Only winds associated with winter frontal pas- 

 sages or hurricanes produce large or sustained waves 

 offshore. Hurricanes usually have a net drift toward 

 the northwest. They can cause considerable modifica- 

 tion to the siielf waters and generally push oceanic 

 waters onto the shore and into estuaries. The intense 

 wave action associated with hurricanes reworks the 

 shelf sediments and can transport large quantities of 

 sediments shoreward, which ultimately affects circu- 

 lation by means of density gradients. 



Tides along the western shelf, especially in the 

 areas of the Sabine and Calcasieu lakes, are as high as 

 0.76 m (2.5 ft) and should produce significantly 

 greater tidal currents than expected around the 

 Mississippi Delta. Locally, significant tidal currents of 

 3.3 kn flood and 4.3 kn ebb develop in restricted 

 passes in the Galveston Bay area, particularly between 

 Galveston and West Bay and between Christmas Bay, 

 Bastrop Bay, and West Bay (Murray 1976). 



Turbidity (suspended solids) is closely related to 

 current velocity, because the faster the current, the 

 greater its potential for carrying sediment. Turbidity 

 is of particular importance to primary productivity 

 because nutrients needed by phytoplankton tend to 

 be adsorbed onto suspended or precipitated clay 

 particles. However, when water turbulence is in- 

 creased, sediments are resuspended and nutrients are 

 released into the water column and become available 

 for plankton. The high turbidity that is observed in 

 shallow inland and nearshore waters is primarily 

 attributable to tidal flow and to local wave conditions 

 which stir up and suspend bottom sediments. Primary 

 productivity (rate of photosynthesis per unit water 

 volume) in turbid waters is greater than in nearby 

 clear waters in south-central Louisiana (Sklar 1976). 



Although productivity may be enhanced by tur- 

 bidity, excessive amounts or prolonged periods of 

 high turbidity may be counter-productive. The depth 

 of the water column that will sustain photosynthesis 

 decreases with increased turbidity because of reduced 

 Ught intensity. 



Mudflats result from the net effect of sedimentary 

 input from local rivers and the erosional forces of 

 waves and longshore currents. When sedimentation ex- 

 ceeds erosion, mudflats may develop offshore of the 

 beach. Alternatively, where the longshore sediment 

 load is very small, severe storms may push the beach 

 ridge back over the marshes behind them. This process 

 also can result in exposed intertidal mudflats, which 

 were former marshes. 



4.8.3 THE IMPORTANCE OF SALINITY 



Sahnity is one of the major variables affecting 

 the abundance and composition of aquatic life. 

 Although a natural salinity gradient persists from land 

 to the ocean, the extent of the gradient at any one 

 time may vary depending upon the depth, rate of 

 freshwater inflow, water circulation pattern, and tidal 

 flow. For the Chenier Plain, the normal gradient may 

 range from near zero s ahnity in and n ear river mouths 



and lakes to 5°/oo to 15°/oo in the mixing zones of 

 the inland open waters, and 10%o to 30%o or higher 

 in the nearshore Gulf waters. 



Despite the tendency for a saUnity gradient in 

 these aquatic habitats, dynamic changes in saUnity 

 are relatively common. Floodwaters from rivers may 

 reduce sahnities over large areas, or strong winds and 

 ocean currents may flush unusually large quantities of 

 saltwater into systems. In some cases a saltwater wedge 

 will penetrate inland open waters, expecially ship 

 channels, and cause wide differences between surface 

 and bottom sahnities (Bowden 1967). 



The significant flow of fresh turbid water from 

 the Atchafalaya River into Louisiana coastal waters 

 keeps the nearshore zone relatively diluted to the 

 Texas border. During the flood season, the sahnity 

 levels along the entire open coast of the Chenier 

 Plain may be as low as sahnity levels in estuaries. The 

 salinity pattern suggests slow shoreward movement 

 of water in the lower saline layer and a circulation 

 dominated by local wind effects in the upper brackish 

 layer. Extreme changes in salinity may reduce or 

 destroy some plant or animal populations. 



4.8.4 ORGANIC DETRITUS DERIVED 

 ADJACENT WETLANDS 



FROM 



In the section on wetlands, it was emphasized 

 that these communities produce vast quantities of 

 detritus. Waters flooding these wetlands carry some 

 of this detritus to inland open waters where it enters 

 the food chain. The magnitude of export depends 

 upon the abundance of detritus and flushing fre- 

 quency, and its impact depends partly on the area of 

 open water relative to the area of adjacent wetlands. 

 The export of organic matter from adjacent habitats 

 into Calcasieu Lake ranges from 1,100 kg/ha/yr(981 

 Ib/a/yr) from fresh marshes to 7,300 kg/ha/yr (6,513 

 Ib/a/yr) from saltmarshes (fig. 4-28). The open water 

 productivity in situ for Calcasieu Lake is indicated in 

 table 4.17. Figure 4-28 also suggests that inland open 

 waters are themselves exporters of organic matter to 

 the nearshore Gulf. This phenomenon, called outwell- 

 ing, is considered an important reason for the high 

 productivity of coastal waters compared to deep 

 oceanic waters. The gradient of decreasing organic 

 carbon concentrations from marshes through the bay 

 to the Gulf (table 4.18) has been demonstrated for 

 Barataria Bay, Louisiana, by Happ et al. (1977). 

 Although outweUing has not been measured in the 

 Chenier Plain, the data in figure 4-28 indicate that the 

 phenomenon must occur. The magnitude of outwelling 

 probably depends to some extent on the flow through 

 coastal passes. Estimating an annual export of 100 kg 

 organic matter per hectare of inland open water 

 (89 lb/a) (a conservative estimate from Happ et al. 

 1977), one can predict that about 46,000 tonnes 

 (50,706 tons) of organic material is flushed from the 

 Calcasieu Basin into the nearshore Gulf habitat each 

 year. 



197 



