Seagrass ecosystems: Their global status and prospects

INTRODUCTION Seagrasses form important underwater marine and estuarine ecosystems on all continents except Antarctica. Seagrass ecosystems are highly productive, forming extensive habitats which support highly diverse communities. The seagrasses themselves assimilate and cycle nutrients and other chemicals. Their extensive above- and below-ground biomass traps sediments, reducing coastal turbidity and erosion, as well as providing habitat for other organisms both attached and free-living (Hemminga & Duarte 2000). Globally, seagrasses are in decline, almost entirely because of human impact. Causes range from changes in light attenuation due to sedimentation and/or nutrient pollution, to direct damage and climate change. Over the last two decades, the loss of seagrass from direct and indirect human impacts amounts to 18% of the documented seagrass area (Green & Short 2003). Seagrass ecosystems are in need of active management to ensure their persistence and long-term survival, requiring education, increased awareness, management and conservation on a global scale. Seagrass meadows are coastal benthic ecosystems found submerged in nearshore waters; the group of plants collectively known as seagrasses consists of about 60 similarly functioning angiosperm species (den Hartog 1970; Hemminga & Duarte 2000). These are all rhizomatous monocotyledons, within two families (Potamogetonaceae and Hydrocharitaceae) encompassing 12 genera, which are restricted to the marine environment to complete their life cycle. Seagrasses are flowering plants that pollinate on or under the water's surface and produce fruits and seeds which are important to maintaining the habitat, although their primary mode of expansion is clonal growth.