139736eo.pdf (20MB) - Japan Oceanographic Data Center

only a partial inflow of Indian Ocean surface water from the Gulf of Aden into the Red Sea. In return,hypersaline Red Sea water exits over the sill into the Gulf of Aden.The inflow of Gulf surface water is significant from November to March, when southeasterlywinds prevail in the southern part of the Red Sea. Further north, the northward flow of surface wateris against the wind field, blowing from the NW. These northwesterly winds prevail throughout theyear north of about 19"N. In summer, i.e. from May to October, the NW wind field covers the entireRed Sea, reversing the surface flow pattern in the south. The induced outflow of the Red Sea surfacewater into the Gulf of Aden from June to September, causes a compensatory current from the Gulf intothe Red Sea. The core of this countercurrent ranges between 30 and 80 m depth, penetrating the RedSea proper to approximately 19"N (PATZERT, 1974).Due to high evaporation, temperature and salinity of the surface water of the Red Sea areconsiderably higher than in the Gulf of Aden. The mean surface temperatures fluctuate between 25"and 32°C in the south, and between 21.3" and 27.9"C in the north. Salinity increases from 37%0 in thesouth to 40950 in the north. In winter, thermal convection in the northern part of the Red Sea makes thecooled (21.3'C) saline surface water sink below 200 m, thereby feeding the homogenous deep water(21.5"C, 40.5%0S) (MORCOS, 1970). Although this deep-water body is not totally isolated, anyexchange of deep water with the surface layer is limited and slow.Nutrient levels of the Red Sea are low. Their decrease from the north reflects the mainnutrient influx from the Indian Ocean and (therefore) limited vertical mixing. Dissloved oxygen ispresent throughout the water column, its concentration is balanced between biological and chemicaloxygen consumption as opposed to oxygen transport with water masses. The upper water layer has anoxygen contents of 4.25 ml 1-1, while 2 ml 1-1 (minimal values of 1.5 ml 1-1) are found in greaterdepths. A fairly sharp oxycline exists around 300 m (MORCOS, 1970). It is assumed that thebiological oxygen demand in deep water is low. Because no measurements of oxygen consumptionare available, replacement times of oxygen and urnover rate of water masses cannot be computed. Theoxygen present at all depths demonstrates that the entire water body is included in this exchange,although the exchange above the oxycline clearly is much faster than below it.PELAGIC COMMUNITYThe pelagic community of the Red Sea (both phyto- and zooplankton) is still poorlyinvestigated, particularly compared to the Mediterranean and the Arabian Seas. Nevertheless, we canobserve some basic features concerning the diversity of species and their distribution and productivityin the Red Sea. These features are derived from incidental sampling of biological data, which areextrapolated on the basis of the slightly better-known hydrographic properties of the Red Sea.In comparison with other deep-sea areas the Red Sea is not particularly deep, nor is the deepsea far from land. Most of the Red Sea lies between 500 and 1000 m, while the greatest inhabitabledepth is only around 2000 m. Surface sediments of the hot brines in the deeper holes may becolonized by a few bacterial species only, while deeper brine layers are anoxic and without life.Estimated standing stock weight of organisms, based on knowledge from other oceans, may amount toa few grams per square meter. However, in the Red Sea productivity is low, and nothing is known onthe fate of organic matter during sinking. Because of the high temperature throughout the watercolumn, it can be assumed that bacterial degradation is fast and little food reaches the deep-sea bottom.In other oceans, the deep temperature below a few hundred meters causes a refrigerator effect, andmore food material is preserved.PRODUCTIVITYThe knowledge on the productivity of the Red Sea is insufficient for both the phytoplanktonand the zooplankton communities. For example no carbon- 14 productivity measurements are availableto calculate primary production in the Red Sea. Chlorophyll values, although somewhat ambiguousfor productivity calculations, suggest that Red Sea productivity is low. Species distribution seems todecrease to the north, and a similar trend is suggested for productivity.There are indications of seasonal large-scale blooms in the Red Sea. In winter, during the NEmonsoon, greater plant pigment concentrations are reported from the northern part of the Red Sea.360