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

OXYGENThe spatial distribution of dissolved oxygen during the major part of the annual cycle (exceptin upwelling areas during the summer) appears to be quite stable and uniform, yet generally atextraordinarily low levels (e.g., SEN GUPTA and NAQVI, 1984; SWALLOW, 1984; WYRTKI,1971, 1973). The western part of the Gulf is significantly better oxygenated than the central andeastern waters, due to input of RSW. Even so, the well-saturated layer (02 concentrations4.5 - 5.2 ml l-l) extends no deeper than 50 m. The next deeper layer (to the depths of 200-300 m)shows a gradual decrease of concentrations to 1.0-1.5 ml l-l; at greater depths they fall below 1.0ml1-1, but never less than 0.5 ml l-1. The deepest measured layers (800-1,000 m) usually show slightincreases (to about 1.0 ml l-l) due to inflow of RSW. Central and eastern parts of the Gulf have, asmentioned, lower contents of available oxygen. The well-saturated surface layer (4.5 - 5.2 ml l-1) isusually shallower, about 20 m, although the concentrations greater than 4.0 ml l-1 may go down to 100m during the winter, particularly in the areas of assumed central gyres. The layer in which O2gradually drops to the level of 1.0 - 1.5 ml 1-1 extends to the depths 100-200 m, while concentrationsless than 1 ml 1-1 are usual in deeper waters, often approaching anoxic concentrations (about 0.5 ml l-1and less; minima reaching 0.08 ml l-1).Sudden changes in oxygenation occur during the summer, with a general decrease of oxygencontent in the whole Gulf except in assumed central gyres and along frontal zones. These are probablyplaces where upwelled surface waters sink, saturated with oxygen from blooming phytoplankton.Actually, this seems to be the only major source of the oxygen for the central and eastern waters of theGulf of Aden, other than Red Sea input in the west. However, drastic changes in oxygenation (orbetter to say deoxygenation) affects the upwelling areas. The semi-anoxic environment appears inless-pronounced western upwellings (as recorded west of Aden and offshore Berbera) up to 50 mbelow the surface (Figs. 8 and but in the east such an environment in extensive upwelling centresextends down from - 20 m (Figs. 9 and 10). The only oxygenated layer occurs in the upper 10 m due tophutosynthesis. .l'his phenomenon can be considered an ecologic catastrophy upon the ecosystem andits communities (including living resources), and it will be discussed in the next chapters.PELAGIC PRODUCTIVITYPhytoplanktonExcluding short-term spring minima and upwelling-induced summer maxima, thephytoplankton abundance and biomass (estimated as chlorophyll a) are very high throughout the year,averaging 150,000 cells dm-3 and 0.5 pM dm-3 chlorophyll a in the euphotic layer. Chlorophyllvalues higher than previously reported for the Gulf of Aden (KREY and BABENARD, 1976 etc.) arethe highest (in average) for the Indian Ocean andseem disproportionally high compared tophytoplankton abundance. This is partially due to the large size of diatoms which can account for amajor part of the chlorophyll content. On the other hand, small-size components (e.g., nakednannoplanktonic flagellates) contribute measured values of chlorophyll, but they are not included inabundance counts since they disintegrate in preserved samples.A marked decrease in standing stock occurs in spring, with abundances falling below 50,000cells dm-3 in the eastern parts of the Gulf. Even during the summer relatively low abundances (in therange between 50,000 - 100,000 cells dm-3) occur Iocally inshore and largely offshore. Asmentioned previously, these small-amplitude oscillations cannot be explained by nutrient limitationssince they hardly exist.Since true surface upwelling (surface temperatures 17-18°C) also were recorded previously (SERIY, 1968) inwestern waters, temporary semi-anoxic conditions most likely appear much closer to the surface.264