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

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

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12.07.2015 Views

Based on the oxidative ratios of the nutrients, the ratios of change of carbon: silicon:nitrogen: phosphorus in the waters of the northern Indian Ocean have been calculated as 108: 40: 16:1 atoms. These agree fairly well with the atomic ratio of concentration of nitrogen: phosphorus inplankton of the Indian Ocean which is 15.4: 1.Silicate concentrations in the deeper waters of the Indian Ocean are higher than those at thecorresponding depths in the other two oceans. Silicate values can be used to identify different watermasses by plotting them against the sigma-t values. This also has been confirmed by examining therelationship between silicate and oxygen consumption. The relationship shows that beyond a certainconcentration of silicate, the relation becomes asymptotic, indicating that the water masses havingsuch silicate and oxygen concentrations originate from a similar source (SEN GUPTA et al., 1976b).BIOLOGICAL,Several atlases on the phytoplankton and zooplankton production in the Indian Ocean werepublished based on HOE data (KREY and BABENERD, 1976; IOBC, 1968a and b). More recentlyboth primary and secondary production rates of the Indian Ocean in relation to various relevant factorswere estimated by QASIM (1977). The gist of the information is presented below.The compensation depth of the euphotic zone (1% of the surface illumination) in the IndianOcean varies between 40 and 120 m, with increasing depth from coastal to offshore regions.The range in column production varies from 0.001 to 6.5 g C m-2 day-1. Areas of highproductivity lie mostly in the coastal and upwelling zones. These include the Gulf of Oman, themouth of the Indus River, Saudi Arabian waters, Somalian and Mozambique coasts, southwest coastof India, the mouth of the River Ganga, the Burma coast, and off northern Sumatra Java (Fig. 4).The average production in the coastal areas works out to be 485 g C m-2 yr-1 as compared to theoffshore average of 229 g C m-2 yr1. Total photosynthetic productivity in the Indian Ocean has beenfound to range from 3 to 6 x 109 tonnes C yrl with an average of 4.5 x 109 tonnes C yr-1 or 258 mgC m-2 day-1.The Arabian Sea and the Bay of Bengal are the two most roductive re ions of the IndianOcean. The total column production in the Arabian Sea is 1.1 x 10 B tonnes C yr K , while in the Bayof Bengal it is 0.4 x 109 tonnes C yr-1. This means that these two areas account for about one-thirdof the total photosynthetic productivity of the Indian Ocean although together they occupy only about14% of the total area. There is, however, a qualitative difference in the productivity of these twoareas. The surface productivity (1 m depth) in the Bay of Bengal is 4.9 tonnes C km-2 yrl while inthe Arabian Sea it is 3.9 tonnes C km-2 yrl. This anomalous situation between column and surfaceproductivity has been attributed to greater cloud cover and higher surface nutrient load, particularlynitrate-nitrogen, in the Bay of Bengal (QASIM, 1977).Zooplankton biomass in the Indian Ocean varies from 15 to 50 ml m-2. The totalzooplankton biomass in the Indian Ocean has been estimated to be 5.2 x lo8 tonnes yrl (PRASAD etal., 1970), and secondary production works out to 69.3 x lo6 tonnes C yr-1. Areas of highabundance mostly coincide with high photosynthetic productivity.Benthic production in the Indian Ocean predictably decreases with water depth. The densityof meiofauna, however, is greater than that of the macrofauna in deeper waters. Benthic population islarger in the areas of high roductivity. Average benthic productivity in the Arabian Sea to a depth of2100 m is 1.8 g C m-2 yr P , while in the Bay of Bengal (to a depth of 1870 m) and the Andaman Sea(to a depth of 1820 m) productivities are 0.5 and 1.2 g C m-2 yr-1, respectively (HARAKANTRA etal., 1980; ANSARI et al., 1977; PARULEKAR and ANSARI, 1981). Extrapolating these values tothe whole of the Indian Ocean, the annual benthic production works out to be 89.9 x 106 tonnes interms of carbon.Recent figures of pelagic, demersal and crustacean resources from the different regionsindicate that the annual catch for the Indian Ocean is about 4 million tonnes (FAO, 1978). Thus, theIndian Ocean, occupying 20.8% of the world oceans area, yields only 2.8% of the global fish catch.The potential yield of the Indian Ocean has been estimated as 14.2 x 106 tonnes yr-1 (QASIM, 1977).Therefore, there is much scope for increasing fish production of the Indian Ocean by increasing theeffort.12

