Survival rate of mangroves: A proxy to assess ecosystem health

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1 Indian Journal of Geo Marine Sciences Vol. 46 (10), October 2017, pp Survival rate of mangroves: A proxy to assess ecosystem health Abhijit Mitra 1*, Nabonita Pal 2, Arpita Saha 3, Tanmay Ray Chaudhuri 2, Pardis Fazli 4 & Sufia Zaman 2 1 Department of Marine Science, University of Calcutta, 35 B.C. Road, Kolkata , India 2 Department of Oceanography, Techno India University West Bengal, Salt Lake Campus, Kolkata , India 3 Indian Institute of Bio-Social Research and Development, Prafulla Kanan, V.I.P. Road, Kestopur, Kolkata , India 4 Department of Biological and Agricultural Engineering, University Putra, Selangor, Malaysia * abhijit_mitra@hotmail.com Received 05 April 2017 ; revised 21 April 2017 Survival rate of mangroves is highly species-specific, which significantly varies between sites. It is observed that survival rate of Sonneratia apetala, Aegiceros corniculatum, Bruguiera gymnorrhiza, Xylocarpus granatum, Nypa fruticans, Heritiera fomes and Derris trifoliate is more in the western sector compared to the central sector of Indian Sundarbans. However, a completely reverse picture with higher survival percentage in the hypersaline central sector is observed for species like Avicennia marina, Avicennia alba, Avicennia officinalis, Acanthus ilicifolius, Excoecaria agallocha, Phoenix paludosa, Ceriops decandra, Rhizophora mucronata and Aegialitis rotundifolia. Salinity seems to be the major driver for mangrove survival. Survival percentages of mangroves seedlings and their transformation into adult forms can be a potential indicator of ambient environment particularly in context to salinity. Such approach can be of importance in the ecosystem health monitoring programme preferably for regions like Indian Sundarbans, where significant spatial variation of salinity exists. [Keywords: Mangrove, survival rate, Indian Sundarbans] Introduction The ecosystem health approach considers the functionality of a system and distinguishes between functional and dysfunctional system states 1. The health of any ecosystem is a function of the behavior and dynamics of all its components in stressed and unstressed conditions. An ecosystem can be regarded as healthy if it can maintain its structure and function under extreme stress condition (resilience). The health of an ideal ecosystem can be judged from various points of view e.g., socio economic status of the people dependent on mangrove ecosystem, species richness, floral biomass, primary and secondary production rates, ecosystem services etc. In most cases, ecosystem health is considered from the perspective of abiotic component of ecosystem, as an unhealthy ecosystem poses an adverse impact on the socio-economic profile of the local inhabitants. The example of Indian Sundarbans can be cited in this context. The western sector of Indian Sundarbans has a relatively good health compared to the central part of the mangrove dominated deltaic complex. In western Indian Sundarbans, the species richness and the mangrove floral biomass are more compared to the central Indian Sundarbans, which may be the effect of significant variation of salinity 2,3,4,5,6,7,8,9,10. In addition to this, anthropogenic activities have resulted in pollution in and around the mangrove ecosystem of Indian Sundarbans, which is the World Heritage Site. In order to sustain functioning and integrity of this unique ecosystem, we require more potential indicators to monitor the ecosystem health, so

2 INDIAN J. MAR. SCI., VOL. 46, NO. 10, OCTOBER that an effective management action plan can be adopted to save guard this valuable ecosystem, which is also the homeland of Royal Bengal tiger (Panthera tigris tigris). Materials and Methods The entire network of the present study consists of three phases. Table 1- Sampling stations with coordinates in Indian Sundarbans Station Harinbari (Stn. 1) Chemaguri (Stn. 2) Longitude & Latitude / // E / // N / // E / // N Phase 1: Site selection Mangroves are influenced by many environmental functions, which shape the abundance, diversity and productivity of each mangrove fores 11. In Indian Sundarbans, the abundance, diversity and biomass are regulated primarily by salinity 4,10 and therefore the entire study area was divided into two major sectors (on the basis of salinity as the primary criterion) namely western and central Indian Sundarbans. A total of 10 stations were selected in these sectors with five in each sector (Table 1, Fig.1). Stations 1 to 5 and 6 to 10 are located in the western and central sectors respectively, which have different profiles of salinity. Earlier studies indicate that the western sector is freshening, whereas salinification is occurring in the central Indian Sundarbans owing to complete blockage of fresh water due to Bidyadhari siltation 12. Sagar South (Stn. 3) Lothian island (Stn. 4) Prentice island (Stn. 5) Canning (Stn. 6) Sajnekhali (Stn. 7) Chotomollakhali (Stn. 8) Satjelia (Stn. 9) Pakhiralaya (Stn. 10) / 0.51 // E / // N / 8.47 // E / // N / 3.62 // E / // N / // E / // N / // E / // N / // E / // N / // E / // N / // E / // N Phase 2: Monitoring the Relative abundance of seedlings The relative abundance (RA) of a species is expressed as RA = 100 The RA of seedlings was monitored from the average values of 10 quadrats (10m 10m) for each station during 2012, when the seedlings were ~ 5-6 months old. After 4 years, during 2016 the RA of the trees were calculated in the same plots. This approach is adopted to identify the dominant species in Indian Sunadarbans, which is the template of the present study. Fig.1- Sampling stations in Indian Sundarbans Phase 3: Evaluation of survival rate The population density of each species (considering both the seedling and adult stage)

