water rise. A quantitative under- 

 standing of the wind, however, was 

 limited by the complications of local 

 geography at each site. A more 

 general result was that the 

 theoretical relationship of 1 cm 

 change in sea level per 1 milli- 

 bar (mb) change in barometric pressure 

 appeared to be correct in this area. 

 Since pressure drops of 20 to 30 mb 

 are not uncommon, this is an important 

 component of storm effects. However, 

 Miller (1958) pointed out that "surge 

 is a rapid rise in water level with a 

 duration of several hours or less, 

 while set-up appears to be a 

 relatively slow rise or fall of water 

 level with durations of hours or 

 days " 



It is less widely realized that 

 there is a seasonal cycle in sea level 

 which passes through a minimum in 

 winter and a maximum during summer 

 (Figure 6). Along the northeast 

 coast, the annual range in monthly 

 mean sea level appears to increase 

 from about 5 cm (2 inches) at East- 

 port, Maine, to over 15 cm (6 inches) 

 at New York (Emery and Uchupi 1972). 

 About 9 cm (3.5 inches) or less of 

 this seasonal variation may be 

 attributed to a seasonal cycle in 

 barometric pressure, and seasonal 

 changes in the wind may also play a 

 role (Emery and Uchupi 1972; Kjerfve 

 et al. 1978), but in general, the 

 remainder is due largely to changes in 

 the in situ density of the sea water. 

 In areas with little freshwater input 

 and a large annual temperature range, 

 much of this density change may be due 

 to heating and cooling (Kjerfve et al. 

 1978), but for most of the northeast 

 coast the density of the water appears 

 to be more strongly regulated by 

 freshwater discharge (Figure 6) (Emery 

 and Uchupi 1972). Changes in fresh- 

 water input also appear to be respon- 

 sible for much of the variation in 

 annual mean sea level (Figure 7), 

 though there must be other factors 

 operating, including secular rise 



10 



and longer-period oscillations (Hicks 

 1968, 1972; Emery and Uchupi 1972). 

 Still other, more permanent changes in 

 water level may result from the 

 dredging of breachways or passes 

 through barrier spits (Lee 1980) or 

 natural coastline changes such as the 

 expansion or contractions of barrier 

 spits or the shoaling of channels 

 (Johnson 1925; Redfield 1972). 



The magnitude of these various 

 short-term changes in sea level 

 appears deceptively large compared 

 with the long-term secular rise of 

 only 1 mm/yr discussed earlier. In 

 the Northeastern United States, it may 

 take about 100 years for the secular 

 rise to equal the seasonal variation 

 in any one year, but the seasonal 

 variation is taking place around an 

 annual mean which is increasing (on 

 the average) throughout the 100-year 

 period. Moreover, on geological time 

 scales the variation due to glaciation 

 is much larger than any of the 

 short-term processes. Nevertheless, 

 the daily, seasonal, and yearly 

 variations in mean sea level (around 

 which the still shorter-term tidal 

 variations must act) may influence the 

 distribution of organisms and 

 sediments on the marsh, as well as 

 chemical exchanges between the tidal 

 waters and the surface of the marsh. 

 The potential for such an effect was 

 described by Kjerfve et al. (1978) in 

 a careful study of a South Carolina 

 marsh where the seasonal range in 

 monthly mean sea level was 26 cm (10 

 inches). 



One consequence of this variation 

 was that the marsh was innundated 42% 

 of the time during October, but only 

 27% of the time in January. Such a 

 difference may well influence the 

 growth rates of small fish or other 

 animals that feed on the marsh surface 

 (Valiela et al. 1977), but it does not 

 necessarily follow that all water- 

 marsh interactions will be greatest 

 during the times of maximum sea level. 



