Seagrass Loss in Belize: Studies of Turtlegrass (Thalassia testudinum) Habitat Using Remote Sensing and Ground-Truth Data

Abstract

Spatial and temporal change in turtlegrass (Thalassia testudinum) habitat of the South Water Caye Marine Reserve (SWCMR) in Belize were analyzed using satellite images backed up with ground-truth data. We had two primary objectives. First, we wanted to determine areal expanse of seagrass across a large area (~12 km by 3 km) of the SWCMR, and address its change over time. We used paired satellite images taken during 2001 and 2005 to determine coverage by seagrass and measure temporal variables. These analyses recorded an overall seagrass loss of 1.8% (52.3 ha) during the 4 yr period. Secondly, we wanted to determine whether seagrass gains or losses were consistent across the study area. Replicate sampling was used as a statistical basis and confi rmed a signifi cant loss of seagrass across the region. It also helped identify two regions of signifi cant seagrass loss; one 600 ha area lost 12.4% of its seagrass; another 240 ha area lost nearly 40%. These components helped us assess seagrass habitat in an area perceived as critical to Belize fi sheries, and provided the scale and statistical rigor necessary to adequately assess a broad region of study. The salient results from our study were not the magnitude of seagrass loss per se, but the loss in seagrass habitat from an area that is thought to be relatively pristine. Seagrass-habitat loss in this region of the Caribbean Sea may be evidence that even near-pristine areas can be impacted by anthropogenic factors. Determining the causes of habitat loss may help prevent loss of productivity, habitat, and livelihood for the associated human and nonhuman communities. INTRODUCTION Seagrass ecosystems are among the most productive on earth, and their ecological and economic importance is becoming obvious as they diminish worldwide. Seagrass habitats are vital primary producers that improve water quality, promote sedimentation, recycle nutrients, and provide structure that serves as refuge and nursery ground for fisheries species (e.g., Moriarty and O’Donohue 1994, Hall et al. 1999, Short and Wyllie-Echeverria 2000, Gillanders et al. 2003, Green and Short 2003, Corlett and Jones 2005). The interest in seagrass is global, as is the research effort to assess changes in seagrass habitat. Many studies centered around areas of long-term loss of seagrass (e.g., Short and Short 2003, Duarte et al. 2008), while others documented regions of seagrass recovery (e.g., Virnstein et al. 2007). Turtlegrass (Thalassia testudinum) is a common seagrass species in waters of the tropical western Atlantic from Venezuela to eastern Florida and the Bahamas (den Hartog 1970), and is one of many species adversely affected by natural and anthropogenic factors. Researchers contend that protecting seagrass habitat may prevent loss of commercial fisheries, improve water quality, and help maintain healthy interrelated communities (Ward 1998). The purpose of this study was to discern the stability of the turtlegrass-dominated seagrass community in the South Water Caye Marine Reserve (SWCMR) of Belize by using satellite imagery. This reserve is generally perceived to be pristine, far from the coastal influences that adversely affect most other seagrass communities, and an area of interest to Belize fisheries and tourism. We had two primary objectives. First, we wanted to determine areal expanse of seagrass, and address its change over time by analyzing satellite images taken 4 years apart. Secondly, we wanted to determine whether seagrass gains or losses were consistent across the study area, and used ground truthing to confirm our observations by visiting sites of concern identified by satellite images. The multiple components of our study helped assess seagrass habitat in an area perceived as critical to Belize fisheries, and provided the scale and statistical rigor necessary to adequately assess a broad region of study.