Venugopalan Ittekkot

Seasonality and interannual variability of particle-fluxes to the deep Arabian Sea

Abstract Long-term sediment trap studies have been carried out since 1986 at three locations in the western, central and eastern Arabian Sea. Here we present total and bulk component fluxes measured for 3 years at the central station and for 4 years at the western and eastern stations. Particulate fluxes to the deep sea are controlled by the monsoons with generally higher fluxes during the SW and NE monsoons and lower fluxes during the intermonsoon periods. The increase of particle fluxes occurs simultaneously with a drop in surface water temperature, induced by wind-or convective-mixing and an associated entrainment of nutrients into the euphotic zone. More than 50% of the annual particle fluxes to the deep sea occurs during the SW monsoon at the western location due to the prolonged influence of the monsoonal upwelling as indicated by increased biogenic carbonate and opal fluxes. However, the opal fluxes peak a month later than the carbonate fluxes. The delayed onset of opal flux peak appears to be controlled by the observed premonsoon silica distribution in the Arabian Sea, where the subsurface waters are silica depleted down to the thermocline at 150 m. At the central location particle fluxes are of similar magnitude during the SW and NE monsoons. The interannual variability of particle fluxes at the eastern location is determined by the NE monsoon. At the western and central locations, on the other hand, maximum interannual variability of fluxes occurs during the SW monsoon and particle fluxes were higher during years of stronger SW monsoon. The results further suggest that, apart from monsoon strength, geographic shifts of the area of maximum wind-stress may produce significant variabilities in particle fluxes to the deep ocean at the western Arabian Sea site.

Silicon Retention in River Basins: Far-reaching Effects on Biogeochemistry and Aquatic Food Webs in Coastal Marine Environments

Abstract Regulation of rivers by damming as well as eutrophication in river basins has substantially reduced dissolved silicon (DSi) loads to the Black Sea and the Baltic Sea. Whereas removal of N and P in lakes and reservoirs can be compensated for by anthropogenic inputs in the drainage basins, no such compensation occurs for DSi. The resulting changes in the nutrient composition (DSi:N:P ratio) of river discharges seem to be responsible for dramatic shifts in phytoplankton species composition in the Black Sea. In the Baltic Sea, DSi concentrations and the DSi:N ratio have been decreasing since the end of the 1960s, and there are indications that the proportion of diatoms in the spring bloom has decreased while flagellates have increased. The effects on coastal biogeochemical cycles and food web structure observed in the Black Sea and the Baltic Sea may be far reaching, because it appears that the reductions in DSi delivery by rivers are probably occurring worldwide with the ever increasing construction of dams for flow regulation.

Seasonality in the fluxes of sugars, amino acids, and amino sugars to the deep ocean: Sargasso sea

The fluxes of sugars, amino acids, and amino sugars as released by acid hydrolysis were determined in the < 37-μm fraction of samples collected during successive two-month sediment trap deployments in the deep Sargasso Sea (3200 ± 100 m) from April 1978 to December 1981. All fluxes varied seasonally and in phase with the flux of the < 37-μm fraction, which has been shown to vary in phase with primary productivity in the surface layers. During the investigation the fluxes were in the range of 0.03 to 1.7 mg m−2 d−1. They contributed 13 to 34% of the measured organic carbon, and 30 to 53% of the measured total nitrogen could be accounted for by amino acids and amino sugars. The relative abundances of sugars and amino acids were, in general, similar to those reported for mineralized tissues of carbonate and silica producers and the cell walls of non-biomeneralizing organisms. However, the amounts of non-protein amino acids, i.e., β-alanine and γ-aminobutyric acid, of aspartic and glutamic acids, and of amino sugars relative to total amino acids varied seasonally. Relative abundances of these compounds appear to indicate the nature and source of organic matter arriving at the sediment trap.

Seasonality in the fluxes of sugars, amino acids, and amino sugars to the deep ocean: Panama Basin

Abstract Time-series sediment traps were deployed for an entire year at depths of 890, 2590, and 3560 m at a station in the Panama Basin during 1980. Fluxes of sugars, amino acids, and amino sugars varied seasonally at each depth. Two peak fluxes were observed: one in February–March, the other in June–July. The peaks were associated with a high productivity period by regional upwelling and an unusual coccolithophorid bloom. There were significant differences in the distributions of sugars and amino acids associated with the fluxes. The peak flux of June/July was characterized by high amounts of arabinose and ribose within the sugar, and high amounts of aspartic acid in the amino acid fractions. The differences were observed at all three depths simultaneously, indicating rapid vertical transport without significant dissolution or decomposition. The observed pattern indicates the utility of specific compounds such as sugars and amino acids as tracers of source materials in the marine environment.