GEOLOGICALBecause of its asymmetric shape, the Indian Ocean is separated from the deep-reachingvertical convection areas of the northern hemisphere (DIETRICH and ULRICH, 1968). Recentgeophysical investigations indicate that the Indian Ocean is the most complex of the three majoroceans in terms of its history of formation, and hence its physiography, probably because of thisasymmetric configuration (McKENZIE and SCLATER, 1971).Measurements of its magnetic fieldshow that the floor of this ocean is spreading at the rate of 2 cdyear and implies that the Indian Oceanis about 108 years old (DIETRICH, 1973).One of the most prominent topographic features of the Indian Ocean is its seismically active,rugged inverted Y-shaped Mid-Indian ridge, which is cut by numerous north-northeast trendingfracture zones (Fig. 5) shown by HEEZEN and THARP (1964). Microcontinents and meridionalaseismic ridges, e.g., the Ninety-East Ridge and the Chagos-Laccadive Plateau, are unique to theIndian Ocean (HEEZEN and THARP, 1964). Although massive, these features are relativelysmooth-surfaced blocks and stand in marked contrast to the rugged features of the Mid-Indian ridge.Like in the other oceans, there are a good number of deep basins between the topographichighs. Sediment cones and abyssal plains, with sediment thicknesses exceeding 1.5 to 2.5 km arepresent in the Bay of Bengal, Arabian Sea, Somali basin and Mozambique basin (EWING et al.,1969). In parts of the Ganges cone, sediment thickness exceeds 12 km (CURRY and MOORE,1971). In the high southern latitudes near the polar front, smooth, 'swale' topography with moderatesediment thickness amears as a result of hieh Droductivitv of the Antarctic Ocean. while in otherY 1areas, and especially bh topographic highs, the sediment c&er is very thin (VENKATARATHNAMand HAYES, 1974).The complex physiography, the asymmetrical distribution of the surrounding continents,their varying geology and climate, and the northward closure of the Indian Ocean (preventing theinfluence of colder northern latitudes) have a considerable effect on the fauna and the sedimentcharacter in the northern part of the Indian Ocean. Moreover, the Indian Ocean receives about 34 xlo8 tonnes of suspended sediment per year from the rivers draining into it (HOLEMAN, 1968).Almost half of this, about 16 x lo8 tonnes, is added by the river systems draining the Indiansub-continen t.In terms of areal extent, the most extensive sedimentary facies in the Indian Ocean iscalcareous sediment which dominates the Mid-Indian Ridge and other more shallower areas notinfluenced by terrigenous influx or the influx of siliceous skeletal remains (VENKATARATHNAMand HAYES, 1974). The critical depth at which calcium carbonate becomes a minor componentvaries with latitude (LISITZIN, 1971). Other Sedimentary facies include: terrigenous clay adjacent tothe major river basins; siliceous clay and ooze in the equatorial and southern latitudes influenced byhigh biological productivity; brown and red clays in deep areas outside the productivity belts and theareas of terrigenous influx (VENKATARATHNAM and HAYES, 1974; SIDDIQUIE et al., 1976aand b). Although the areal extent of terrigenous sediment is relatively small, because of its enormousthickness its volume exceeds 70% of the total sediments of the Indian Ocean (EWING et al., 1969).Within the limits of the broad features noted above, VENKATARATHNAM and HAYES (1974)mapped the following materials in the Indian Ocean: (i) hard rocks (volcanic, and limestones) ontopographically high areas, especially on the Mid-Indian Ridge; (ii) Mn-nodules in the deep basinsand Agulhas Plateau, far removed from terrigenous sediment supply; and (iii) silicic volcanic ash inthe eastern Indian Ocean adjacent to the Indonesian archipelago. In addition, deposits of ilmeniteplacers have been mapped in the coastal regions of several of the countries bordering the IndianOcean, (e.g., India, Australia and Sri Lanka).NATURE OF ENVIRONMENTAL PROBLEMSAlmost all the countries bordering the Indian Ocean are developing countries. Their majorsource of revenue is agriculture and, in some countries, also mining. The effects of pollution in themarine environment, largely arising out of the economic activities, began to be felt only recentlyalthough the practices have continued for a long period of time. These problems, however, are13

