lar intervals for up to days at a time until the 

 wind direction shifts or the wind velocity declines 

 greatly. Wind direction in coastal North Carolina 

 varies strongly with the seasons. Winters are char- 

 acterized by north, northwest, and northeast 

 winds, while southwest winds prevail during 

 summer months. In response to the seasonal shifts 

 in wind direction different shorelines tend to be 

 exposed at different seasons in these large sounds. 

 Northern shorelines are frequently uncovered in 

 the winter months, while southern areas are 

 exposed during the strong winds of summer 

 (Roelofs and Bumpus 1954). 



1.3 CHEMICAL ENVIRONMENT 



Geologists usually recognize a fairly distinct 

 dichotomy among semienclosed coastal water 

 bodies by distinguishing true estuaries from mar- 

 ine lagoons (Warme 1971). An estuary is an area 

 where freshwater runoff from land meets the sea. 

 Along its length an estuary exhibits a gradient in 

 salinity, and usually includes large expanses of 

 brackish waters in the middle of this salinity gra- 

 dient. Estuaries usually extend in a direction 

 more or less perpendicular to the coastline and 

 are inhabited by euryhaline organisms, tolerant of 

 brackish waters and variable salinities. Marine 

 lagoons, on the other hand, tend to be fully saline 

 with little freshwater input. They usually lie par- 

 allel to the shoreline behind a barrier island. A 

 marine lagoon is populated by stenohaline or- 

 ganisms typical of marine waters and physiolo- 

 gically incapable of withstanding low salinities. 



Although marine lagoons are characteristic 

 of continental coastlines in desert and Mediter- 

 ranean climates and of the coasts of islands where 

 the land drainage basin is small, there are occa- 

 sional marine lagoons along the coast of North 

 Carolina. Boguc Sound, Back Sound, and to some 

 extent even Core Sound (Figure 3) are marine 

 lagoons. A number of true estuaries can be found 

 on the North Carolina coast, such as the mouths 

 of the Newport and Cape Fear Rivers. 



Estuarine waters are characteristically variable 

 in virtually all of their physico-chemical pro- 

 perties. For example. Rice and Ferguson (1975) 

 describe the variability in temperature and sali- 

 nity in the Newport River estuary in North Caro- 

 lina. .\nimals and plants that inhabit estuaries 

 must be able to tolerate this extreme environ- 



mental variability. Much of the variability, in tem- 

 perature and salinity especially, is on a very short 

 time scale in that it is produced by the change of 

 tides and the alternating influence of the sea and 

 of terrestrial runoff (Roelofs and Bumpus 1954). 

 Along the .Atlantic coast of North America tides 

 are semidiurnal, meaning that there are two low 

 and two high tides daily. Consequently, every 

 6 hr or so, conditions within an estuary are likely 

 to change radically. Water temperature and salini- 

 ty within an estuary or lagoon vary with tidal 

 movements. As the incoming tidal current pene- 

 trates the estuary, salinity rises. In winter, water 

 temperature also rises abruptly on incoming tides; 

 in summer, the ocean is colder and provides a 

 cooling influence at high tide (Roelofs and 

 Bumpus 1954). Because of these sudden changes 

 in the pysical conditions that recur with each 

 tidal change, daily changes in many environmental 

 variables in an estuary or marine lagoon are often 

 a large fraction of the total annual variation. This 

 renders the estuary a harsh physical environment 

 for both plants and animals. 



The movement of tidal currents through the 

 complete ebb and flood cycle is rarely symmetri- 

 cal in any sound or estuary. Inlets at the mouths 

 of sounds and estuaries are usually either ebb- 

 dominated or flood-dominated. In a system domi- 

 nated by flaod tides, the incoming tidal currents 

 usually last a shorter period of time than the out- 

 going currents, but the incoming flood currents 

 are much faster. Ebb-dominated systems possess 

 the opposite pattern. In both ebb- and flood- 

 dominated systems, the flow velocities vary 

 through the tidal cycle. The alternating directions 

 of flow produce short periods (up to 20 or 30 

 min) of slack water inside sounds and estuaries 

 near the time of the change of the tides. Dining 

 these periods turbidity drops to a minimum only 

 to rise again with the resinnption of tidal currents. 



Turbidity of the water column is, in part, 

 related to the degree of input from rivers into the 

 coastal waters. Estuaries tend to be more turbid 

 than coastal lagoons, where terrestrial input is 

 negligible. Some estuaries like those along the 

 Georgia-South Carolina coast experience greater 

 freshwater flows and have higher turbidity levels 

 than others, like the Newport River estuary in 

 North Carolina. There is a general gradient of in- 

 creasing turbidity from the central North Carolina 

 estuaries and coastal lagoons to those of the south- 



