthyofauna were split into trophic groups: piscivores (46%), zooplankton feeders (10%), meiobenthos feeders (27%), and macrobenthos feeders (17%).     

Seasonal Abundance of Acanthamoeba rhysodes (Singh, 1952) (Protozoa: Gymnamoebia) in a Mangrove Litter-Soil Ecosystem of Gangetic-Estuary,India1,2

Acanthamoeba rhysodes has been found to be a predominant intertidal benthic gymnamoeba in the mangrove ecosystem of Sundarbans of lower deltaic Bengal, facing the Bay. The sampling zones under study were the highest high tide regions, with characteristic mangrove litter-soil, inundated twice per month during the highest ebb of spring tide. Population abundance of this species, both in its trophic and cystic forms in the three distinct seasonal periods of pre-monsoon (March to June), monsoon (July to October), and post-monsoon (November to February) has been surveyed for over two years. These seasonal periods affect the physico-chemical parameters of the habitat substrata, including temperature, pH, and salinity. It has been found that the overall number of organisms per gram of soil attains peak value during the monsoon period. This value comes down in post-monsoon samples and is the least in pre-monsoon ones.     

Flowering patterns in three neotropical mangrove species: Evidence from a Caribbean island

The present study sought to identify the factors that drive flowering in the main neotropical mangrove species. We evaluated the effects of water regime variables and foliar meristematic activity on the flowering intensity of Rhizophora mangle, Laguncularia racemosa, and Avicennia germinans in three physiographic types of San Andres Island, Colombia. The results show that pore salinity regulates flowering intensity and periodicity in all three mangrove species. All species flowering showed significant correlations with water balance and air vapor pressure deficit (VPD). In the fringe and interior mangroves, R. mangle flowering was explained linearly by salinity (25%) and monthly change in salinity (47%), respectively. L. racemosa flowering was linked with stronger periods of foliar meristematic activity and occurred during months of relatively high water balance (54-233 mm) and low VPD (1.18-1.29 kPa). The flowering of A. germinans was triggered by water deficit conditions when the monthly pore salinity increased over 30 g L⁻¹ and, with a month delay response, when the water column height (WCH) was below ground. The flowering of A. germinans was also explained by these variables at 65% and 39%, respectively. The flowering patterns of the studied mangrove species indicate that reproduction within the neotropical mangrove community depends on seasonally contrasting water conditions on an annual basis.     

无瓣海桑与乡土红树植物混交对林地大型底栖动物的影响

选择广东省雷州市附城镇和珠海市淇澳岛沿海1年生无瓣海桑人工林,分别在林下混种乡土红树植物红海榄或木榄幼苗,对混交林和无瓣海桑纯林林地大型底栖动物群落进行比较,探讨无瓣海桑与乡土红树植物混交对林地大型底栖动物的影响。结果显示,混交林和无瓣海桑纯林之间大型底栖动物群落的优势种存在差异;相似性分析检验(One-Way ANOSIM)、等级聚类和非参数多变量标序结果均表明1年无瓣海桑+红海榄混交林、1年无瓣海桑+木榄混交林和1年无瓣海桑纯林之间大型底栖动物群落结构差异显著。研究还发现在无瓣海桑人工林林下混交红海榄或木榄这两种乡土红树植物,可提高林地底栖动物的生物量和物种多样性。BIOENV分析说明大型底栖动物分布与红海榄或木榄这两种混交的乡土树种的凋落物量密切相关,这进一步证实了混交乡土红树植物对林地底栖动物多样性和分布的影响。两个研究地实验结果均显示,在无瓣海桑林下种植木榄的效果要优于红海榄,表现在木榄的平均苗高、凋落物量、凋落物量占群落凋落物总量百分比和幼苗成活率均高于红海榄,其对提高林地大型底栖动物生物量和物种多样性的效果也明显优于红海榄。     

