Pamlico Sound Estuarine System (PSES)
An estuary is a place where freshwater mixes with the sea in a semi-enclosed coastal body of water. The Pamlico Sound is home to a host of unique habitats and is the largest of the North Carolina estuaries. Water from the marshes, forests and grasslands of eastern North Carolina and southeastern Virginia flow into and mix with ocean water fed from Oregon and Hatteras inlets. The combination of fresh and salt water creates brackish water, which varies in its salinity or salt content hour to hour and day to day.
There are three types of estuaries found in North Carolina:
- Tributary estuaries – begin at the initial interface of fresh and sea water in a river
- Trunk estuaries – begin downstream of tributary estuaries and meet at the widening of a river into a sound
- Back Barrier Sounds – lie parallel to the coast, sandwiched between the mainland and the barrier islands
Each type of estuary is unique and offers conditions that are incredibly changeable, yet remarkably suitable for wildlife. Some creatures move through the different types of estuaries based on the stage of their life cycle. This project zeroes in on the back barrier Pamlico Sound area and its habitats, ecosystems, and ecosystem services.
Sub-Aquatic Vegetation: Sea Grass
Sea grass grows in shallow salty and brackish water like that of the Pamlico sound. Unlike seaweed, sea grass shares the same composition as terrestrial plants, similar to what you might find on a golf course or in your backyard. This means that they have roots, stems, and leaves and can also produce flowers and seeds. Every spring throughout the sound, sea grass appears in a patchwork of underwater meadows, some of which can grow so large they can be seen from space.
These meadows of underwater grasses provide an important habitat for a wide variety of animals. Many of the fish that are caught locally, both in the sound and out in the ocean, start their life in the sanctuary of the sea grass beds. This makes sea grass an important ecosystem service provider for the commercial and recreational fisheries in North Carolina. The habitat created by sea grass acts as a nursery for juvenile creatures of all shapes and sizes including invertebrates, fish, crabs and turtles.
Sea grass offers other ecosystem services such as sediment trapping. Waves and currents are capable of whipping up sand and silt from the sound floor suspending the sediment up in the water column. The suspended sediment impairs water clarity and prevents sunlight from reaching the sound bottom where life depends on it to grow. As water rushes over these sub-aqueous meadows, sand and silt become trapped by the flowing grasses and settle to the bottom. Not only does the sea grass act as a net containing clouds of suspended sediment sweeping across the sound floor, it also acts as an anchor keeping what sediment lies beneath it in place. In the absence of sea grass, waves and currents can rush through undeterred and stir up more sediment to be carried and deposited elsewhere, like one of the inlets. The presence of sea grass mitigates the ever constant process of erosion and improves water clarity.
Sea grass plays another important role vital to the health of an estuarine ecosystem and that is denitrification. Rainwater washes nutrients on land into rivers and streams which flow into the sound. These nutrients include nitrogen in the form of nitrites and nitrates which cycle through the estuarine ecosystems. When large quantities of these nutrients inundate an ecosystem the health of the system is threatened. Algae, another photosynthetic organism (meaning it relies on the sun for food), propagates in greater numbers when higher concentrations of nutrients are present. This results in algal blooms. Algae, which absorbs many of the nutrients cycling through the system, are short-lived and die off quickly. When this happens the organic material that remains begins to decay, a process which strips the water of dissolved oxygen. As oxygen levels drop species which depend on O2 to live are threatened and may even result in large fish die-offs. Studies have shown that sea grass beds produce favorable conditions for a balanced nitrogen cycle maintaining the health of the ecosystem and guarding it against the negative impacts of the influx of nutrients from rainwater.
Scientists at the UNC Coastal Studies institute are examining this process at work. Nutrient cycling is bio-mediated, meaning that the act of denitrification is occurring at the microbial level. Both biotic (living organisms) and abiotic (physical, non-living) factors play a role in affecting the way nutrients are processed and where they are stored. This research is concerned with how much is coming in and how much is going out at the areas of sediment-water interface. To study this process scientists take core samples at each of the different habitats at the Rodanthe site starting first with the shoreline that has marshes and then shoreline without marshes, and eventually areas without any vegetation at all. Next, they focus on the sea grass by taking cores from densely vegetated areas of sea grass, to less densely vegetated, to areas where sea grass is totally absent. By taking an array of samples, researchers hope to better understand the role marshes and sea grass play in nutrient cycling. To see the process of taking a push core sample first-hand check out the video below.
As more people move to the coast, as weather continues to impact the inhabitants of the coast and shape the landscape, as water levels continue to rise, it is important to consider these ecosystems and monitor how they are affected by human and natural elements. One of the suggestions for the deposition of dredge spoil is the creation of an island. Creating an island would result in an intertidal area, where the tide sometimes covers the shore and sometimes leaves it exposed. In those areas creating a marsh may be the best option to develop a new habitat and area for nutrient cycling. Like sea grass, marshes are known to have high rates of denitrificaiton and in some cases higher than sea grass beds. Oyster beds are also being considered for the fringes of the island as a way to mitigate erosion and contribute another ecosystem service of value to humans. Ultimately, when a major engineering project is undertaken on the coast, it is important to keep in mind how we impact the area. NC DOT is working with UNC Coastal Studies Institute to discover the best options, so that if a habitat will be affected by dredging, that habitat can be replaced, or the spoil can be used to augment an already existing productive ecosystem. To learn more about Estuarine Ecology at UNC Coastal Studies Institute check out the video below.