Stratified estuaries

The biodegradation of pure external isomers is faster than that of isomer mixtures due to the steric impediment [7,22], The faster biodegradation rate for internal isomers of different LAS homologues is shown for the freshwater layer in a stratified estuary in Fig. 5.3.5 (from Terzic et al. [7]). 

This stratified estuary has a depth of 40 m, with an upper fresh or brackish water layer of 0.2-4 m, depending on the river flow. The main source of pollution is untreated municipal wastewater, which is discharged into the estuary. Water samples were collected at different distances from these sewage outlets at two water depths from the fresh and the marine water layers. Furthermore, at one location, a vertical profile of the water column was made, including a sample of the water surface micro layer. Total A9PEOn, and individual AgPEOi, A9PEO2 and NP concentrations were determined with normal phase HPLC-FL analysis. 

Kvestak and Ahel investigated the biotransformation kinetics of A9PEO in the Krka estuary in Croatia [35]. Static die-away tests were performed with autochthonous bacterial cultures originating from the two compartments of the stratified estuary the upper fresh/brackish water layer and the lower saline water layer. Experiments were performed at three different temperatures, and at two concentrations. Samples were taken daily and all separate ethoxylates (1-16) were quantified by normal phase HPLC-FL analysis. No other metabolites were analysed. 

In stratified estuaries, surfactants tend to accumulate at the phase boundaries air-water and freshwater-saline water, leading to a complex vertical distribution pattern in the water column [5,6]. 

Mason RP, Fitzgerald WF, Hurley J, Hanson AKJr, Donaghay PL, Sieburth JM. 1993. Mercury biogeochemical cycling in a stratified estuary. Limnology and Oceanography 38 1227-1241. 

Borsuk, M., Stowe, C., Luettich, R.A., Paerl, H., and Pinckney, I. (2001) Modeling oxygen dynamics in an intermittently stratified estuary estimation of process rates using field data. Estuar. Coastal Shelf Sci. 52, 33 19. 

Etemad-Shahidi, A., and Imberger, J. (2002) Anatomy of turbulence in a narrow and weakly stratified estuary. Mar. Freshwater Res. 53, 757-768. 

Fisher, H.B. (1972) Mass transport mechanisms in partially stratified estuaries. J. Fluid Mech. 53, 671-687. 

Geyer, W.R., and Nepf, H. (1996) Tidal pumping of salt in a moderately stratified estuary. Coast. Estuar. Stud. 53, 213-226. 

Malone, T.C., Crocker, L.H., Pike, and Wendler, B.W. (1988) Influences of river flow on the dynamics of phytoplankton production in a partially stratified estuary. Mar. Ecol. Prog. Ser. 48, 235-249. 

Sempere, R., and Cauwet, G. (1995) Occurrence of organic colloids in the stratified estuary of the Krka River, Croatia. Estuar. Coastal Shelf Sci. 40, 105-114. 

Harding, L. W., Jr., MaUonee, M. E., and Perry, E. S. (2002). Toward a predictive understanding of primary productivity in a temperate, partially stratified estuary. Estuar. Coast. Shelf Sd. 55, 437—463. 

Meeuwig, J. J. (1999). Predicting coastal eutrophication from land-use An empirical approach to small non-stratified estuaries. Mar. Ecol. Prog. Ser. 176, 231—241. 

FIGURE 2-9 (a) An idealized estuary in cross section. In this well-stratified estuary, a distinct... 

The results of Sumer s work show that the transverse mixing in a uniform channel is decreased by the existence of a stable density gradient in about the same proportion as the vertical mixing is reduced. In addition, a relationship between the coefficient and a Richardson number can be discerned. However, few natural channels are uniform, and Sumer s results show that Dj, is increased if the channel is irregular. Sumer and Fischer (90) note that it seems likely that the same holds for but they did not investigate this aspect Most numerical models of stratified estuaries hypothesize a relationship between mixing coefficients and some form of Richardson number. The results of this study suggest that in natural channels no such relationship may exist, or if it does, it may be in the opposite direction from what has usually been assumed. ... 

These nutrients that are supplied from the seabed during summer are transported to the head of the bay by the estuarine circulation. Furthermore, they are again uptaken into the upper water, and are used for the increase in the phytoplanktons in the euphotic zone. Estuarine circulation plays a key role in the nutrient cycles of stratified estuaries by increasing the retention, recycling the nutrients, and stabilizing the ecosystem. ... 

FIGURE 2.9 (a) An idealized estuary in cross section. In this well-stratified estuary, a distinct salt wedge extends upstream beneath fresher water at the surface. The freshwater/saltwater interface moves upriver at high tide and seaward at low tide. Data from Silver Bay, Alaska, USA show (b) a steep average salinity gradient (salinity is shown in parts per thousand) and (c) upstream advection of saltwater at depth. More strongly mixed estuaries exhibit weaker vertical stratification. Data from Rattray (1967). 

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