Simply put, estuaries are coastal zones where fresh water flowing from a land mass meets and mixes with sea water. Estuaries are typically brackish water systems, which means the salinity therein is only half that of the sea, which is, on average, 35 parts per thousand (ppt) by volume.
The Port Royal Sound system is the exception to the rule. The two small fresh water rivers that flow into it originate in Jasper County and drain a relatively small watershed. The distance between the mouths of these rivers and the low volume of fresh water carried by them precludes the establishment of a contiguous and stable brackish zone. Salinity throughout the majority of the system averages 29 to 32 ppt.
Port Royal Sound is home to the vast majority of species found in other Atlantic Coast estuaries. Its inhabitants, however, have evolved to tolerate the vertically mixed high salinity environment. This adaptation accounts for some species being more wide spread and in greater populations than that of their cousins in stratified salinity systems. This partially accounts for the claim of higher biomass productivity. Another contributing factor is that it is also home to a number of species that prefer a higher salinity and are not found in brackish water systems.
One denizen of the deep not normally associated with estuaries, and yet a frequent visitor to local waters, is the elegant manta ray. In the 19th century there was even a manta ray fishery here. The grand creatures were harpooned and processed into pet food and fertilizer.
Another species not typically found in estuaries, cobia, draw sport fishermen from near and far to the Broad River each spring.
Next to the water, the second most important element in a salt marsh system is the marsh grass itself. It stabilizes vast areas of intertidal geography and provides habitat and cover for numerous species. White shrimp and blue crabs spawn off shore where the offspring feed on plankton throughout their larval stages. In their post larval forms (shrimp, 3 to 4 mm in length and crab the size of the head of a pin) they migrate to the estuary where their first food is detritus. Hence, no marsh grasses – no shrimp or crab.
Detritus is also a primary food source for the fry of most estuarine and many near shore fish species. Black sea bass and red drum are two of many game fish species that spawn in the ocean with the young entering the estuary as fry. They feed and grow in the food rich and relatively protective waters until ready to face the sea.
Because they are filter feeders detritus forms a large part of the diet of oysters, clams and mussels also. They, in turn, are fed upon by three of the four native whelk species which are found on local menus listed as conch.
The Port Royal Sound ecosystem is arguably one of the richest on earth. Those riches, however, are not locked within its boundaries. Outgoing tidal currents deliver nutrients beneficial to populations of near shore species which in a web of interconnectivity are a food source for large open water species.
Here we have only scratched the surface in beginning to understand the importance of Port Royal’s estuarine environment. It is both unique and beautiful, and certainly a big part of what makes Port Royal so special. We invite you to learn and experience more of Port Royal’s estuarine environment at the Lowcountry Estuarium located on 16th Street in the Old Village of Port Royal. Visit www.lowcountryestuarium.org for infomation.