Miniature inland seas offer refuge to a rich gallery of marine life.
Story & Images By Dr. Eric Cole, Biology Professor, St. Olaf College, Northfield, MN
Ten thousand years ago the last of the great glaciers receded from the continents in the North, and their meltwaters drained into the ocean basins. Far to the South, the rising seas crept upwards gradually submerging the Lucayan high-plateau. Their brilliant white dunes, fossilized by rain and sun, slowly sank beneath the rising sea.
Today, only the highest sweeps of the Turks & Caicos Islands remain above water. The carbonate islands have seen this languorous drama unfold many times before. Glaciers grew, and sea levels dropped far below today’s low-tide line. The steep-walled, flat-topped islands rose proudly 400 feet above their historic shores. Rainstorms raked the forested heights, leaching the carbonate soils as their waters percolated down, dissolving channels that would become vertical caves. As the rainwaters filtered through ages of former dune sediments, they pooled and floated on the denser marine stuff that forms the islands’ water table. Here the interface between fresh and salt waters digested lateral caves, creating an underground labyrinth. As glaciers continued to hoard their snowfalls, sea levels dropped further and the abandoned, dripping chambers grew draperies of flowstone, stalagmites, and stalactites. Occasionally, a cavern grew so vast that it collapsed inward, creating a sinkhole.
When the glaciers melted once again, lifting the oceans, they flooded these caverns and gradually filled the sinkholes and deeper depressions across the landscape—creating ponds and blue holes.
This pinkish salt flat forms the shores of one of Middle Caicos’ hypersaline ponds. A pristine, cavern-fed shallow pond lies deep in the interior of Middle Caicos.
The ponds that populate the Turks & Caicos Islands exist along a spectrum of connectivity with the sea. Some ponds have no direct connection to the sea. In such ponds, evaporation exceeds rainfall and they become “hypersaline.” Though impoverished in the more familiar marine life, hypersaline ponds provided refuge for strange, pre-cambrian microbial reefs called stromatolites and occasional blooms of “sea monkeys,” or brine shrimp. The Islands’ flamingos and spoonbills value these occasional buffets, acquiring a richer pink pigment as a result.
The Islands also boast ponds that are richly connected to the distant ocean through submarine caves. These caves can be so substantial that their so-called “anchialine ponds” rise and fall with the ocean’s tides, even though the coast may be a mile away. This can create substantial, even alarming, currents at the cave’s entrance. Tidal ponds also have enough turnover that their waters remain clean and fully marine. Rainfall may soften the salinity a bit, favoring the growth of marine life even more.
Anchialine (“near the sea”) caverns help their connected ponds maintain a constant marine environment, serving as an umbilicus to the Mother ocean. Twice-daily seawater exchanges through the unseen caverns prevent over-salination by evaporation during dry spells, and counter the effects of storm and hurricane-delivered rainwaters. One can almost imagine them “breathing.” Such ponds represent perfectly maintained miniature seas.
These are scenes from cavern-fed ponds at Northwest Point, Providenciales.
The same subterranean caverns that balance and protect their ponds from change, also serve as wildlife corridors for colonization. Ponds with substantial cave connections can support incredibly rich marine communities with living sponges, jellyfish, macro-algae, annelids and mollusks. Some even provide harbor for pupfish, needlefish, mojarra, and the occasional barracuda, sea turtles, or juvenile Atlantic tarpon that somehow found their way through the dark, anchialine caves, establishing a colony within the ponds as far as a mile from the sea. Orphaned oceans indeed.
During our studies, we have learned that each pond represents a unique, precious “natural experiment” in marine colonization and community structuring. Some ponds are incredibly simple. A single species of cave shrimp, having negotiated passage through the network of submarine fissures and caves, finds a tiny surface pond with an unlimited supply of rich, green sediment. Their population explodes. In the absence of competitors, and with no marine predators, they abandon their troglodytic (cave-dwelling) ways and forage the sunlit fields of phytoplankton in broad daylight. Another pond has become refuge to a single species of jellyfish. The enigmatic, “upside down jellyfish” has adopted the lifestyle of a plant, tentacles up, harvesting sunlight just as their distant coral relatives do with the same symbiotic algae living within their tissues.
Erosion of karst along the shores of St. Thomas Hill Pond in North Caicos led to its attendant anchialine cavern. Terrestrial Indian Cave in Middle Caicos displays its dissolution holes and roof collapse.
In North Caicos’ Cottage Pond, a favorite with visitors to the Islands, the pond is deep, cavern fed, and supports a substantial body of fresh water riding over the deeper, anaerobic layer of salt water. Snorkeling Cottage Pond, we discovered an incredibly simple world with one species of freshwater algae (Chara) and in the complete absence of fish, a population explosion of the endearing water bug, (Notonecta), a “back-swimmer.” This insect’s only concern seems to be a single pair of diving grebes. Cottage Pond is another example of a “species jackpot,” an anchialine Garden of Eden in which some lucky creature escapes the pressures of a more complex environment.
It should be noted (and perhaps come as no surprise) that the greatest threats these endearing “orphaned oceans” face, each with their one-of-a-kind experiment in marine colonization, are man-made. Development without thoughtful planning can compromise not only the pond communities, but the subterranean network of caverns that serve as their circulatory system—their gills. Heavy equipment and incautious excavation near such fragile wonders can (and has) accidentally collapsed the caverns, severing their circulation with the sea. Even visiting them, one hopes to observe without impact. My team covers our skin with fabric, not sun lotion or insect repellent, recognizing that in close quarters, our very presence constitutes a bio-hazard. With respect, we hope to continue exploring and documenting this enchanting anchialine archipelago.
Professor Cole teaches and conducts research with undergraduates at St. Olaf College in Northfield, Minnesota. During business hours, he conducts research into cellular and developmental Biology, but moonlights as an explorer of the natural history of marine habitats. One can visit his website (always woefully under-constructed) at: https://pages.stolaf.edu/colee/.