Tropical Studies

While CCS is dedicated to research and conservation efforts within Cape Cod and the larger the Gulf of Maine region, our science staff contributes to globally significant conservation issues in related fields of study. On such effort is a tropical wetland restoration initiative in the Lesser Antilles, specifically on the island of Carriacou (Grenada) West Indies. The tropical coastal wetlands are in many ways the functional analogue of temperate coastal salt marshes and are similarly threatened by encroachment, development and other anthropogenic impacts. A brief outline of the project is provided below.

Mangroves are the principal vegetation of low-energy intertidal zones of coastlines and when occurring as coastal forests, or mangal, represent a highly productive and threatened habitat within the Caribbean. As the initial barrier between the energy of the coastal ocean and the up-gradient terrestrial ecosystem, the dense root systems of fringe mangroves play an integral role in storm damage control, contributing to stabilization of shorelines, while the above ground portion of the plants provide wildlife shelter, substrate, and safe breeding grounds for a host of economically important marine organisms. The density and continuity of the mangal canopy may also provide extensive habitat and breeding areas for a diversity of resident and migratory bird species. Additionally, mangal comprise a significant proportion of the world’s forested wetland area, estimated to be 16-30 x 106 ha (Lugo et al. 1990), thus making valuable contributions to the global carbon cycle, fisheries production and biogeochemical cycles.

Increasingly, concern for the maintenance of mangrove ecosystems is being expressed in the scientific literature. This concern extends from protection of direct economic applications such as fisheries, to conservation of wetland. While natural short-term impacts such as hurricanes may cause temporary destruction of mangroves, the forests are readily re-established by seedling recruitment and/or epicormic sprouting by local mangrove species. Anthropogenic wetland impacts, however, such as oil spills and eutrophication, harvest of tree and wood products, and alteration of hydrology and clear-cutting for development threaten the long-term integrity and functional values of these coastal wetlands. Within the Grenadine region, added threats to the few remaining mangal include land development, encroachment, and coastal erosion posing a serious threat to the sustainability of this fragile and significant resource.

Within Carriacou, a small 13-square mile island in the Grenadines, three individual mangal remain as productive, ecologically significant habitats albeit somewhat impacted by encroachment, development and arguably unsustainable uses. These systems occur within Lauriston Point, Tyrrel Bay, and L’Appelle. The latter, L’Appelle, was significantly impacted by a hurricane and subsequent fire roughly 60 years ago. Since this time, pronounced dieback has occurred within the central portion of the system. We speculate that this loss of mangroves was initiated by the storm damage and fire, and maintained through the deposition of sand in and around the system causing a partial tidal restriction. This restriction is evident today and results in episodic tidal flooding, prolonged inundation, and the development of hypersaline surface water and interstitial conditions. Additionally, the lack of regular diurnal tidal flooding may be limiting propagule distribution and seedling recruitment.

In 1997, a community-based mangrove restoration effort was initiated to attempt to reestablish mangroves within this impacted central area. A preliminary survey of the existing water biogeochemistry was conducted to confirm general site suitability, however now long-term examination of pore water chemistry has been maintained. During this initial period of data collection we recorded pore water salinity, pH, redox potential, and sulfide concentration within the rhizosphere (root zone) as well as generall characterized the sediment (grain size analysis). Then, with the help of local school children and volunteers, over several hundred propagules of red mangrove (Rhizophora mangle) were harvested from nearby mature trees.
young red mangrove at low tide.

Of these, 500 of relatively similar size and presumed maturity were selected and planted along ten permanently marked linear transects. Three years later our data showed approximately 60% of the planted red mangrove propagules has survived and demonstrate stem growth, first and second order branching, and prop root (rhizophore) development in limited cases. Presently, survival is estimated to be much lower (<20%); However, those that survived demonstrate considerable growth, branching, and in many cases, flowers and fruit.

Because mangal continue to be threatened throughout the Caribbean, we propose to resume this restoration and study initiative within L’Appelle by conducting a thorough and statistically robust study of the biogeochemistry of this system while conducting further mangrove restoration and planting. This would involve re-examination of pore water chemistry as described above, as well as a full sediment analysis included determination of percent sand, silt and clay, percent organic matter, and sediment porosity as these parameters have been shown to significantly affect mangrove species establishment and success.

Biannual data collection over a 2-3 year period would measure pore water biogeochemistry to monitor the conditions within the rhizosphere as well as above ground measures of planting success (e.g. survivorship, stem growth, branching, etc.). Furthermore, we propose that the mangal of Laurestan and Tyrel Bay are also generally studied (e.g., baseline data obtained to serve as comparison and to assess relative system health and productivity). The goals of this work will be 1) to more fully restore the mangal at L’Appelle; 2) provide an empirical basis and explanation of the restoration process within a regional context; and 3) use these data to develop a model for restoration or enhancement of other mangal within the Lesser Antilles.

As briefly described above, the primary outcome of the proposed project will be to continue with the restoration of a storm-damaged mangal on the island of Carriacou. Of equal significance however is the involvement of local volunteers and pertinent government agencies in the execution and continued stewardship of this effort. Through continuing with an existing project that has already enjoyed the benefit of local volunteer involvement, we can best achieve sustainable restoration of the site by encouraging continued local support, field assistance, and continued monitoring.

Specifically, the project would seek to engage local volunteers in the re-examination of soil biogeochemistry throughout the existing mangal at L’Appelle. Additionally, local involvement would be sought for the evaluation of the restoration site, including counting surviving plants, measurement of stem height, crown diameter, branching, and associated metrics. Finally, volunteers would assist in the gathering of additional propagules from this and other mangal sites within Carriacou and be involved in the establishment of additional plantings within L’Appelle as well as contributing to the on-going monitoring and evaluation of the restoration over the course of two to three years.

The data collected would be used to statistically evaluate the restoration success, as well to provide an empirical basis for varied restoration strategies that may be suitable for the region. As various planting arrangement have and will be established along environmental gradients (such as salinity, soil texture, tidal elevation), this study will attempt to develop model strategies for most effectively re-establishing mangroves species for site-specific applications throughout the region. Finally, working together with the local forestry department, the project seeks to identify critical areas within Carriacou, Grenada, and the larger Grenada Bank region that may warrant further study, restoration, or habitat enhancement.