What's Going On At Great Salt Pond, St. Croix?
Great Salt Pond
Great Salt Pond Bay on St. Croix’s south-east coast is a salt pond located within the St. Croix East End Marine Park, and one of just a few mangrove ecosystems on the island. Alarmingly, it supports the only remaining juvenile fish mangrove habitat on the south side of the island! While all mangrove nurseries are essential, St. Croix’s remote location makes this one particularly important.
St. Croix is isolated from the US and British Virgin Islands by a 15,000 foot deep-water trench and wide open ocean. St. Thomas, the closest landmass, is 43 miles away. This distance ensures limited larval fish connectivity especially when compared to the easy migration of marine species between St. Thomas and St. John, and even between St. John and the British Virgin Islands. It is therefore critical that St. Croix’s reef fish populations continue to be self sustaining. The Department of Planning and Natural Resources (DPNR) is greatly concerned about the quality of juvenile fish habitats in St. Croix and in particular the habitat at Great Pond; they have provided funding for researchers at the Center For Marine & Environmental Studies at the University of the Virgin Islands (UVI) to look at the impact of Hurricane Hugo’s influence on this salt pond.
Hurricane Hugo
Hurricane Hugo devastated St. Croix in 1989 and Great Pond was not spared. The storm infilled and closed the channel that connected the salt pond to the lagoon, changing the hydrology of the system and allowing mangroves to root and expand. Before the hurricane, the bay was flushed by tides which helped maintain water quality and salinity. Now, with water flow restricted, the ecosystem has changed significantly resulting in a dramatic increase of red mangrove trees. Seedlings once flushed to sea instead took root, increasing the red mangrove stand by approximately 65% to 85%. This is great news for the birds that now nest there in abundance, but the effects on fish habitat quality are unknown.
Allie Durdall and Andrew Reeve, Ph.D. setting up the GPS equipment over the groundwater wells to get precise elevations which are needed to understand groundwater movement.
Allie Durdall, Master of Marine and Environmental Studies candidate at UVI has centered her graduate research on Great Pond. Allie was particularly drawn to this project because of its demanding multidimensional scope of work. The project brings together researchers from UVI, the University of Maine, and the University of Connecticut.
Biological
Allie began this research by replicating a previous survey conducted by DPNR twenty years ago. Using the same methods and materials as they did then, she did fish surveys using traditional traps and seine nets to gauge the number and species of the fish currently using the pond. Water quality samples were also taken. Juvenile fish need water quality to fall within certain parameters in order to thrive. Great Pond may be pushing those limits. This project is trying to measure that environmental variability. These data will be compared to the previous survey results.
Geological
This June, a team of researchers including Allie, her advisor Kristin Wilson Grimes, Ph.D., watershed specialist Sydney Nick and visiting hydrologist Andrew Reeve, Ph.D. from the University of Maine installed eight ground water well clusters to ascertain ground water movement into and out of the pond. Often the importance of ground water is overlooked when thinking about mangroves, yet it helps scientists to understand vegetation patterns and predict how those might change in response to changing environmental conditions, like drought, or management actions, like re-opening the berm.
Allie Durdall and Dr. Grimes are ready to head out to one of the field sites with their kayak filled with gear
hydrology
The wells being installed are basically PVC tubing with slats cut into the bottom to allow water to flow through. The tubes are installed in eight clusters; ideally, each cluster consists of one 10’ deep well and one shallower 5’ deep well. The tubes are equipped with water level sensors which automatically record the fluctuating water levels every 12 minutes. These sensors are checked using manual measurements collected with a hand-held electronic water level meter. Water quality samples are also taken to gauge changes in salinity as fresh and sea water flow through the area. Carefully positioned, the well clusters will help provide a picture of ground water in the area.
Great Pond Wildlife
Much like a bowl filling with sediment, so is Great Pond filling in. Trees have flourished and taken root; a variety of birds have taken advantage. Various species like black-necked stilts, Wilson’s plovers, snowy and great egrets, and green and little blue herons, snack on the fish that are in the lagoon while egrets, herons and white-crowned pigeons nest in abundance in the mangrove trees. For better or worse, some fish benefit as well.
When the earlier Great Pond study took place, researchers caught primarily schoolmaster and mangrove snapper, mohara, horse eyed jack and lots of other species of fish. Now, while more fish are caught in terms of numbers, diversity has decreased and of the fish caught are completely different species. Primarily tilapia.
Tilapia are not a native species but are currently abundant in Great Pond.
Tilapia are not native to the Virgin Islands and it is suspected they may have escaped from a tilapia farm on the island, possibly as a result of a heavy rain event. They thrive in the now brackish Great Pond. Why tilapia? These hyper-adaptable fish can easily manage extreme shifts in water quality and can even gulp air if the water’s salinity becomes toxic. Traditionally bottom feeders, they can change their habits as necessary. Allie has caught close to 1,000 tilapia during her monthly surveys since September!
Possible solutions
Ultimately, land management authorities need to determine if dredging the lagoon to restore its conditions to pre-hurricane levels is the best decision for the island. There are varying opinions about this, but the goal of Allie’s research is to provide all the data needed to make an informed decision. One final piece of that puzzle will be provided with the help of ground penetrating radar. This summer, Peter Leach from the University of Connecticut, will be with the team on St. Croix with a machine that basically looks like a lawn mower but has the ability to send and receive “pings” creating a picture of the earth below ground. This ground penetrating radar will inform researchers what the berm is made of, how porous it is and even determine historical channels into the lagoon.
Allie is pictured here with Andrew Reeve, Ph.D.
Allie
Allie was introduced to this research and is advised by VI-EPSCoR-supported scientist Dr. Kristin Wilson Grimes, Research Assistant Professor of Watershed Ecology at UVI and Director of the VI Water Resources Research Institute. After earning her MMES degree she is looking forward to beginning a Ph.D. This research has introduced Allie to many different areas of Environmental Science and carbon storage in particular has peaked her interest. It will certainly play a role in her future.
In the process of doing this work Allie has spent a great deal of time on the island of St. Croix. Coming from Minnesota, she had to adjust to a slower pace, driving on the left and the intense (stinky, yet somehow wonderful) sulfuric mangrove odor. She has found Crucians to be very curious about the work, asking lots of questions about the mangroves and fish species in the pond. Because the location is so remote, generally speaking, only senior fishermen are aware the pond has filled in at all. Still, she gleefully notes that the food on St. Croix is amazing!
A hand-held Dutch Auger was used during well installation. The cores have been archived for future analysis in the lab at UVI and Allie aims to be part of that team. These cores can provide a great deal of information on the history of Great Pond as well as stored carbon data. Here are Sydney Nick and Dr. Grimes as they collect a core.