3 2048 MITRA et al.: SURVIVAL RATE OF MANGROVES: A PROXY TO ASSESS ECOSYSTEM HEALTH is used to estimate the survival percentage as per the expression Survival percentage = 100 Result Mangroves are unique vegetation at the land-sea interface that can withstand tidal actions with variations in salinity. They can tolerate deterioration of ambient water and soil, and the reflection can be evaluated through several indicators like above ground biomass values, growth rate, survival rate etc. This paper aims to study the ecosystem health of Indian Sundarban mangroves in 10 selected stations using survival rate of the true mangrove species as proxy. For this we estimated the population density of the mangrove flora for the seedling and adult stages in the same quadrates and computed the survival percentage as per the standard expression. It is interesting to note that survival rate of the mangrove species are strikingly site-specific or in other words sites/location/environment have regulatory influence on the survival of mangroves and such influence varies with species. It is observed that survival rate of Sonneratia apetala, Aegiceros corniculatum, Bruguiera gymnorrhiza, Xylocarpus granatum, Nypa fruticans, Heritiera fomes and Derris trifoliate is more in the western sector compared to the central sector of Indian Sundarbans. However, a completely reverse picture is observed for species like Avicennia marina, Avicennia alba, Avicennia officinalis, Acanthus ilicifolius, Excoecaria agallocha, Phoenix paludosa, Ceriops decandra, Rhizophora mucronata and Aegialitis rotundifolia. These species exhibit more survival percentage in the central sector compared to the western Indian Sundarbans (Fig.2). Fig. 2(a)- Survival rate of S. apetala Fig. 2(b)- Survival rate of A. marina

4 INDIAN J. MAR. SCI., VOL. 46, NO. 10, OCTOBER Fig. 2(c)- Survival rate of E. agallocha Fig. 2(d)- Survival rate of A. alba Fig. 2(e)- Survival rate of A. officinalis Fig. 2(f)- Survival rate of A.ilicifolius

5 2050 MITRA et al.: SURVIVAL RATE OF MANGROVES: A PROXY TO ASSESS ECOSYSTEM HEALTH Fig. 2(g)- Survival rate of A. corniculatum Fig. 2(h)- Survival rate of B. gymnorrhiza Fig. 2(i)- Survival rate of X. granatum Fig. 2(j)- Survival rate of N. fruticans

6 INDIAN J. MAR. SCI., VOL. 46, NO. 10, OCTOBER Fig. 2(k)- Survival rate of P. paludosa Fig. 2(l)- Survival rate of C. decandra Fig. 2(m)- Survival rate of R. mucronata Fig. 2(n)- Survival rate of H. fomes

7 2052 MITRA et al.: SURVIVAL RATE OF MANGROVES: A PROXY TO ASSESS ECOSYSTEM HEALTH Fig. 2(o)- Survival rate of A. rotundifolia Fig 2 (p)- Survival rate of D. trifoliate Discussion Survival rate is a signature of the degree of the environmental stress on a particular/group of species. More the stress posed by environmental variables, less is the chance of survival. Depending on the adaptive capacity of species, the percentage of survival varies greatly among different sites. The western Indian Sundarbans has a different set of environmental parameters compared to the central sector, particularly with respect to salinity 2,3,4,510,13,14. This variation in salinity is attributed to complete blockage of the fresh water in the central sector of Indian Sundarbans due to Bidhyadhari siltation since the late 15 th century 4,12,15, which has made the region hyper-saline compared to the western sector. The western Indian Sundabans, on contrary receives fresh water through Farakka barrage discharge. Such a variation of salinity is the major driving force on survival percentage in the present geographical local as mangrove flora are capable of adjusting in different salinity gradient depending on their salt regulating capacity. The overall results thus point out very clearly the different degree of adaptation of the species to ambient salinity and suggests for an interlinking of the Hooghly River (in the western Indian Sundarbans) with Matla River (in the central sector of Indian Sundarban) for a better homogeneous salinity throughout the deltaic lobe. Apart from flood control in the western sector, the benefits of interlinking may be perceived through eco-restoration of the central sector in terms of salinity, which may be a smart approach for increment of mangrove diversity in this world heritage site. Species like Sonneratia apetala, Aegiceros corniculatum, Bruguiera gymnorrhiza, Xylocarpus granatum, Nypa fruticans, Heritiera fomes and Derris trifoliate.can survive under this scenario in the central Indian Sundarbans and thus can be a road map for eco-restoration of mangrove ecosystem health. Conclusion The entire study leads us to conclude that survival rate of mangroves can serve as potential indicator of ambient environment, particularly in terms of salinity. It is also extremely salinity specific, due to which contrasting variation is observed in the survival rate of the studied species thriving in the western and central sectors of Indian Sundarbans. The two sectors are strikingly different in terms of salinity which caused variation in the survival pattern of mangroves. It seems from the study that the mangrove species that survived better in the central

