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

The maximum standing stocks in the Gulf of Aden occur during the summer andpost-monsoon autumn, with avera e abundances above 200,000 cells dm-3 and chlorophyll a valuesin the range of 1.0 to 2.0 pM dm-? However, in upwelling areas, extensive blooms can occur, withabundances of 1-6 millions cells dm-3 and chlorophyll a values up to 5.1 pM dm-3 . To ourknowledge such extremes have not been recorded previously in the open Indian Ocean, although theymost probably do occur in other upwelling zones in the northern Arabian Sea. Nevertheless, such amassive production of organic matter (a considerable part of which is probably “wasted” by bacterialdecomposition) may contribute a great deal not only to the richness of higher trophic levels, but also tothe marked oxygen depletions discussed above.The taxonomic composition of phytoplankton during the blooming phase shows very lowdiversity, as is typical for “immature” upwelling ecosystems (MARGALEF, 1978). Few species ofdiatoms(Chaetoceros curvisetus, Ch. compressus, Thallasionema nitzschioides, Rhizosoleniafragilissima, Nitzschia delicatissima, Skeletonema costatum and Asterionella japonica) account for70-80% of the total abundance. In contrast, throughout most of the year the phytoplanktoncommunities have quite diverse compositions; seventy species in a sample is not unusual.ZooplanktonJudging from the present data base, standing stocks of zooplankton exhibit much lesspronounced seasonal oscillations, (Figs. 13 and 14) and more homogeneous spatial distribution, evenduring summer upwelling (Fig. 16). However, in the eastern upwelling center the maximum standingstocks occur during the period of upwelling and consequent phytoplankton blooms. This is not thecase in central and western waters, where zooplankton populations actually fall in the initial period ofNE monsoon.Two smaller peaks in populations occur at the end of winter and during the SW monsoon insummer. A significant minimum is evident in spring, which hydrographically is characterized byoverheating (up to 32°C in the mixed layer).The average range of zooplankton standing stocks throughout the year, expressed asdisplacement volume er haul (to a depth of 30 m), is 5-10 cm3, with minima of 2 to 3 cm3 andmaxima of 14-24 cm !. Although still preliminary, dry weights and ash-free dry weights forzooplankton biomass range from 52 to 301 mg m-3 and 23-111 mg m3, respectively. These valuessupport previous statements which consider zooplankton stocks of the NW Arabian Sea as the richestin the Indian Ocean (GAPISHKO, 1971; LENZ, 1973; RAO, 1973). Moreover, our values exceedpreviously reported ones and indicate the Gulf of Aden (in terms of zooplankton standing stocks) to beone of the richest areas of the world ocean.Although it is rather premature to report on the composition of studied zooplanktoncommunities, some currently available information seems worthy of mention. Average abundances,both during the winter and the summer periods range from 1,000 to 6,000 specimens m-3, the maximabeing 8,000-16,500 specimens m3. The most common taxonomic groups are as follows:TaxonCnidaria 0.1 - 3.5Pteropoda 0.1 - 2.6Appendicularia 0.5 - 18.3Chaetognatha 0.5 - 8.1Thaliacea 0.2 ’ 4.3Ostracoda 0.1 - 2.5Copepoda 65.5 - 96.8YO Relative Abundance265