Status of Mangrove Forests in Trinidad and Tobago, West Indies

Status of Mangrove Forests in Trinidad and Tobago, West Indies

Caribbean Journal of Science, Vol. 47, No. 2-3, 291-304, 2013 Copyright 2013 College of Arts and Sciences University of Puerto Rico, Mayagüez Status of Mangrove Forests in Trinidad and Tobago, West Indies Rahanna Juman and Deanesh Ramsewak, Institute of Marine Affairs, Hilltop Lane, Chaguaramas, Trinidad and Tobago [email protected] ABSTRACT- Much of the mangrove forests in the Caribbean have been impacted by human activities, and now they are projected to be negatively affected by sea-level rise especially where they are constrained on the landward side by built development, or starved of sediment. This study assessed the status and trends of mangrove forests in Trinidad and Tobago so that response to human induced changes and climate changes can be determined. Mangrove forests in Trinidad and Tobago were mapped between 2008-2010 using high resolution satellite imagery (2000-2007), geographic information systems (GIS) technology and extensive ground-truthing surveys. Their sizes were determined. In Trini- dad, mangrove coverage was estimated at 7,532 ha on the west coast, 1,132.8 ha on the east coast, 481.3 ha on the south coast, 0.3 ha on the north coast and 222.9 ha in Tobago. Mangrove coverage was higher than anticipated, perhaps because of inaccuracies in historical data and in some cases because of re-growth following past disturbances. While some mangrove forests are negatively impacted by land-use changes and erosion, there are instances where forests have overgrown freshwater marshes, or are expanded onto mudflats. KEYWORDS: Mangrove forest, baseline maps, land-use change, coastal erosion, climate change INTRODUCTION an would be variable since there is a wide range Coastal areas worldwide are vulnerable of wetland types and geomorphic settings in the to an unprecedented combination of climate region. Mangrove forests in the insular Carib- change associated disturbances (e.g., storm surg- bean are of four main functional types based on es, flooding, drought), and other local change edaphic and hydrologic conditions (Lugo and drivers (e.g., land use change, pollution, over- Snedaker 1974): - riverine, fringe, basin and exploitation of resources) (Gilman et al. 2006; scrub. Bacon (1994) stressed the importance IPCC 2007). The Caribbean has experienced on of site-specific analysis and recommended that average a mean relative sea-level rise of 1 mm more attention be paid to site physiography, hy- year-1 during the 20th century, although there is drology and ecology in predicting responses to extensive local variation. Sea-level is projected sea level rise by tropical coastal wetlands. More to rise between 0.18 – 0.59 m by 2099 (IPCC importantly, if sedimentation rate keeps pace 2007). with rising sea level, mangrove forest could re- Global climate change is expected to exac- main largely unaffected (Snedaker 1993; Ellison erbate loss and degradation of mangrove forests 1996). and loss or decline of their species, as well as Accurate predictions of changes to man- harm to human populations dependent on their grove area and health, including those originat- services (Millennium Ecosystem Assessment, ing from climate change effects, enable advanced 2005). Small island states like Trinidad and To- planning to minimize and offset anticipated bago, have limited capacity to adapt to relative losses and reduce threats to coastal development sea level rise, including accommodating land- and human safety for specific sections of coast- ward migration of mangroves and other coastal line (Gilman et al. 2006). Establishing man- ecosystems. This is a result of their small land grove baselines and monitoring gradual changes mass, high population densities and growth will enable the separation of site based influence rates, poorly developed infrastructure, and sus- from global climate change, and will provide a ceptibility to damage from natural disasters better understanding of mangrove responses to (Nurse et al. 2001). sea level rise and global climate changes. The Bacon (1994) suggests that responses of study assesses the status and trend of mangrove coastal wetlands to sea level rise in the Caribbe- forests in Trinidad and Tobago, and establishes 291 292 R. JUMAN AND D. RAMSEWAK a baseline so that response to human induced ers, mainly the Orinoco River, while its terres- changes and climate changes can be determined. trial biota is largely South American. The land areas of Trinidad and Tobago are 4823 km2, and 300 km2 respectively. MATERIALS AND METHODS In 2010, the population of Trinidad and To- bago was estimated at 1,317,714 persons (http:// Site Description cso.gov.tt). Approximately 90 % of the popula- tion lives along the west coast and the foothill of Trinidad and Tobago is situated in the the Northern Range. Most of the population of southernmost end of the Caribbean island chain Tobago is concentrated in the southwest part of between 10° 02’ -10° 50’N latitude and 60° 55’- the island. Some 80% of industrial