Salt ponds are enclosed or mostly enclosed water bodies that occur within coastal mangrove wetlands. They are typically hypersaline, as defined by Hammer, 1986 , with water salinities typically in excess of 50 parts per thousand (ppt) . Salt ponds and their surrounding mangrove forests, together known as "basin mangrove forests" , are the predominant type of coastal wetland in the Caribbean . These wetlands provide important ecological services, including storm protection and flood mitigation, shoreline stabilization, erosion control, and retention of nutrients and sediments [5-7]. They also provide critical habitat and food resources for resident and migratory birds in the Caribbean .
Mangrove wetlands throughout the Caribbean are being replaced by coastal developments, and they are now considered to be one of the most threatened habitats on Earth . Despite their ecological importance, salt ponds have received little scientific attention and remain poorly understood [10,11]. A thorough description of salt ponds is urgently needed to provide baseline information for wetland conservation efforts in the Caribbean and to establish frameworks for conservation and management protocols.
This study describes the physical characteristics of salt ponds in the British Virgin Islands (BVI), a small archipelago at the eastern end of the Greater Antillean island chain in the Caribbean (Figure 1). Nearly sixty salt ponds occur along the semi-arid coastlines of the BVI, which provided an opportunity to compare multiple habitats within a small geographical area. In this paper, we examine spatial and temporal variation in hydrology and salinity in 17 salt ponds (Figure 2; Table 1). Monitoring change, both cyclical and progressive, is essential for management and restoration of these vulnerable habitats [12,13]. This paper establishes a scientific baseline for such work.
Salinity is the predominant abiotic factor limiting aquatic communities in hypersaline water, and it influences both dissolved oxygen concentrations and temperature [14-16]. Salinity is known to fluctuate widely in shallow hypersaline water bodies because their high surface to volume ratio makes them especially sensitive to seasonal and shorter-term environmental changes . Weather patterns control the concentration of salts by evaporation and dilution . In coastal ponds, seawater input and flushing can also influence salinity [19-21].
We anticipated salinity fluctuations in BVI salt ponds similar to those reported for saline lakes and coastal salt ponds elsewhere in the world, and we expected the amplitude of these salinity fluctuations to be influenced both by weather patterns and by a pond's degree of hydrological connection with the sea.
Salt ponds are dynamic ecological systems that evolve through a series of natural changes. Succession via sedimentation and changing hydrological conditions should occur in the salt ponds of the BVI, and, consequently, present-day ponds should represent stages in a natural hydrological progression, from near-marine systems to near-terrestrial systems. Similar processes have recently been described in Belize .
Factors that influence the hydrology of wetlands include precipitation, catchment size, groundwater flow, surface flow, permeability of sediments, and vegetation. Precipitation in the BVI is seasonal, with a rainy season from August through December. Salt ponds occur at the bottom of steep watersheds on all islands except Anegada, which is flat. They are not associated with estuaries, which are generally absent from dry Caribbean islands. Rainfall tends to run down hillsides over the surface rather than through the ground because the soil layer in the Virgin Islands is thin and the underlying rock has low permeability . Surface flow occurs only after heavy rainfall, as there are no permanent rivers, creeks or streams in the BVI. Groundwater resources, which are limited to narrow alluvial valleys, scattered sand deposits and fractured volcanic rock, recharge at a rate of only 3 to 8 cm/yr . Seawater, on the other hand, may enter salt ponds over the surface or through the ground because the berms that separate salt ponds from the sea are composed of permeable sediments, mainly sand and coral rubble. Seawater input is predicted to be an important force in controlling pond hydrology and salinity.
Four mangrove species, Rhizophora mangle, Avicennia germinans, Laguncularia racemosa, and Conocarpus erectus, make up the predominant vegetation along the shores of salt ponds. Their distribution among ponds in the BVI is described in  and will not be addressed here.