A revised nitrogen budget for the Arabian Sea

Despite its importance for the global oceanic nitrogen (N) cycle, considerable uncertainties exist about the N fluxes of the Arabian Sea. On the basis of our recent measurements during the German Arabian Sea Process Study as part of the Joint Global Ocean Flux Study (JGOFS) in 1995 and 1997, we present estimates of various N sources and sinks such as atmospheric dry and wet depositions of N aerosols, pelagic denitrification, nitrous oxide (N2O) emissions, and advective N input from the south. Additionally, we estimated the N burial in the deep sea and the sedimentary shelf denitrification. On the basis of our measurements and literature data, the N budget for the Arabian Sea was reassessed. It is dominated by the N loss due to denitrification, which is balanced by the advective input of N from the south. The role of N fixation in the Arabian Sea is still difficult to assess owing to the small database available; however, there are hints that it might be more important than previously thought. Atmospheric N depositions are important on a regional scale during the intermonsoon in the central Arabian Sea; however, they play only a minor role for the overall N cycling. Emissions of N2O and ammonia, deep-sea N burial, and N inputs by rivers and marginal seas (i.e., Persian Gulf and Red Sea) are of minor importance. We found that the magnitude of the sedimentary denitrification at the shelf might be ∼17% of the total denitrification in the Arabian Sea, indicating that the shelf sediments might be of considerably greater importance for the N cycling in the Arabian Sea than previously thought. Sedimentary and pelagic denitrification together demand ∼6% of the estimated particulate organic nitrogen export flux from the photic zone. The main northward transport of N into the Arabian Sea occurs in the intermediate layers, indicating that the N cycle of the Arabian Sea might be sensitive to variations of the intermediate water circulation of the Indian Ocean.

Nature of particulate organic matter in the River Indus, Pakistan

Abstract Suspended sediments from the Indus River collected during 1981 through 1983 were analyzed for POC and its constituent fractions including amino acids, amino sugars and sugars. Percentage of POC decreased with increasing suspended matter concentrations, which suggested dilution of organic matter by mineral matter. The concentrations of amino acids, amino sugars and sugars varied, respectively, between 180 and 2000 μg/l, 5 and 125 μg/l, and 60 and 1100 μg/l. Their contributions to POC varied between 2 and 60% for amino acids and amino sugars, and between 2 and 15% for sugars. They were high during low sediment discharge (February to June), and low during high sediment discharge (August and September). Suspended sediments associated with high sediment discharge periods were characterized by low ratios of: 1. (i) aspartic acid:β-alanine 2. (ii) glutamic acid:γ-aminobutyric acid 3. (iii) amino acids:amino sugars 4. (iv) hexoses:pentoses. These and the relative distribution pattern of the monosaccharides such as galactose, arabinose, mannose and xylose indicated that, not only dilution, but also differences in the sources and processes affect the POC transport in the Indus River. These result in transport of biodegraded organic matter during high sediment discharge periods: this appears to be common to other major rivers of the region, with depositional centers in deep sea areas. These rivers, with their high sediment loads, could contribute up to 8 to 11% of the global annual organic carbon burial in marine sediments.

Coupling between SW monsoon-related surface and deep ocean processes as discerned from continuous particle flux measurements and correlated satellite data

Particle flux data obtained by time series sediment traps deployed at water depths of approximately 3000 m in the western, central, and eastern Arabian Sea since 1986 were compared with wind speeds derived from measurements made by microwave radiometer flying on polar orbiting satellites and sea surface temperatures (SSTs) provided by the Physical Oceanography Distributed Active Archive Center at Jet Propulsion Laboratory. This comparison has allowed us to trace the link between the oceanographic and biological processes related to the development of the SW monsoon with the pattern and interannual variability of particle fluxes to the interior of the Arabian Sea. We could recognize the well-known upwelling systems along the coasts of Somalia and Oman as well as open ocean upwelling at the beginning of the SW monsoon. Both open ocean upwelling and coastal upwelling off Oman cause a cooling of surface waters at our western and central Arabian Sea stations. When SSTs fall below their long-term average, an increase in fluxes which are dominated by coccolithophorid-derived carbonates occurs. The timing of this increase is determined by the rate of surface water cooling. Further intensification of upwelling as the SW monsoon progresses causes additional increases in biogenic opal fluxes denoting diatom blooms in the overlying waters. The total fluxes during this period are the highest measured in the open Arabian Sea. At the central Arabian Sea location the fluxes are only randomly affected by these blooms. The particle flux in the eastern Arabian Sea is as high as in the central Arabian Sea but is influenced by a weaker upwelling system along the Indian coast. The observed interannual variability in the pattern of particle fluxes during the SW monsoons is most pronounced in the western Arabian Sea. This is controlled by the intensity of the upwelling systems on the one hand and the transport of cold, nutrient-poor, south equatorial water into the Oman region on the other. The latter effect, which is strongest during the SW monsoon with highest recorded wind speeds, reduces the influence of upwelling and the related particle fluxes at the western Arabian Sea station, where highest fluxes occur during SW monsoons with moderate wind speeds. Thus coastal and open ocean upwelling are most effective in transferring biogenic matter to the deep sea during the SW monsoons of intermediate strength.