Based on the oxidative ratios of the nutrients, the ratios of change of carbon: silicon:nitrogen: phosphorus in the waters of the northern Indian Ocean have been calculated as 108: 40: 16:1 atoms. These agree fairly well with the atomic ratio of concentration of nitrogen: phosphorus inplankton of the Indian Ocean which is 15.4: 1.Silicate concentrations in the deeper waters of the Indian Ocean are higher than those at thecorresponding depths in the other two oceans. Silicate values can be used to identify different watermasses by plotting them against the sigma-t values. This also has been confirmed by examining therelationship between silicate and oxygen consumption. The relationship shows that beyond a certainconcentration of silicate, the relation becomes asymptotic, indicating that the water masses havingsuch silicate and oxygen concentrations originate from a similar source (SEN GUPTA et al., 1976b).BIOLOGICAL,Several atlases on the phytoplankton and zooplankton production in the Indian Ocean werepublished based on HOE data (KREY and BABENERD, 1976; IOBC, 1968a and b). More recentlyboth primary and secondary production rates of the Indian Ocean in relation to various relevant factorswere estimated by QASIM (1977). The gist of the information is presented below.The compensation depth of the euphotic zone (1% of the surface illumination) in the IndianOcean varies between 40 and 120 m, with increasing depth from coastal to offshore regions.The range in column production varies from 0.001 to 6.5 g C m-2 day-1. Areas of highproductivity lie mostly in the coastal and upwelling zones. These include the Gulf of Oman, themouth of the Indus River, Saudi Arabian waters, Somalian and Mozambique coasts, southwest coastof India, the mouth of the River Ganga, the Burma coast, and off northern Sumatra Java (Fig. 4).The average production in the coastal areas works out to be 485 g C m-2 yr-1 as compared to theoffshore average of 229 g C m-2 yr1. Total photosynthetic productivity in the Indian Ocean has beenfound to range from 3 to 6 x 109 tonnes C yrl with an average of 4.5 x 109 tonnes C yr-1 or 258 mgC m-2 day-1.The Arabian Sea and the Bay of Bengal are the two most roductive re ions of the IndianOcean. The total column production in the Arabian Sea is 1.1 x 10 B tonnes C yr K , while in the Bayof Bengal it is 0.4 x 109 tonnes C yr-1. This means that these two areas account for about one-thirdof the total photosynthetic productivity of the Indian Ocean although together they occupy only about14% of the total area. There is, however, a qualitative difference in the productivity of these twoareas. The surface productivity (1 m depth) in the Bay of Bengal is 4.9 tonnes C km-2 yrl while inthe Arabian Sea it is 3.9 tonnes C km-2 yrl. This anomalous situation between column and surfaceproductivity has been attributed to greater cloud cover and higher surface nutrient load, particularlynitrate-nitrogen, in the Bay of Bengal (QASIM, 1977).Zooplankton biomass in the Indian Ocean varies from 15 to 50 ml m-2. The totalzooplankton biomass in the Indian Ocean has been estimated to be 5.2 x lo8 tonnes yrl (PRASAD etal., 1970), and secondary production works out to 69.3 x lo6 tonnes C yr-1. Areas of highabundance mostly coincide with high photosynthetic productivity.Benthic production in the Indian Ocean predictably decreases with water depth. The densityof meiofauna, however, is greater than that of the macrofauna in deeper waters. Benthic population islarger in the areas of high roductivity. Average benthic productivity in the Arabian Sea to a depth of2100 m is 1.8 g C m-2 yr P , while in the Bay of Bengal (to a depth of 1870 m) and the Andaman Sea(to a depth of 1820 m) productivities are 0.5 and 1.2 g C m-2 yr-1, respectively (HARAKANTRA etal., 1980; ANSARI et al., 1977; PARULEKAR and ANSARI, 1981). Extrapolating these values tothe whole of the Indian Ocean, the annual benthic production works out to be 89.9 x 106 tonnes interms of carbon.Recent figures of pelagic, demersal and crustacean resources from the different regionsindicate that the annual catch for the Indian Ocean is about 4 million tonnes (FAO, 1978). Thus, theIndian Ocean, occupying 20.8% of the world oceans area, yields only 2.8% of the global fish catch.The potential yield of the Indian Ocean has been estimated as 14.2 x 106 tonnes yr-1 (QASIM, 1977).Therefore, there is much scope for increasing fish production of the Indian Ocean by increasing theeffort.12

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