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

RED SEA BIOLOGICAL OCEANOGRAPHY AND RELATEDPROBLEMSHJALMAR THIEL AND HORST WEIKERTInstitut fur Hydrobiologie und Fischereiwissenschaft derUniversitat Hamburg, 2000 Hamburg 50, Zeiseweg 9, FRGINTRODUCTIONIsolated from the Indian Ocean water by the shallow Hanish Sill (137 m depth) north of Babel Mandeb, the Red Sea has unique hydrographic conditions and resulting plant and animal life. Ourstudy of the Red Sea has been in the context of risk assessment from mining the Atlantis II Deep inthe central region of this long and narrow sea (=BE et al., 1981; THIEL et al., 1986). W e presenta short ecological characterization of the Red Sea, and we then identify problems that could be studiedby small research groups or through international cooperative programs. Much of the ecologicalinformation concerning the pelagic ecosystem has been extracted from a review by WEIKERT (inpress); results on the benthos are summarized by THIEL (in press).THE CENTRAL RED SEAHYDROGRAPHYAlthough the hydrographic conditions of the Indian Ocean and the adjacent Gulf of Aden canbe termed "normal", about 10 m below sill depth Red Sea water is 7OC warmer and 4Ym more salinethan normal Ocean water at a similar depth. At 2000 m depth these differences increase to about 19OCand 6%0. Surface water transport in summer is directed south throughout the Red Sea by theprevailing northerly winds. During winter, however, the southern half is governed by southernwinds, and the current direction is to the north. The net northern transport is mainly due toevaporation. Our main study area was situated in the transition zone, 19-21'N.The 100-m thick surface layer exhibits high temperatures and salinities; oxygen concentrationsare around saturation, but nutrient levels are low (Figs. 1 and 2). Temperature and nutrientsdecrease towards the north, contrasting to the salinity and oxygen gradients. In deeper layers,temperature and salinity are markedly elevated as compared to the Indian Ocean, averaging around21.6OC and >40.'7%0. Inorganic nutrients and oxygen are inversely related to each other:Concentrations of the former reach maximum values at depths of 350-450 m in the central Red Sea(Fig. 2). The concurrent minimum oxygen concentrations are due to culminating degradation of theorganic matter derived from sinking and vertical transport. This zone occurs at about 300-400 m inthe south and 400-500 rn in the north of the basin, reflecting the decline of biological productivity inthe surface water. Beneath the intermediate waters, oxygen increases to >2 ml/lwith depth, whereasthe nutrient levels remain more or less constant. Except for the extreme north and south of the basin,the thermocline and the halocline prevent the nutrients in the deeper waters from being recycled intothe euphotic zone. This is a characteristic feature for oligotrophic tropical Ocean regions.BIOLOGYThe central Red Sea can be described as oligotrophic, with a mean daily rimary productionper year of 170 mg C m-2 d-1 (WEIKERT, 1987, in press) and 258 mg C m-2 d- P near the reefs offJeddah (DOWIDAR, 1983). Adequate coverage in neither time nor space, however, has beenachieved for the total basin. HALIM (1984), unaware of the above data nor of earlier Russian studies(KHMELEVA, 1970), gives an average for the total Red Sea of less than 100 mg m-2 day-1, which299