Millennial‐scale oscillation of denitrification intensity in the Arabian Sea during the Late Quaternary and its potential influence on atmospheric N2O and global climate

The intensity of denitrification in the Arabian Sea during the last 65 kyr is reconstructed using high-resolution δ 15 N records of three sediment cores in conjunction with other geochemical tracers for water column oxygenation and productivity. The results reveal a close link to the Greenland ice core record with low or absent water column denitrification during the Last Glacial Maximum, the stadials, and at the time of the Heinrich Events including the Younger Dryas. In contrast, denitrification was high during the Holocene and the interstadials. The intensification of denitrification is related to stronger SW monsoonal upwelling, which enhances organic matter flux and degradation, resulting in a strengthening of the midwater oxygen deficiency. Such a combination of enhanced upwelling and denitrification has also been implied for the Eastern Tropical North Pacific (ETNP), where these events occur during the Holocene and to some extent during the interstadials, too. Today, the Arabian Sea and the ETNP together contribute substantially to the global marine water column denitrification, and a significant fraction of the ocean-atmosphere N 2 O flux originates from these areas. Changes in N 2 O emissions from these areas could thus have effected the recently described stadial/interstadial variations in the atmospheric concentration of this greenhouse gas as deduced from ice cores. Moreover, denitrification is the major sink for oceanic nitrate and provides a primary control for the oceanic nutrient inventory, which in turn influences global primary productivity and CO 2 sequestration by the biological pump. Short-term switches between a nondenitrification mode and a denitrification mode in these marine regions therefore have an impact on global climate.

Interannual variability in particle flux in the southwestern Black Sea

Abstract Vertical particle flux was measured at two sites in the southwestern Black Sea using automated time-series sediment traps over a period of 4.5 years. The particle flux between both sites varied considerably. (1) At site BSC (80 km from shore) the dominant fraction of the annual flux was deposited during short blooms; at site BS (40 km from shore, but still beyond the shelf break), the particle flux was less dominated by short-term blooms. (2) At site BSC, plankton blooms were the dominant cause for removal of suspended lithogenic matter; at site BS, vertical transport of lithogenic matter was linked also to the occurrence of storms and to high discharge periods of local rivers. Upwelling in the southwestern Black Sea may play an important role in triggering plankton blooms.

Variations of dissolved organic matter during a plankton bloom: qualitative aspects, based on sugar and amino acid analyses

Abstract Water samples collected during a spring plankton bloom in the northern North Sea were investigated for dissolved organic carbon (DOC), total dissolved sugars (TDS) and dissolved amino acids (TDAA). Their concentrations varied in the range of 0.6 to >4 mg 1 −1 for DOC, 40—400 γg 1 −1 for TDS and 100—850γg 1 −1 for TDAA. TDS and TDAA contributed to less than 20% of the DOC, but showed variations independent of each other. Changes in the efficiency of heterotrophic utilization and its selectivity seem to have caused these variations. These, together with differences in the sources of dissolved organic matter — phytoplankton and detrital materials — were also reflected in the qualitative composition of TDS and TDAA in the water column. In the TDS fraction glucose and mannose showed maxima during the phytoplankton bloom and galactose, rhamnose, xylose, arabinose and fucose after it. Glutamic acid registered maximum in the early stages of the phytoplankton bloom, histidine towards its end and aspartic acid, glycine, alanine and lysine after it. Serine was present in large amounts throughout the investigated time interval. The results of the present study are discussed in the light of previously published data on the formation and utilization of dissolved organic matter in the sea.