activities of 61° 56’W longitude (Figure 1). It is located on strategic national importance are located within the continental shelf of South America, and im- coastal areas (Central Statistical Office (CSO) mediately adjacent to the outflow of the Orinoco 2007). River. The country is less exposed to tropical In Trinidad and Tobago, mangrove forests storms and hurricanes than most of the Caribbe- are typically riverine/ estuarine, fringe or ba- an nations because of its southerly location, and sin systems, and are associated with other plant has a tropical climate with two distinct seasons; communities such as tidal marshes and swamp dry -January-April and wet - June-November forest. Seven mangrove species has been report- (Henry 1990). Its marine ecosystems are influ- ed in Trinidad and four in Tobago. Rhizophora enced by discharge from South American Riv- FIG 1 Map showing location of Trinidad and Tobago MANGROVE FORESTS IN TRINIdad AND TOBAGO 293 mangle (L.), Avicennia germinans (L.) Stearn, agery was utilized to map the boundaries of the Laguncularia racemosa (L.) Gaernt and Cono- mangroves. The Spatial Analyst extension of carpus erectus (L.) are found on both islands, ArcGIS 9.0 was then used to calculate the area while Rhizophora racemosa (G.F.W. Meyer), of each of the mangrove regions mapped. Rhizophora harrisonii (Leechman) and Avicen- It should be noted here that the IKONOS nia schauriana (Stapf & Leechman) have a lim- satellite imagery utilized for this study was ited distribution in Trinidad. missing the near infrared band. This essential- ly reduced the effectiveness of computer aided classification techniques. Trials of supervised Methods and unsupervised classification on the red green In order to separate site-based influences and blue channels yielded results that were less from impacts due to global climate change, accurate than the manual delineation should be particularly sea-level rise, mangrove baselines less accurate than the manual delineation. were established as of 2007. The mangrove ar- eas were delineated using IKONOS satellite im- agery. This imagery had a spatial resolution of RESULTS AND DISCUSSION one metre (panchromatic) and four metres (mul- In Trinidad, mangrove coverage was esti- tispectral). Spatial reference for the dataset was mated at 7,532 ha on the west coast, 1,132.8 ha the WGS 84 UTM Zone 20 N coordinate sys- on the east coast, 481.3 ha on the south coast tem. The dataset consisted of thirty eight (31 for and 0.3 ha on the north coast (Figure 2; Table 1). Trinidad) and (7 for Tobago) individual satel- In Tobago, mangrove coverage was estimated lite scenes captured over the period 2000-2007. at 222.9 ha encompassing 11 systems, most of These scenes were mosaicked together using ER which are located on the Windward Coast (Fig- Mapper 7.1 image processing software to pro- ure 3; Table 1). Although Bacon (1993) and Al- duce complete coverage for both islands. leng (1997) provided sizes for some mangrove Field surveys were conducted to verify the forests, the imagery and mapping technology boundaries of the mangrove forests. Surveys used in this study were not available to them. were conducted in Tobago in September 2008 Maps were not provided so that differences in and between November 2008 and March 2010 in sizes could be validated. It is therefore assumed Trinidad. During field surveys information was that the estimates for mangrove coverage in collected on mangrove forest type using Bacon’s this study are the most accurate and up-to-date. (1993) classification, flora and fauna, hydrology Although comparisons between past and pres- and adjacent land uses and impacts. ent mangrove coverage give some indication of GIS software (ArcGIS 9.0) was used in con- change, they are not exact. junction with field verification data and ancillary The majority of mangrove forests are found data such as topographic and hydrographic maps on the sheltered west coast of Trinidad; which is to delineate mangrove areas from the IKONOS the coastline that is occupied by more than 70% imagery. The GPS field points collected were of the population and has experienced the most imported into ArcGIS and overlaid onto the intense development activities within the past satellite imagery to assist in determining the five decades (CSO 2007). Seventy percent of the margins of the mangrove vegetation. Delinea- mangroves on this coast are found in the Caroni tion was then carried out based on visual inter- Swamp, 10% in the Godineau Swamp and the pretation to produce maps showing the spatial remaining 20% amongst smaller systems. While distribution and extent of the mangrove areas. In mangroves were cleared for housing, industries, certain areas, mangroves validated by the field agriculture, roads and ports, there has been some visits could not be delineated on the satellite im- regeneration or new growth, but at the expense agery due to the occurrence of cloud cover. For of other wetland communities in most instances Chacachacare and Mucurapo, Google Earth im- (Table 2).

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