Other Estuaries

A useful way of describing the forcing of estuarine sedimentary processes is in terms of the specific dissipation (watts/square meter), which will be a function of both time and location throughout the estuary. Direct, systematic measurements of the specific power are not likely to be available for many (if any) estuaries, so estimates of the specific power must be based on the characteristics of the forcing mechanisms. The most im- 

The specific dissipation due to wave power is strongly dependent on water depth and, therefore, will have sharply defined bounds in most estuaries. It Is determined by the depth, the available fetch, and the intensity of the winds having sufficient duration to raise a fully developed sea. For Long Island Sound the wave-dominated zone is that in water shallower than 18 m this constitutes 54% of the total area of the Sound. Within the wave-dominated zone the particle motion due to waves at the water surface is more effective in exciting sediment from the bottom than other causes of water movement. Large quantities of sediment may be set in motion by the waves and relatively small currents can then effect substantial transport of the material so excited. An example of an estuary in which wave-excited sediment is an important fraction of the total sediment available for estuarine processes is the Tay, where wave erosion followed by overland flow on bare mudflats exposed on the ebb of the tide results in large sediment concentrations in the water of the estuary (Buller et al., 1975). 

The specific dissipation due to river power is most likely to be important near the head end of an estuary, but in some estuaries where the discharge is very large and the tide weak, power from the inflow of fresh water may dominate throughout. The specific dissipation due to the fresh water flow is 7 dSu, where is the mean flow speed, S the slope of the water surface, d the depth, and 7 the unit weight of water. Long Island Sound has no significant area where the specific dissipation due to fresh water inflow is dominant. In the estuary of the Connecticut River it is expected that river power will be a significant fraction of the tidal power when the river is in spate, but detailed calculations have not been done. 

In addition to raising waves on the water surface, winds will set the surface layers of water in motion in the direction of the wind stress or, if the water is sufficiently shallow, set up a circulation pattern extending to the bottom. Pickard and Rodgers (1959) have shown, for example, how an up-estuary wind can set the surface layer of water in the Knight Inlet (B.C.) in motion against the estuarine circulation. Elliott (1978) has demonstrated the importance of wind stress in determining the circulation in the Potomac estuary. To have much influence on estuarine sedimentary processes, however, the wind-driven circulation must penetrate to the bottom, which is likely to happen only in relatively shallow estuaries. In 

Long Island Sound wind-driven water movements of local origin do not influence the water flow over the deeper parts where the deposits of muddy sediment are located. Their effects in the wave-affected zone have not been investigated. 

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Atchafalaya and Mississippi Rivers. Florida Bay waters, which overlie U-rich sediments, contain much higher ( Ra/ " Ra) activity ratios than other estuaries. The increased ( Ra/ " Ra) values observed at high salinities in the Mississippi/Atchafalaya systems indicate preferential decay of the shorter-lived ""Ra over Ra during estuarine mixing. 

The input of terrestrial DOM via rivers and aeolian transport was discussed in Chapter 23.3. Riverine concentrations of DOC range from 2 to 20mgC/L. In contrast, little or no terrestrial DOM is detectable in seawater, leading to the cmrent consensus that most is removed close to its point of input. In some estuaries, removal is associated with flocculation reactions promoted by the large increase in ionic strength that occms when river water mixes with seawater. In other estuaries, DOC exhibits conservative behavior, leaving marine chemists with a mystery as to how and where DOC is removed. 

Chesapeake Bay, USA, is the largest estuary on Earth and almost all of the arsenic entering the headwaters is As(V). Although inorganic As(V) is consistently the most abundant arsenic species in the estuary, extensive arsenic reduction and methylation occur during warm months (Sanders, Riedel and Osman, 1994), 295 (Millward et al., 1997), 53. The appearance of As(III) and methylarsenic species correlates well with phytoplankton production. Similar seasonal patterns involving arsenic reduction and methylation are seen in other estuaries (Sanders, Riedel and Osman, 1994), 295. 

Fig. 4. Electrophoretic mobilities (Ug)of natural (untreated) - curve A - and treated particles as a function of salinity (S°/°<>) for two sets of samples from Keithing Burn (KB 1 open symbols - 31 March 1982 KB 2 closed symbols - 30 dune 1982). Shaded area B indicates the spread of results from other estuaries (redrawn from Fig. 3 of Hunter and Liss 1979). Curve D - suspended particles centrifuged and resuspended in UV- oxidized sample supernatant and then UV-oxidized. Curve C - natural samples (particles plus supernatant) UV-oxidized. Curve E - sample supernatant UV-oxidized to form new particles (UV-PPT). Several UV-PPT samples from KB2 were centrifuged and the particles resuspended in their original untreated sample supernatant. The resulting changes in Ug are indicated by the dashed lines (asterisks - final values). Keithing Burn suspended matter is mostly composed of iron oxides (after Loder and Liss, 1985).

Buller, A. T., Green, C. D., and McManus, J. (1975). Dynamics and sedimentation The Tay in comparison with other estuaries. In Nearshore Sediment Dynamics and Sedimentation (J. Hails and A. Carr, eds.), pp. 201-249. Wiley, New York. 

We feel that we now have a much better understanding of how one estuary functions as a whole. This understanding yields a sense of satisfaction, but does not leave us in a state of complacency much remains to be learned about the complexities of Long Island Sound, let alone about the other estuaries of the world. Nonetheless, we believe that many of the methods we have used to examine the Sound will prove useful in studies elsewhere. 

Because the sea water obtained in Freeport, as in many other estuarial locations, varies in salinity, a standard must be set against which the concentrations are measured both in the diluted sea water and in the concentrated brine. 

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