Current velocities in these lagoons are functions of size of the lagoon, its shape, size of the inlet and tidal range etc. Tides are of primary importance, providing periodic exchange. Wind stress plays a sometimes dominant but variable role, depending on the strength of the local tide and the characteristic wind speed (Sultan and Ahmad, 1990 and Ahmad and Sultan, 1992). The tides in the Red Sea are probably a combination of an independent oscillation of the water within the basin and a forced co-oscillation induced by the tides in the Gulf of Aden (Morcos 1970). The independent oscillations are semidiurnal and of small

amplitude. Towards the central part of the Red Sea the tidal range decreases (Edwards & Head 1987); at about 20°N there is a nodal point. Between 19°N and selleckchem 21°N the tidal currents are weak and variable. However, at the inlets

of the coastal lagoons, the tidal currents may be as high as 1 m s− 1. But in the main body of the lagoons the currents are weak and vary depending on the spring-neap cycle and the variation in sea level. The Red Sea level is strongly influenced by the rate of evaporation and balance between in- and outflowing water at Bab-el-Mandab. In winter the inflow exceeds the combined effect of outflow and loss by evaporation in spite of the fact that evaporation is higher in winter, at least in the central Red Sea (Ahmad & Sultan 1989). Consequently, the mean sea level rises over the entire Red Sea in winter. In summer the reverse occurs and mean sea level is lower (Morcos,

1970, Ahmad and Sultan, 1993 and Smeed, 2004). The use of some coastal lagoons R428 in vivo as discharge areas for industrial and municipal waste and some others as sea water intakes for desalination purposes in Saudi Arabia makes an assessment of water column Idoxuridine stability indispensable. Lying to the north of Jeddah (Figure 1), Rabigh Lagoon is over 17 km long and has an average width of about 4 km. Urban and industrial development has begun to exert an impact upon the lagoon’s ecology. The objective of this study is to predict the water column conditions in Rabigh Lagoon. It is generally shallow but in some parts depths may reach about 20 m. The mean rate of change of the potential energy of the water column is applied to evaluate the water column condition, i.e. whether it is stratified or vertically mixed (Simpson and Hunter, 1974, Simpson et al., 1978, Simpson and Bowers, 1981, Yanagi and Takahashi, 1988, Yanagi and Tamaru, 1990 and Simpson, 1997). The potential energy ‘v’ is relative to the mixed conditions, and positive and negative signs are assigned to the term that increases and decreases water column mixing. The equation is: equation(1) dvdt=−αgQH2cp−βgSHR2A+4ϵKbρwut33π+δKsγρaw3. The 1st term is the contribution of the surface heat flux, while the 2nd, 3rd and 4th terms are the respective contributions of fresh water discharge, tidal mixing and wind mixing.