Near-bottom layers

The shallow-water character of the sea provides rapid propagation of wind and convective mixing down to the bottom, which leads to equalizing the vertical temperature distribution in most cases, the temperature difference is less than 1 °C. Meanwhile, during summertime calm periods, the thermocline is formed which prevents the near-bottom layer from water exchange. 

The absolute values of T , S, and oq in the layer 1500-2100 m significantly vary from one survey to another, which may be explained by the biased calibrations of CTD profilers [14]. In our case, it is manifested in the vertical homogeneity of T and S standard deviations (respectively, 0.031-0.033 °C and 0.021-0.027 psu) over the depth range from 1500 to 2100 m. According to the measurements carried out at the beginning of the 1990s, the mean values of T and S in the near-bottom layer of the Black Sea were 8.895 °C and... 

The high intensity of anthropogenic load on ecosystems of the Black Sea affects, first of all, the viability of organisms confined to the phase interface coastal zone, near-surface and near-bottom layers of the water. Adverse environmental consequences of the Black Sea pollution are revealed in the reduced populations of hydrobionts that were previously widespread. 

The near-surface currents at about 10 m depth predominantly flow toward northwesterly directions (outflow), and the flow 3—4 m above the bottom is predominantly directed toward east and southeast (inflow). The magnitude in the near-bottom layer is smaller than the magnitude within the surface layer. The net mean currents (speed) are comparable, but in opposite directions. Opposite pressure gradient forces determine the water exchange through the Fehmambelt within both layers. 

The ADCP current profiles at the same site revealed an interesting structure. Between the surface layer (net outflow) and the near-bottom layer (net inflow) there is an intermediate layer with an upper boundary fluctuating between 12 and 18 m (Table 6.5). At this upper boundary, the mean vector speed is minimum and exhibits a great directional variability (sf). At its lower boundary between 16 and 23 m depth, the intermediate layer exhibits relatively high vector speeds and directional stability. No matter whether the mean values indicate outflow within the entire water column (the opposite case—inflow within the entire water column—was never observed on the long run) or outflow and inflow prevail in the upper and... 

At the eastern flank of the relatively shallow and topographically even Darss SiU is the MARNET station Darss Sill (DS, see Chapter 3), where operational ADCP current measurements have been carried out since 2004. This is the region from where the most important proportion of saline and oxygenated water masses from the Kattegat occasionally spreads within the near-bottom layer down the slope into the deep of the Arkona Sea. Table 6.8 shows the annual and monthly means of the year 2005. 

Wind and density induced currents near the bottom temporarily are very weak. If there were no tidal currents, which are weak but are always ( ) present oxygen depletion in the near-bottom layers of the Baltic Sea probably would occur more frequently. Land uplift and subsidence play a major role in current studies of sea level rise. However, when evaluating gauge data, also the tides have to be taken into account, especially in historical datasets (Liebsch, 1997),... 

The temperature of the near-bottom layers of the whole Gotland B asin decreased by about 1 C during the 10 year of stagnation. The MBls between 1960 and mid-1970 caused... 

The moderate MBI in November-December 1976 led to very high temperatures in the near-bottom layers of the central Baltic, the highest on record in the Gotland Deep (cf. Figs. 10.11 and 10.14). The moderate MBI in September 1997 resulted in exceptionally high temperamres in the Eastern Gotland Basin (7.0°C in January 1998 at the 200 m level of the Gotland Deep and 6,1°C in May 1998 in the 150 m level of the Faro Deep). 

FIGURE 12.14 Time series of DOC and POC (top) and DON and PON (bottom) in the near bottom layer of station 271 in the central Eastern Gotland Basin. Oxygen concentrations characterizing the redox state are added. H2S was converted into negative oxygen equivalents. (Updated from Nausch et al., 2005.)... 

FIGURE 14.26 Model situation for mean current velocities of >5.5 cm/s in the near-bottom layer of the Arkona Basin. The current directions and velocities are described by black arrows (The figure was provided by Seifert, lOW). 

To explain the observed peculiarities of the vertical structure of the ionic content, we recall that, as we substantiated in our previous publications, the water in the bottom part of the western trench usually contains a significant admixture of the water originating from the shallow eastern basin [2, 9,10]. Saltier, denser, and chemically altered to a larger extent, this eastern water penetrates into the western basin under favorable wind conditions and then sinks into the near-bottom layer, while gradually mixing with the ambient waters. In fact, this mechanism is likely to be the principal controller of the western basin stratification. As shown in [2], typically, 10-20% of the water mass in the western basin is associated with recent intrusions from the eastern basin. Because the advected eastern water transports negative buoyancy, its core must be located in the bottom layer and little or none of it is manifested at the surface. The deeper a sample is taken, the larger is the content of the eastern water in it. 

These results show that thermocapillary forces generate a complicated circulation liquid motion in the layer, and, the flow changes its direction at the depth equal to 1/3 of the layer depth. Just as one can expect, the flow is symmetric with respect to the plane X = 0 with temperature To the fluid flows out from the near-bottom layer along this plane. 

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