Three-layer water column

Figure 2. Three-layer water column transfer of a conservative species from the lower into upper layer...

The ocean system is separated into three major reservoirs that best represent the dominant pools and pathways of P transport within the ocean. The surface ocean reservoir (5) is defined as the upper 300 m of the oceanic water column. As discussed in an earlier section and displayed in Fig. 14-6, the surface layer roughly corresponds to the surface mixed layer where all... 

The model (Fig. 23.6) consists of three compartments, (a) the surface mixed water layer (SMWL) or epilimnion, (b) the remaining open water column (OP), and (c) the surface mixed sediment layer (SMSL). SMWL and OP are assumed to be completely mixed their mass balance equations correspond to the expressions derived in Box 23.1, although the different terms are not necessarily linear. The open water column is modeled as a spatially continuous system described by a diffusion/advection/ reaction... 

Pacanowski and Philander, 1981 Peters et al., 1988). More sophisticated methods are based on prognostic equations for the turbulent kinetic energy k and a second quantity, which is either the dissipation rate e or a length scale in the turbulent flow see Burchard (2002) for a recent review and applications of two-equation turbulence closures for onedimensional water column models. A two-equation turbulent closure has been applied by Omstedt et al. (1983) and Svensson and Omstedt (1990) for the Baltic Sea surface boundary layer under special consideration of sea ice, whereas the application in three-dimensional circulation models is described by Burchard and Bolding (2002) and Meier et al. (2003). 

To simulate the transport of sedimentary material in the water column over realistic topography, it is necessary to run a three-dimensional circulation model, which is extended by submodels describing the surface waves, (Schwab et al., 1984), the shear forces within the bottom boundary layer, and the resulting deposition and erosion processes at the seabed. 

Temperature and salinity data are obtained with SBE 16 Seacat thermosalinometers at four depth levels (7, 12, 17, and 19.5) and cover the entire water column from the mixed surface layer to the sea bed. Additional temperature sensors are installed at 2 and 5 m depth since 2003. Intercalibrations of the instruments with a SeaBird SBE 9 CTD are carried out during regular maintenance cruises approximately every three months. The accuracy of temperature and salinity is 0.01 K and 0.02 psu, respectively. Occasionally, failures of instruments caused a partial loss of data in a few periods of time. 

In water and sediments, the time to chemical steady-states is controlled by the magnitude of transport mechanisms (diffusion, advection), transport distances, and reaction rates of chemical species. When advection (water flow, rate of sedimentation) is weak, diffusion controls the solute dispersal and, hence, the time to steady-state. Models of transient and stationary states include transport of conservative chemical species in two- and three-layer lakes, transport of salt between brine layers in the Dead Sea, oxygen and radium-226 in the oceanic water column, and reacting and conservative species in sediment. 

The three-dimensional dispersion of a completely soluble organic solute within a volume of pure water will be governed by its rates of diffusion within the water column and by the flow characteristics of the water itself (also called convection or advection). In actual water bodies, complicating factors include the presence of particles of various sizes within the aqueous, phase and the effects of boundary layers such as those associated with the air-water and sediment-water interfaces. Further complications occur in soil-water and groundwater systems in which the aqueous phase is a minor component in the presence of an excess of solid material (Thibodeaux, 1979). 

Figure 32.10 shows the variations of (a) wind, (b) daily solar radiation, chlorophyll a [Stn. 1 (c), Stn. 2 (d), and Stn. 3 (e)], water temperature [Stn. 1 (f), Stn. 2 (g), and Stn. 3 (h)], and salinity [Stn. 1 (i), Stn. 2 (j), and Stn. 3 (k)]. Station 1 is located at the inner most part of east side of the bay, Stn. 2 at the center part of east side of the bay, and Stn. 3 at the west part of the inner part of Tokyo Bay (see Fig. 32.2). Chlorophyll a content varied rapidly between 0 and 100/xg/l. Chlorophyll a increased during the five periods marked by A-E in Fig. 32.10(c)-32.10(e). The increase of chlorophyll a was observed almost simultaneously at the three stations, except at Stn. 3 during A and E. So, the general variation of phytoplankton is almost the same inside the bay. The maximum chlorophyll a occurred in period B at the three stations. For these five periods, when the north wind began to blow, the blooms stopped rapidly. This is because a strong north wind caused outward transport from the inner bay and upwelling at the east side of the bay, resulting in advection and dispersion of ph doplankton in the water column. For example, when a north wind blew, the surface water temperature at Stn. 1 fell rapidly as shown in Fig. 32.10(f) (on April 20, 25, 30, May 15, June 9, and 19). In contrast, the bottom water temperature at Stn. 2 rose sharply as shown in Fig. 32.10(g) (on April 25, 30, May 15, June 9, and 19). On the other hand, the south wind caused the contrary phenomena, and the bottom layer temperature rose sharply at Stn. 1 (on May 5, 20, 28, June 3, 11, and 24). In contrast, the surface temperature fell at Stn. 2. These responses to the wind seemed to have occurred because the two stations are located on the east and west sides of the inner bay while downwelling occurs at one station, upwelling occurs at the other. These responses are particularly clear at Stn. 2, since it is located at the innermost part...

In particular, horizontal advection and horizontal diffusion in the Chesapeake Bay are comparable while vertical difiiision is a fast process that acts over short distances, and a model must account for all three. In this environment, atrazine that is discharged to the surface waters could be horizontally distributed over a distance of 1 km over a period of one week, since the time scale of horizontal advection-difiusion processes is 10 -10 s (approximately 3 hours). As atrazine is distributed horizontally, it also mixes vertically down the water coluitm. With the estimates of verticd diffiisivity for the Bay that are available in the literature, for a depth of 10-20 m the time scale for vertical diffusion processes is on the order of 15 minutes, and can be as short as 3 minutes. The sidfidic vraters are in the sediment porewaters and atrazine needs to be transported to the water-sediment inter ce in order to encounter and react with reduced sulfiir species. The characteristic horizontal and vertical scales that describe the flow in the Bay indicate that it is possible for atrazine to reach the depth of the water-sediment interface before it is horizontally transported out of the system. The subsequent exchange at the water-sediment interface depends on many factors, including half-life of atrazine, the hydraulic residence time of the bottom layer, turbulent processes, and other characteristics of the water column above the sediment layer. Simple box models cannot capture the dynamics necessary to describe these exchanges that ultimately govern the te of atrazine in the Bay. 

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