Hydrography of a hypersaline coastal lagoon in the Red Sea

Hydrography of a hypersaline coastal lagoon in the Red Sea

Estuarine, Coastal and Shelf (1987) 24,167-175 Science Hydrography of a Hypersaline Lagoon in the Red Sea Amin Faculty Coastal H. Meshal” of M...

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Estuarine,

Coastal

and Shelf

(1987) 24,167-175

Science

Hydrography of a Hypersaline Lagoon in the Red Sea

Amin Faculty

Coastal

H. Meshal” of Marine

Received

29June

Keywords:

Science,

King

Abdulaziz

1984 and in revised

hydrography;

form

hypersaline;

University,Jeddah, 21 January

Saudi

Arabia

1986

coastal lagoons; time series; Red Sea

Temporal and spatial variations in salinity, temperature, oxygen and pH were studied in a coastal lagoon located on the eastern coast of the Red Sea during 11 cruises taken in the period from May 1983 through April 1984. The lagoon has an area of about 30 km2 and a maximum depth of about 3 m. The study showed that the water in the lagoon had a very high salinity that varied between 51% in February to 113k in October and was believed to reach higher values in summer. Rapid fluctuations in salinity were observed in the lagoon especially in positions near its mouth due to the daily exchange of water with the Red Sea accompanying the tides. During high tide, a layer of relatively cold and low salinity water from the Red Sea entered the lagoon and spread over its warmer and more saline water. This led to the existence of a strong vertical stratification in this shallow lagoon. During low tide, the subsurface warm and high salinity water was exposed resulting in the existence of one layer of water in the lagoon. Temporal variation of salinity was correlated with the monthly changes of the mean sea level of the Red Sea and with evaporation. Spatial variations in this property at any position in the lagoon depended upon its location from the mouth of the lagoon. Temperature variation was controlled by meteorological conditions in the region and by the volume of the water in the lagoon. The marine environment of this lagoon is under natural stress due to its high temperature and to the extreme fluctuations of its high salinity. This can be reduced by increasing the rate of exchange of water between the lagoon and the Red Sea.

Introduction The eastern coast of the Red Sea has a series of coastal lagoons which are still in their natural state. However, urban and industrial activities will soon begin to exert an impact upon the ecology of these lagoons. The objectives of this study was to make basic hydrographic investigations that can be considered as the background against which the situation in future may be monitored and assessed. The coastal lagoon under investigation is of considerable interest because of its potential for developing mariculture. The work reported herein offers useful information necessary in making rational decisions concerning the development of mariculture in the lagoon. “Present

address:

Unesco

Regional

Office,

P.O. Box 3945 Doha,

Qatar.

167 0272-7714/87/020167

+ 09 $03.00/O

0 1987 Academic

Press Inc. (London)

