Saltwater ponds can also be called marine ponds, as they are filled with saltwater like the sea, and contain some of the same species. Most salt water ponds lie at low points in Bermuda’s topography and they were filled as sea level rose following the last glaciation (Ice Age). Some of the ponds, e.g. Walsingham Pond, were formed by cave roof collapse, so they are technically sinks with cliffs around the edges. Others have gently sloping edges with mangroves or salt marshes along their shores.
Marine ponds are still connected to the sea through caves or tunnels in the rock; for this reason they can also be called Anchialine ponds. The degree of water exchange between the pond and the open sea when the tide rises and falls depends on the size of the connection and distance to the sea. Because of these variations, the environmental conditions in all saltwater ponds are different. Bermuda’s ponds have a range of temperature, salinity (saltiness), light, sediment in the water and inputs of plantkton and plant material. Also the depth of the pond, steepness of the sides, what type of material is on the bottom of the pond all influence what species are using the pond as habitat.
Mangrove Lake is the largest of Bermuda’s saltwater ponds at 30 acres (12.3 ha). Other large saltwater ponds include Trott’s Pond, Lover’s Lake, Walsingham Pond, Evan’s Pond, and Coney Island Pond. There are also a number of man-made saltwater ponds in Bermuda, such as Bartram’s Pond, Blue Hole Pond and the pond at Cooper’s Island Nature Reserve. Spittal Pond differs from all other ponds in not having a permanent connection to the sea. It is generally considered a brackish pond, as it receives significant saltwater input from tidal flooding and storms, but after heavy rain it becomes very fresh.
Why are Saltwater Ponds Important?
Saltwater ponds are important as they support a high diversity of marine and brackish water species. Saltwater ponds are home to some of Bermuda’s threatened wildlife, such as the rare native Diamondback Terrapin, which is only found in Mangrove Lake and Trott’s Pond. Saltwater ponds also contain some endemic species, for example the endemic Bermuda Sargassum Weed (Sargassum bermudense) which is found in Walsingham Pond.
Saltwater ponds are a key habitat for resident and migratory birds. Ponds are used by migrant species of songbirds, waterfowl and occasionally birds of prey like ospreys and owls. They are also nesting sites for resident herons and gallinules.
Anchialine ponds are a relatively rare habitat type worldwide, as they are only found on coasts with limestone or volcanic rock. For this reason alone, Bermuda’s marine ponds are considered an important habitat.
Threats to Bermuda’s Saltwater Ponds
Pollution is the most serious threat to saltwater ponds. Pollutants such as pesticides, herbicides, fertilisers, oil and other chemicals enter the ponds by surface runoff from roads, golf courses, gardens and farmland. Pollutants, especially from sewage, also enter ponds by seeping into the groundwater. This is especially problematic in ponds that do not flush much when the tide changes, which is most of Bermuda’s ponds. Many of Bermuda’s wetlands were historically used for dumping garbage or were intentionally filled to make new land. Most of the historically dumped material is still present in the ponds.
Protecting Saltwater Ponds
Many of Bermuda’s salt water ponds lie within Nature Reserves or Parks. Evan’s Pond, Coney Island Pond, Blue Hole Pond, Cooper’s Island Pond and Lover’s Lake are all protected in the Bermuda National Parks and Nature Reserves system. Other ponds, notably Walsingham Pond, West Walsingham Pond, portions of Mangrove Lake and Bartram’s Pond are held in private or NGO-owned nature reserves.
Bermuda has 7 designated Ramsar sites under the Convention on Wetlands of International Importance Especially as Waterfowl Habitat (Ramsar Convention). One of these sites, Lovers Lake Nature Reserve, is a salt water pond.
Potential Species in Saltwater Pond HabitatsThe potential species in a saltwater pond varies and depends on the saltiness of the pond (salinity) and the type of connection it has with the sea. Another factor that influences species in a pond is the topography, for example a pond with steep sides will not have mangroves around the edge, while a gentle sided pond may have them. The presence of mangroves will attract other species to the pond, like birds. Some ponds reach very high water temperatures in summer, particularly if they are not shaded, which can reduce biodiversity
Pond-fringing mangroves are described on the mangrove habitat page. Mangroves are also important as a species inhabiting ponds, as they provide valuable ecological services to the pond; namely providing shade and shelter, providing firm attachment points for other species and providing leaf litter (falling leaves). When leaves fall into the pond they break down into nutrients that pond animals will eat.
Walsingham Pond, as a collapsed cave, has lots of hard surfaces for attachment of animals and plants on its rocky walls. It also has mangroves of both species, giving another layer of habitat complexity. For these reasons, it is therefore not surprising that Walsingham Pond is Bermuda’s most biodiverse saltwater pond.
Plant and animal species found in Bermuda's saltwater ponds include:
- Red Mangrove (Rhizophora mangle)
- Black Mangrove (Avicennia germinans)
- Upside-Down Jellyfish (Cassiopea xamachana)
- Bermuda Killifish (Fundulus bermudae)
- Lover's Lake Killifish (Fundulus relictus)
- Diamondback Terrapin (Malaclemys terrapin)
- Green Heron (Butorides virescens)
- Yellow-crowned Night Heron (Nyctanassa violacea)
- Mallard Duck (Anas platyrhynchos)
- Great Egret (Ardea alba)
- Common Moorhen (Gallinula chloropus)
- American Eel (Anguilla rostrata)
- Widgeon Grass (Ruppia maritima)
- Bermuda Sargassum Weed (Sargassum bermudense)
- Mangrove Oysters (Isognomon alatus)
- Sea Squirts (Tunicates)
- Spirorbid and Sabellid Tube Worms
- Marine Snails
- Marine Fish
Salt marshes occur on the edge of saltwater ponds and in sheltered coastal locations, like the heads of bays. They contain mostly coastal plants that are adapted to occasional flooding by seawater.
Plant and animal species found in pond edge salt marshes include:
- Sheathed Paspalum Grass (Paspalum vaginatum)
- Marsh Samphire or Woody Glasswort (Salicornia perennis)
- Sea Rush (Juncus maritimus)
- Saltmarsh Oxeye (Borrichia frutescens)
- Sea Oxeye (Borrichia arborescens)
- Seaside Heliotrope (Heliotropium curassavicum)
- Seaside Purslane (Sesuvium portulacastrum)
- Bermuda Biodiversity Project Profile for Pond Habitats
- Many of Bermuda's important saltmarsh and saltwater pond habitats are recognised as Ramsar Sites.
- Article - Colonization and Conservation of the Green Heron on Bermuda, the Result of Partnerships in Species and Wetland Restoration