Estuarine circulation

Ocean prevents eutrophication. Much more water flows into the Mediterranean Sea than is required to replace evaporation from it. The excess, high salinity water exits Gibraltar below the water flowing in af fhe surface. Nufrients that enter the Mediterranean Sea from pollution sources are utilized by marine phytoplankton that sinks and exits with the outflow. Another example is that estuaries often have lower salinity or even freshwater at the surface with a denser saline layer at the bottom. An estuarine circulation occurs with nutrients being trapped in the saline bottom water. 

The water column distribution of particulate " Th in partially mixed estuaries aids in assessing the transport of particles throughout the system, as a consequence of tidal mixing or the estuarine circulation. Feng et al. (1999a) took advantage of the fact that the... 

Pollutant trapping in estuaries is the result of two related phenomena patterns of water circulation and patterns of sediment deposition and resuspension. As shown in Figure 28.4, estuarine circulation is characterized by a subsurface inflow of dense saline... 

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.)...

B., and Barron E. (1996) Estuarine circulation in the Turonian Western Interior Seaway of North America. Bull. Geol. Soc. Am. 108, 941-952. 

This positive water balance determines the basic hydrographic and ecological properties of the Baltic Sea as the estuarine circulation, the deepwater formation and ventilation, and the stratification and the nutrient balance. Outflow of brackish surface water and inflow of saline water combined with upwelling and vertical mixing of saline bottom water with brackish surface water closes the estuarine circulation. 

The freshwater balance of the Baltic Sea drives an estuarine circulation that determines the basic properties of the hydrography and the ecosystem of the Baltic Sea on a longer timescale. 

The Straits of Sicily represent a physical barrier which lies at the entrance to the Eastern Mediterranean. It is approximately 500 m deep and represents a significant control on the biogeochemical processes occurring within the eastern basin. There also is an anti-estuarine circulation through these straits because evaporation exceeds precipitation in the eastern basin. A surface and subsurface flow of modified Atlantic waters, low in nutrients, flows eastwards into the eastern basin while LIW with relatively higher nutrients flows westwards out of the basin. This is the fundamental feature that causes the Eastern Mediterranean as a whole to be oligotrophic. 

The Black Sea shows up as an area of very high chlorophyll throughout the year. This is expected since it is dominated by river inputs and estuarine circulation. The sea of Marmara also has high chlorophyll throughout the entire year. While there is a higher chlorophyll plume from the Black Sea into the northern Aegean, the amount seems to be relatively small and certainly smaller than the plumes observed from the Po and the Nile. Qualitative observations on the biomass produced by these extra-basin inputs are entirely compatible with recent estimates for the amount of nutrients fluxed into the basin from the sources (Krom etal., in preparation). In addition to these natural increases in biomass there is a major increase in colour off the Tunisian coast. This corresponds to an area of shallow water where there is both an increase in chlorophyll and particulates in the water column. 

Fig. iO. Velocity vectors recorded for successive 20-min time intervals by a current meter 2 m above the bottom near the geometrical center of Long Island Sound. The rotary character of the tide and the net drift of bottom water due to the estuarine circulation are shown. 

Sound is the Connecticut River, which drains most of central New England and enters the Sound near its eastern end. This is an area of strong mixing (because of a high level of tide-stream turbulence) and sediment brought down the river is mixed into the bottom water that subsequently flows into the central Sound. The extensive deposits of silt-clay sediment in the central basin of the Sound are formed, therefore, as a consequence of the estuarine circulation. 

Gordon, R. B. (1979). Denudation rate of central New England determined from estuarine circulation. Am. J. Sci. 279, 632-642. 

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