No. 17–Unique Port Royal Sound

Except for the Antarctic, all continents have coastal estuaries. One function of estuaries is that they serve as nursery grounds for near shore fish populations. 70% of all commercially harvested species either spend part of their life in an estuary or feed on those that do.

Estuaries of the North American Atlantic Coast are the most productive ecosystems on earth with regard to total biomass created within the system. While it has not been established scientifically, anecdotal evidence leads many to believe that the Port Royal Sound system is the most productive of these.

Simply put, estuaries are coastal zones where fresh water flowing from a land mass meets and mixes with sea water. Estuaries typically are brackish water systems which means the salinity therein is only half that of the sea which is 35 parts per thousand (ppt) by volume.

An easy way to understand the concept of ppt is to imagine a container that holds 1,000 cups liquid. If it is filled with seawater and the water allowed to evaporate, there will be 35 cups of sea salt left behind. The same container filled with brackish water would yield 17 1/2 cups of salt.

Virtually all North American Atlantic Coast estuaries are fed by river systems which deliver a relatively constant flow of fresh water. This maintains a relatively stable brackish zone much to the liking of the species that evolved to live within it.

The Port Royal Sound system is the exception to the rule. The few small fresh water rivers that flow into it originate in Jasper County to the north 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 typically reads at 29 to 32 ppt.

Another factor contributing high salinity is the sheer volume of sea water delivered by the extraordinary tidal forces this section of the coast is subject to. Picture, if you will, the South Atlantic Coast from North Carolina to Florida. How it bows inward with Port Royal Sound at the center of the concavity. The oceanic tide moving landward is concentrated toward the center as it approaches. This phenomena account for the highest tidal amplitudes found this side of the Bay of Fundy.

Also contributing to the uniqueness of Port Royal Sound is the fact that its waters are vertically mixed. This means that except for during periods of excessive rain, the salinity is constant from surface to bottom. The water column in most estuarine systems is either slightly or highly stratified with the highest salinity most dense water at the bottom. The amount of mixing occurring within a system is determined by such variables as the volume of fresh water being introduced; the size, shape and depth of the system; bottom topography and tidal forces. A stratified system brings a stratification of species within it. This can pose a problem for sessile (can’t move around) species if normal conditions are disrupted for even relatively short periods of time.

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.

To be continued…