Limited

168

Amin H. Meshal

No name was given to the lagoon in any of the available charts including those made by the British Admiralty but the area where the lagoon occurs is locally called Sheik Salman Region. In the British Admiralty chart the lagoon lies directly eastof Ras Hatiba (Hatiba Head) which is a land intrusion found at the mouth of the lagoon (Figure 1) hence, it is appropriate to name it Hutiba Lagoon. Hatiba Lagoon (Figure 1) is located between latitudes 21”50’-21’57’N and longitudes 38”58’-39”Ol’E on the eastern coast of the Red Sea about 80 km north of Jeddah, Saudi Arabia. It is an isolated water body extending between the NNW and the SSE directions with a length of 13 km. The lagoon consistsof two parts: The northern basin had roughly a squared shapeof an areaof about 20 km2 and the southern basin consists of a narrow channel of about 7 km long and has an average width of about 1.5 km. The total area of the lagoon was estimated as30.25 km2. Hatiba Lagoon is connected from its western sideto the Red Seaby a narrow (700 m width) and a very shallow opening. The depth of this opening depends on the tide and varies from 20 cm to 50 cm during the ebb and the flood, respectively. At the northern end of the lagoon there is another opening to the Red Seabut it is blocked by sandshoals.Hence, the exchange of water between the lagoon and the Red Seais restricted to the western opening. The lagoon is extremely shallow with a maximum depth of about 3 m. Its water level undergoes diurnal and seasonalvariations following the daily fluctuations and the seasonal changesin the water level of the Red Sea. Hatiba Lagoon lies in an arid zone where precipitation is very scarce(6 cm year ‘) and evaporation is intense (250 cm year -I). As a result, it contains hypersaline water with salinities in excessof those of the adjacent Red Sea. Methods Field trips were made to the lagoon during the period from May 1983 through April 1984 (excluding July, August and September). In eachtrip, the spatial and vertical distribution of salinity, temperature, oxygen and pH in the lagoon were measured at 8 hydrographic stations (Figure 1). Three stations were located on section AB which lay in front of the mouth of the lagoon while the remaining five stations were distributed on section CD extending along the main axis of the lagoon parallel to the direction of prevailing winds. In addition, observations were taken at station 9 to represent the conditions of the coastal water of the Red Sea in front of the mouth of the lagoon. Measurements of depth were recorded by a portable echosounder along the two transects AB and CD. Due to the shallowness of the lagoon, observations at each station were made at two levels, i.e. the surface and immediately above the bottom. Surface sampleswere collected by filling glassbottles directly from the surface water. Subsurface sampleswere obtained either by Nansen bottle or, in very shallow regions, by immersing a stoppered glassbottle in water to the desired depth, removing the stopper to allow water to fill the bottle and then sealing it. The salinity was determined by measuring the conductivity ratio using an inductive salinometer (Beckman) calibrated with standard seawater. Since all the lagoon’s water sampleshave high salinities out of the range of the salinometer ( > 43%0),it wasnecessary to dilute the samples.Dilution was made by taking a known weight of the water sample, adding a known weight of distilled water and mixing them thoroughly. Temperatures were measured at the surface by bucket and thermometer and at the subsurface level by protected reversing thermometers (Kahlsico Co.). Values of pH were measured immediately and on-the-spot by portable pH meter (Tacussel). Samples for dissolved

Hydrography

169

of a hypersaline lagoon

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oxygen were collected, fixed and analysed according to the Winkler method (Grasshof, 1976). For time-series information, hourly measurements of salinity and temperature were taken for a 24 hour period in the mouth of the lagoon. Results The results indicate that the most striking features of the hydrographic parameters were: 1. The salinity of the water wasexceptionally high compared to the salinity of the adjacent water of the Red Sea. During the period of investigation the salinity in the lagoon fluctuated between 51’??in February and 113%0in October (Figure 2, 3) compared to 39-40K for the Red Seawater. 2. The spatial distribution of salinity showed a rather wide range of variation in the same day. Positions near the lagoon’smouth had the least salinities (Figure 2) while positions in the southern tip of the lagoon had the highest salinities (Figure 3). The difference between

170

Amin H. Meshal

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the two extreme values may reach in one day asmuch as 50% at the surface and 10%0for subsurface water. 3. The vertical distribution of salinity at any position in the lagoon showed a marked stratification and was dependent on its relative location from the inlet. The difference between surface and bottom salinities increased sharply at positions near the mouth of the lagoon where a larger portion of the inflowing Red Seawater was received. The difference reached 38%0 at station 1 where the depth was about 2m (Figure 2). On the other hand, the water column at stations away from the mouth of the lagoon was lessstratified especially at positions in the southern and northern part of the lagoon which received a minimum share of the Red Sea water inflowing into the lagoon (Figure 3). 4. Rapid and large fluctuations in salinity were observed in a short time. During successive cruises these temporal fluctuations were 50/win 2 days and 30%0in 27 days. 5. The water temperatures in the lagoon were high, being 33.4 “C in June and 24.8 “C in February compared to 28.6 “C and 22.3 “C in the sametwo months in the neighbouring coastal water of the Red Sea.

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6. The bottom temperatures were generally higher than the surface temperatures at most of the occupied stations. 7. In spite of the high salinity and high temperatures of the water of the lagoon, the values of dissolved oxygen were high. However, these values were reduced sharply when the salinity exceeded 100X. An anchor station located in the mouth of the lagoon (Figure 1) was occupied for 24 hours in March 1984. The hourly variations of temperature and salinity are illustrated in Figure 4. The temperature variation was influenced by the diurnal insolation and by the exchange of the water between the lagoon and the Red Sea.The hourly variation of salinity was confined to the surface and was controlled by the tidal cycle in the region while the bottom salinity remained virtually constant. This indicates that mainly water from the surface layer exchanged with the seaon the tides. The vertical distribution of the salinity and temperature showed a wide range of fluctuations in a very short time especially at positions near the mouth. This was because

