uppermost portion of the sound and the 

 main rivers feeding the sound. This zone 

 is influenced by both marine and riverine 

 processes. Bottom sediments in the upper 

 reaches of the estuary are characteristi- 

 cally muddy sands or interbedded fine- 

 grained sands and muds. Farther inland, 

 if the river transports a significant 

 amount of coarse-grained material, bottom 

 sediments contain a decreasing percentage 

 of mud (Dorjes and Howard 1975). Turbid- 

 ity levels are generally higher in this 

 zone (the middle region) during all por- 

 tions of the tidal cycle (Day 1951; Howard 

 et al. 1975). These higher turbidity lev- 

 els in part reflect the fact that tidal 

 currents (especially ebb currents) attain 

 the highest velocities in the middle re- 

 gions of the estuary before they are 

 slowed in the open sound. The importance 

 of turbidity to oyster populations will be 

 explained in Section 2.3. 



The complex network of smaller tidal 

 creeks that drain extensive areas of salt 

 marsh forms the third division of the es- 

 tuarine sedimentary environment. Tidal 

 creeks exhibit highly sinuous channel pat- 

 terns; laterally migrating point bars on 

 the convex inner sides form depositional 

 banks. The concave outer banks of tidal 

 creeks are areas of net erosion, where 

 water currents attain their highest veloc- 

 ities. This estuary zone can be classi- 

 fied as a low-energy, sedimentary environ- 

 ment. Current velocities in tidal creeks 

 depend on the extent of marsh drainage 

 area. Fine-grained mud-silts and, less 

 frequently, fine sands are the most common 

 bottom sediments. Despite the relatively 

 fine grain size of bottom sediments in 

 tidal creeks, the bottom includes all gra- 

 dations, from extremely soft and organi- 

 cally rich to hard mud and clay (Galtsoff 

 and Luce 1930). The degree of bottom sed- 

 iment consolidation is a function of the 

 interaction between depositional and ero- 

 sion forces. Hard mud bottoms form in 

 areas where tidal creeks erode into con- 

 solidated marsh sediment. 



Physico-Chemical Environment 



The chemical environment of the estu- 

 arine ecosystem is strongly influenced by 

 local hydrography. The three general divi- 

 sions (Figure 2) of the estuarine system 

 used in the discussion of sedimentation 



also provide a convenient framework for a 

 discussion of the chemical environment. 



In the study area, estuaries are 

 characterized by highly variable lateral 

 and vertical salinity gradients. Within 

 any particular estuary, however, salinity 

 trends are best described by the degree of 

 vertical mixing taking place between fresh 

 and saline water masses. Three relatively 

 well-defined salinity zones exist in the 

 majority of estuarine systems: (1) a 

 stable, well-mixed, and marine-dominated 

 lower zone; (2) an unstable intermediate 

 zone where large changes in the vertical 

 salinity gradient occur with each tidal 

 cycle; and (3) a stable upper region domi- 

 nated by riverine fresh water influence 

 (Howard et al. 1975). The juxtaposition of 

 these three zones depends upon the inter- 

 action and relative magnitude of riverine 

 and tidal influences. In the lower sound 

 and inlet entrance, corresponding to zone 

 1, mean salinities are high, ranging from 

 approximately 20 °/oo (parts per thousand) 

 to 32 °/oo, and the water column tends to 

 remain well mixed throughout the tidal 

 cycle. In estuaries receiving large river- 

 ine inflows, the well-mixed, high-salinity 

 zone may be displaced seaward several kil- 

 ometers (Oertel 1974). The upper sound in 

 the vicinity of the river mouths is influ- 

 enced by both marine and riverine process- 

 es. Salinity in this region varies, rang- 

 ing from 5 °/oo to over 20 °/oo, and 

 strong vertical salinity gradients are 

 common. Upstream of the river mouths 

 (zone 3), salinities reflect riverine in- 

 fluence. The water column remains well 

 mixed at all times, and salinities vary 

 from °/oo to 10 °/oo. Salinity varia- 

 tions in marsh tidal creeks correspond to 

 that of the tidal water mass flooding the 

 marsh. As might be expected, these values 

 are lowered significantly during periods 

 of local precipitation in the marsh and 

 resultant runoff from adjacent uplands. 



In general, thermal mixing of estua- 

 rine water masses occurs rapidly (Oertel 

 1974). Hence, over most of the lower 

 sound, vertical temperature gradients in 

 the water column are not pronounced 

 and are subject to daily fluctuations 

 (Oertel 1974). In summer, lower tempera- 

 ture ocean waters have a cooling influence 

 on the estuary. Water temperatures in 

 marsh creeks are slightly higher during 