无瓣海桑和白骨壤植株根系时空分布特征

无瓣海桑和白骨壤是我国红树林造林中较具代表性的两种植物,通过对10龄无瓣海桑纯林、10龄白骨壤纯林、6龄无瓣海桑纯林和6龄白骨壤纯林的调查分析发现:1)无瓣海桑和白骨壤植株根系水平分布的半径和地下根系垂直深度均随树龄的增加而增加,其中10龄无瓣海桑和白骨壤植株的水平分布半径分别为30.6 m和3.85 m,6龄无瓣海桑和白骨壤植株的水平分布半径为9.47 m和2.23 m; 10龄和6龄无瓣海桑和白骨壤植株的地下根系分布深度分别为60 cm和40 cm; 2)无瓣海桑和白骨壤植株地表呼吸根系密度、高度、基径的分布范围会随其树龄的增加而增加,但其根系密度、高度和基径总体表现为由树冠向外逐渐减小; 3)无瓣海桑和白骨壤植株地下根系主要分别于0—20 cm表层,地下根系密度随树龄的增加而增多,具体如10龄无瓣海桑和白骨壤根系分布深度为60 cm,其中76.3%和77.6%根系分布于0—20 cm深度; 6龄无瓣海桑和白骨壤根系分布深度为40 cm,其中91.9%和91.6%根系集中于0—20 cm土层。这些结果能为进一步理解红树林的促淤保滩功能提供新启示。     

Hydrographic Status of Sundarbans Mangrove, Bangladesh with Special Reference to Post-larvae and Juveniles Fish and Shrimp Abundance

Environmental factors, including water parameters were correlated with assemblages of shell and finfish post-larvae and juveniles in the 5 rivers of Sundarbans mangrove for 2 years, located in the south-west of Bangladesh. Fifteen species of shellfish belonging to 6 families and 37 species of finfish belonging to 27 families were recorded. Four species of shrimp (Metapenaeus monoceros, M. brevicornis, Macrobrachium villosimanus and Acetes spp.) and megalopa of crabs (79.3–96.83%), and 4 species of finfish (Panchax melastigma, Liza parsia, L. tade and Gobidae) (40–76%) were the most abundant species. Significant (p<0.05) inter-month variations in temperature, transparency and chlorophyll-a; inter-month and inter-river variations in salinity, conductivity and TDS were observed in the Sundarbans water. Ordination on correlation coefficients between temperature and salinity with the abundance of dominant species showed that Acetes spp. and Gobidae were abundant in high salinity and high temperature areas; Stolephorus tri in low salinity and high temperature areas; Macrobrachiun villosimanus, Leiognathus spp. and Liza tade in low salinity and low temperature areas, however, other species showed variations among 2 years of study. Out of 7 dominant shellfishes, 4 species showed positive correlation to temperature, pH and TDS; 3 species to salinity, transparency, conductivity and chlorophyll-a in the river water. Among 8 dominant fish species, 4 species were found to positively correlate with temperature and pH; 3 with salinity, conductivity and chlorophyll-a; 6 with transparency and 5 species showed positive correlation to TDS in Sundarbans water. Abundance of Penaeus monodon also showed positive correlation to observed water parameters except chlorophyll-a.     

Increasing fluctuations of soil salinity affect seedling growth performances and physiology in three Neotropical mangrove species

Background

Micro-tidal wetlands are subject to strong seasonal variations of soil salinity that are likely to increase in amplitude according to climate model predictions for the Caribbean. Whereas the effects of constant salinity levels on the physiology of mangrove species have been widely tested, little is known about acclimation to fluctuations in salinity.

Aims and methods

The aim of this experiment was to characterize the consequences of the rate of increase in salinity (slow versus fast) and salinity fluctuations over time versus constant salt level. Seedling mortality, growth, and leaf gas exchange of three mangrove species, Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle were investigated in semicontrolled conditions at different salt levels (0, 685, 1025, and 1370 mM NaCl).

Results

Slow salinity increase up to 685 mM induced acclimation, improving the salt tolerance of A. germinans and L. racemosa, but had no effect on R. mangle. During fluctuations between 0 and 685 mM, A. germinans and R. mangle were not affected by a salinity drop to zero, whereas L. racemosa took advantage of the brief freshwater episode as shown by the durable improvement of photosynthesis and biomass production.

Conclusions

This study provides new insights into physiological resistance and acclimation to salt stress. We show that seasonal variations of salinity may affect mangrove seedlings’ morphology and physiology as much as annual mean salinity. Moreover, more severe dry seasons due to climate change may impact tree stature and species composition in mangroves through higher mortality rates and physiological disturbance at the seedling stage.