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the salinity in the lagoon was highly correlated with the inflowing water to or from the lagoon accompanying the daily tidal cycle. During the tidal flood, colder and lower salinity water from the Red Sea entered the lagoon and spread over the warmer and higher salinity water of the lagoon. Thus, in the period of high tide two distinct layers existed in the lagoon giving a large difference between the surface and bottom salinities and temperatures (Figure 4). The vertical fluctuation, due to the tide, at any position in the lagoon depended upon its location from the mouth of the lagoon and hence upon the amount of Red Sea water which reached that position. Thus, stations near the mouth of the lagoon were more subject than others to the dilution effect of the inflowing Red Sea water and hence had lower surface salinities. Bottom water at these stations were less affected by the dilution. During low tide, the surface layer in the lagoon flowed out to the sea leading to the exposure of the subsurface layer of the water in the lagoon which had high salinity and warm temperature. It follows that the time at which the observations were taken at any station was very important. If the measurements were made at a position in the lagoon during high tide, there would be a marked difference in the salinity and temperature between the surface and the bottom. This difference was higher in positions nearer to the mouth of the lagoon and on spring tides than in positions away from the mouth and on neap tides. An example of the first case was station 1 in October and November (Figure 2). On the other hand, when measurements were taken during low tide, only one layer of high salinity water existed in the lagoon. The rapid fluctuations in salinity observed in May and June were due to taking observations during high tide in some cruises and during low tide in others.

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The monthly variations in the salinity of the water of the lagoon were influenced by evaporation and by the monthly changes in the mean sea level of the Red Sea. The author estimated evaporation from the coastal and open waters of this part of the Red Sea (Meshal et al., 1984). The bulk aerodynamic method was applied using the monthly means of the meteorological parameters observed at a station 40 km to the south of the lagoon. It was shown that these parameters represent the average conditions in this region (Meshal et al., 1984). Hence, these meteorological parameters were used to estimate evaporation from the lagoon by the same formula and using the surface temperature of the water of the lagoon. Evaporation from the lagoon was found to be 250 cm year-’ i.e. 1.22 times the evaporation from the coastal sea water. The only available measurements of sea level in this region are those taken at Port Sudan (Figure 5) on the western side of the Red Sea (Patzert, 1974). It is believed that the mean

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H. Meshal

sea level follows the same trend at both the eastern and western sides of the Red Sea in this region. The salinity of the lagoon was inversely related to the variations in the mean level of the Red Sea (Figures 2,3,5). Unfortunately, no measurements were taken in the lagoon during July, August and September when the mean sea level was very low. It is expected that the salinity of the lagoon’s water would be higher in these months than during the rest of the year. The mean level of the Red Sea fell to its lowest value in August (Figure 5) and hence the salinity of the lagoon’s water should have reached its highest value. The inverse relationship between the salinity in the lagoon and the mean level of the Red Sea may be explained as follows: when the mean sea level in the region rises, the daily exchange of the water between the lagoon and the Red Sea increases. In this case, some salts are washed out from the water of the lagoon during each tidal cycle leading to a decrease in its salinity since the inflow of salt to the lagoon is less than the outflow. On the other hand, when the mean sea level falls, only a small quantity of Red Sea water enters the lagoon during high tide and a smaller quantity leaves the lagoon during the ebb. The variations in oxygen (Figures 2,3) were affected by the variations in temperature and salinity and showed a general increase when the temperature and salinity decreased. The marine environment of the lagoon seems to be under natural stress from the high temperature of the water (24” to 33 “C) in addition to the extreme fluctuations in its high salinity (50 to 113X). To reduce this stress on the marine life in the lagoon, the rate of exchange of water with the Red Sea should be increased in order to lower the salinity and to minimize its extreme fluctuations. This can be achieved by clearing the blocked opening in the northern part of the lagoon. Moreover, a proposal of creating another opening connecting the southern part of the lagoon to the sea should be investigated. Discussion Hatiba Lagoon can be considered as a coastal hypersaline lagoon according to the definition of Barnes (1980). The rate of fluctuation of its high salinity may reach more than 2%0 per day in some months depending on the rate of evaporation and the exchange of water with the Red Sea. Edwards (1978) reported a similar increase of salinity to hypersaline levels, in Caimanero Lagoon in Mexico. Hypersaline water is found also in Dowhat as Sayh Lagoons, Arabian Gulf, Saudi Arabia where the salinity in its outer part ranged from 53 to 56%0 in 1974-1975 while in the inner part it ranged from 54 to 70%0 (Jones et al., 1975) in Laguna Madre, Texas, the salinity varied from 44 to 70%0 (Hedgepeth, 1953). Colombo (1972) found that the water of Comacchio in northeastern Italy showed a variation in salinity from 26 to 48%0. Levy (1980) found that the salinity of the water of Bardawil lagoon may reach up to three times that of the Mediterranean Sea. The pH values of the lagoon’s water ranged from 8.03 in October (S = 113%0) to 8.14 in February (S = 5 1%0) indicating that in hypersaline solutions, the pH is slightly lower than in natural sea water. This agrees with the conclusion of Krumgalz (1980) which shows that when sea water is concentrated its pH decreases slightly. In spite of the shallowness of Hatiba Lagoon (maximum depth 3 m), strong vertical stratification was observed at high tides especially in positions near the mouth of the lagoon. Barnes (1980) indicated that in lagoons with a relatively small size entrance compared to their large volumes, longitudinal salinity gradients were stable and did not fluctuate on a semidiurnal basis in relation to tide. This agrees with what we have found in Hatiba Lagoon but the salinity and temperature fluctuated with tides especially at its mouth. Also, Lara-Lara et al. (1980) studied a coastal lagoon on the Pacific Coast on Baja

