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

Low oxygen content in subsurface waters suggests a long residence time, but the reportedhigh productivity in the euphotic layers, which may be significantly deeper than the upper thermocline(reported values are 60 - 80 m for the euphotic zone in the Gulf of Aden), may cause a high oxygenconsumption through the subsequent remineralization processes. The high silicate content suggests,however, a relatively long residence time. This, in turn, has a consequence not only for the oxygenconsumption and nutrient accumulation processes but also for the eventual accumulation of harmfulsubstances, such as chlorinated hydrocarbons, petroleum hydrocarbons and trace metals.The upwelling of subsurface waters is the principal mechanism by which the upper layers ofthe west Arabian Sea are enriched with nutrient salts. Though the data coverage leaves large gaps intime and space, this part of the Arabian Sea appears to be one of the most productive parts of theIndian Ocean. The mean production of the entire west portion of the Indian Ocean, up to severalhundred kilometers offshore, is several times that for the worlds oceans. Primary production is aslarge, if not larger than that encountered in upwelling regions along the coast of Peru and off WestAfrica. One of the most important findings of the IIOE is the extremely high rate of primaryproduction and the large standing crops of phytoplankton and zooplankton in the Arabian Sea,especially along the west side (WOOSTER et al., 1967).The inner waters of the Gulf of Aden and those off Ras-Fartak and Socotra show chlorophylla values of about 0.5 mg m3. The potential productivity in the upper 50 m exceeds 1.00 mg C m-3hr-l. In autumn, surface photosynthesis is moderately high throughout the area, but is decreasing. Itis, however, higher off Ras-Fartak and Socotra. During the NE monsoon (December-February),chlorophyll a is low, and so is the potential productivity (McGILL and LAWSON, 1966).The highest zooplankton volumes for the Indian Ocean are found in the Arabian Sea off theSomali coast and Cape Fartak and further east. Here average values as high as 54.7 ml m-2 have beenobtained; the rest of the Indian Ocean ields values less than 15 ml m-2 (RAO, 1973). During the NEmonsoon, a biomass of 20 - 40 ml m- r occurs in the region of the Gulf of Aden, but occurrences ofhigher volumes up to 80 ml m-2 have been identified more offshore, between Socotra and Ras-Fartak.The period of the rising SW monsoon (May-June) yields a comparable volume of 20 - 40 ml m-2between Ras-Fartak and the Gulf of Aden. During the full SW monsoon period (July-September) highbiomass values of 40 ml or above are found around Socotra. Unfortunately data from the rest of theYemeni waters are too scanty for comparison. The transition period between the end of the SWmonsoon and the beginning of the NE monsoon shows a general rise in the biomass, with a densepatch of up to 80 ml m-2 off and around Cape Fartak. From this brief account of the seasonaldistribution of the zooplankton biomass, it appears that Democratic Yemen waters, particularly in thearea between Socotra and Cape Fartak, show a consistently high level of secondary production in spiteof the seasonal fluctuations in primary productivity. It also appears that the area of higher densityspreads out from the area of upwelling, in the direction of the prevailing currents.Unfortunately, the authors of the above review (HALIM et al.) were probably aware of morerecent results, obtained by Soviet investigations of the Gulf of Aden (references of available sourcessee above). On the one hand these more recent and extensive data have substantiated many of thereview's statements, particularly for primary and secondary productivity. On the other hand theyprovide clear evidence on some oceanographic phenomena of a crucial importance, such as thefour-layer structure of the Gulfs water masses, development of upwellings in the Gulf itself, etc.Nevertheless, one has to agree fully with the following introductory statement of the abovequoted review: ". . development and management of the fisheries resources is (sic) dependent on asound and comprehensive knowledge of the physical, chemical and biological conditions whichcontrol those resources. Unfortunately, these conditions are, as yet, inadequately known . . basichydrographic information is still fragmentary and knowledge about water structure and circulation isscarce" (HALIM et al., 1980).Recognizing these gaps in knowledge, both as related to applied fisheries and to the generalobjectives of basic marine sciences, the recently established Marine Science and Marine ResourcesResearch Center (MSRC)' started in 1984 with systematic oceanographic and pelagic bioproductivityinvestigations of the Gulf of Aden. Since then a considerable amount of data has been gathered whichis used in this paper to discuss oceanographic and bioproductivity conditions in this region.1 MSRC as a unit of PDRY Ministry of Fish Wealth was established in 1983, partly through UNESCOFund-in-Trust Project 703/PDY/40, financed by the Islamic Development Bank, Jeddah.257