8 INDIAN J. MAR. SCI., VOL. 46, NO. 10, OCTOBER sector are resilient to climate change and subsequent sea level rise in the present geographical locale. Acknowledgement The authors acknowledge the support of the project entitled Vulnerability Assessment and development of adaptation strategies for Climate Change impacts with special reference to coasts and island ecosystems of India (VACCIN). funded by CSIR for undertaking the experimental works on the seedling samples collected from the islands of Indian Sundarbans. References 1. Somerville, MA. Planet as patient in Rapport, D., Costanza, R., Epstein, P., Gaudet, C., Levins, R. (editors), 1998, Ecosystem Health, Blackwell Science, Malden, Massachusetts, USA; (1995) Mitra, A., Gangopadhyay, A., Dube, A., Schmidt. A.C.K. & Banerjee, K. Observed changes in water mass properties in the Indian Sundarbans (Northwestern Bay of Bengal) during Current Science, (2009) Mitra, A., Banerjee, K. & Sinha, S. Shrimp tissue quality in the lower Gangetic delta at the apex of Bay of Bengal. Toxicological and Environmental Chemistry, 93 (3) (2011) Mitra, A. In: Sensitivity of Mangrove Ecosystem to changing Climate. (Springer DOI: / ) (2013) pp Banerjee, K., Sengupta, K., Raha, AK. & Mitra, A. Salinity based allometric equations for biomass estimation of Sundarban mangroves. Biomass & Bioenergy, (Elsevier), 56 (2013) Raha, AK., Bhattacharyya, SB., Zaman, S., Banerjee, K., Sengupta, K., Sinha, S., Sett, S., Chakraborty, S., Datta, S., Dasgupta, S., Chowdhury, M.R., Ghosh, R., Mondal, K., Pramanick, P. & Mitra, A. Carbon census in dominant mangroves of Indian Sundarbans. The Journal of Energy and Environmental Science (Photon), 127 (2013) Mitra, A. & Zaman, S. Carbon Sequestration by Coastal Floral Community; published by The Energy and Resources Institute (TERI) TERI Press, India, ISBN ; (2014). 8. Mitra, A. & Zaman, S. Blue carbon reservoir of the blue planet, published by Springer, ISBN (Springer DOI / ) (2015). 9. Mitra, A. & Zaman, S. Basics of Marine and Estuarine Ecology, 2016, Springer, ISBN ; (2016). 10. Trivedi, S., Zaman, S., Ray Chaudhuri, T., Pramanick, P., Fazli, P., Amin, G. & Mitra, A. Inter-annual variation of salinity in Indian Sundarbans. Indian Journal of Geo-Marine Science, 45 (3) (2016) English, S., Wilkinson, C., Baker, V. (eds.). Survey Manual for Tropical Marine Resources, 2nd Ed., Australian Institute of Marine Science (AIMS), Townsville, Australia, (1997). 12. Chaudhuri AB. & Choudhury A., Mangroves of the Sundarbans. The World Conservation Union, Dhaka (1994). 13. Raha AK., Mishra AV., Das S., Zaman S., Ghatak S., Bhattacharjee S., Raha S. & Mitra A. Time Series Analysis of forest and tree cover of West Bengal from 1988 to 2010, using RS/GIS, for monitoring afforestation programmes. The J Ecology (Photon), 108 (2014) Mitra A., Sundaresan J., Banerjee k., Agarwal SK., In: Environmental coastguards. Published by CSIR- National Institute of Science Communication and Information Resources (CSIR-NISCAIR), ISBN ; (2017). 15. Pal N., Gahul A., Zaman S., Biswas P. & Mitra A. Spatial Variation of Stored Carbon in Avicennia alba Seedlings of Indian Sundarbans. Int. J. Trend Res. Dev., 3(4) (2016)