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lagoon

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California, Mexico and found that the temperature and salinity at the mouth of the lagoon had a semidiurnal behaviour with high values corresponding to low tide and vice versa. Acknowledgements This work was funded by a grant from the research funds at King Abdulaziz University. The assistanceof the Marine Engineering Unit at the Faculty of Marine Science is appreciated. The field work, laboratory analysis and drafting were made by Mr S. ElNady and S. Sabra. Their valuable assistancein preparing this manuscript is also acknowledged with thanks. References Barnes, R. S. K. 1980 Coastal Lagoons, the Natural History of a Neglected Habitat. Cambridge: Cambridge University Press, 104 pp. Colombo, G. 1972 Primi risultati delle ricerch future. Boll. Zool., 39,471478. Edwards, R. R. C. 1978 Ecology of a coastal lagoon complex in Mexico. Estuarine, Coastal and Shelf Science, 6,75-92. Grasshof, K. 1976 Methods of Sea Water Analysis. Weinheim, New York: Verlay Cheme, 317 pp. Hedgepeth, J. W. 1953 An introduction to the zoogeography of the northwestern Gulf of Mexico with reference to the invertebrate fauna. Publications of the Institute of Marine Science 3,107-224. Jones, D. A., Price, A. R. G. & Hughes, R. N. 1978 Ecology of high saline lagoon Dawhat as sayh, Arabian Gulf, Saudi Arabia. Estuarine and Coastal Marine Science 6,253-262. Krumgalz, B. 1980 Salt effect on the pH of hypersaline solutions. In Hypersahne Brines and Ewaporitic Environments (Nissenbaum, A., ed). Proceedings of the Bat-Sheva Seminar on saline lakes and natural brines. Developments in Sedimentology 28,73-83. Lara-Lara, J. R., Varez-Borrego, S. Al. & Small, L. F. 1980 Variability and tidal exchange of ecological properties in a coastal lagoon. Estuarine and Coastal Marine Science l&613-637. Levy, Y. 1980 Evaporitic environments in northern Sinai. In Hypersaline Brines and Evaporitic Enoironments (Nissenbaum, A., ed). Proceedings of the Bat-Sheva Seminar on saline lakes and natural brines. Dewelopmerits in Sedimentology

28,31-143.

Meshal, A. H., Osman, M. M. & Behairy, A. K. A. 1984 Evaporation from coastal and open waters of the central zone of the Red Sea. Atmosphere-Ocean 22(3), 369-378. Patzert, W. C. 1974 Wind-induced reversal in Red Sea circulation.3oumal of Deep-Sea Research 21,109-12 1.