Fjords stratification

Figure 3.7 The general categories of estuarine circulation identified as Type A, well-mixed estuaries, where there is minimal vertical stratification in salinity Type B, partially mixed estuaries, where the vertical mixing is inhibited to some degree Type C, highly stratified with lower freshwater discharge than the salt wedge system and Type D, salt wedge estuary and many fjords. (Modified from Bowden, 1980.)...

Figure 3.9 Stratification-circulation diagrams used to describe a spectrum of circulation and geomorphometric types of estuaries that can be defined by stratification. Estuarine types are as follows Type 1 estuaries are those without upstream flow requiring tidal transport for salt balance Type 2 estuaries are partially mixed (e.g., Marrows of the Mersey (NM) (UK), James River (J) (USA), Columbia River estuary (C) (USA) Type 3 estuaries are representative of fjords [e.g., Siver Bay (S), Strait of Juan de Fuca (JF) (USA)] and Type 4 estuaries indicative of salt wedge estuaries [e.g., Mississippi River (M) (USA)]. The basic classification parameters are as follows the stratification is defined by SS/Sq where SS is the difference in the salinity between surface and bottom water and So is the mean-depth salinity, both averaged over a tidal cycle and Us/Uf, where U is the surface velocity (averaged over a tidal cycle) and Uf is the vertically averaged net outflow. The subdivisions a and b represent values where SS/Sq <0.1 and SS/Sq >0.1, respectively subscripts h and 1 refer to high and low river flow. The curved line at the top represents the limit of surface freshwater outflow. (From Hansen and Rattray, 1966, as modified by Jay et al., 2000, with permission.)...

Hansen and Rattray (1966) introduced a general classification scheme for estuaries based on stratification/circulation that is divided into the following four estuarine types Type 1 estuaries well-mixed estuaries with mean flow in the seaward direction and the salt balance being maintained by diffusive processes—via tidal transport Type 2 estuaries partially mixed estuaries where the net flow reverses at depth and the salt flux is maintained by both diffusive and advective processes Type 3 estuaries these estuaries include fjords with two distinct layers and advection accounting for the majority of the salt flux Type 4 estuaries these are salt-wedge estuaries where freshwater flows out over a stable more dense bottom layer. 

An example of sulfate reduction in a shallow fjord estuary is the upper basin of the Pettaquamscutt River (Rhode Island, U.S.A.) studied by Orr and Gaines (1973). It is normally well-stratified water but, every few years, there is overturn which mixes near-surface oxygenated water with the bottom water containing biogenic sulfide. In a time span of just over one month after an overturn in November 1971, 80% of the sulfide was oxidised. After stratification was re-established, the sulfide remained below 4 m depth and accumulated at a calculated rate of 48 g S m" y. ... 

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