Salt ponds of the British Virgin Islands : investigations in an unexplored ecosystem

Salt ponds occur within the coastal mangrove forests of Caribbean islands, where they are threatened by coastal development. Despite extremes of salinity, temperature and dissolved oxygen, these ponds support highly productive aquatic communities upon which most waterbirds depend. Salt ponds also provide valuable services, such as trapping sediments and nutrients that can damage near-shore marine ecosystems. Nevertheless, salt ponds have received little scientific attention, resulting in a misunderstanding of their ecosystem services and little guidance for their appropriate management.

This thesis presents an ecosystem characterization, based on extensive sampling of hydrological, chemical and biological parameters in salt ponds of the British Virgin Islands. Sampling results showed levels of temporal and spatial variation previously undescribed in mangrove habitats. Patterns of variation among ponds indicated at least 3 distinct types of ponds and reflected a geological transition from open lagoons through successive pond stages with increasing isolation from seawater.

Pond salinities fluctuated in response to seasonal evaporation and precipitation patterns. Mean salinities in most ponds ranged from 52 ± 33 to 160 ± 69 ppt. Populations of widely-distributed aquatic taxa responded to temporal salinity variations. Most taxa did not occur at salinities above 120 ppt, but a few tolerated salinities greater than 150 ppt. High-salinity species were generally absent, due to predatory and competitive interactions, at salinities below 70 ppt. Transitions between communities of high-salinity and low-salinity taxa occurred at salinities between 75 and 110 ppt.

Salinity fluctuations and community responses were not synchronized among ponds, leading to a shifting mosaic of aquatic populations. Waterbirds depended on these fluctuating prey populations and regularly moved between ponds. These findings showed that salt ponds exist as a network of habitats within a wider ecosystem. As a result, effective conservation efforts must protect a full range of salt ponds rather than individual ponds.