Primary productivity and its regulation in the Arabian Sea during 1995 (original) (raw)
Related papers
Physical control of primary productivity on a seasonal scale in central and eastern Arabian Sea
Journal of Earth System Science, 2000
Usingin situ data collected during 1992–1997, under the Indian programme of Joint Global Ocean Flux Study (JGOFS), we show that the biological productivity of the Arabian Sea is tightly coupled to the physical forcing mediated through nutrient availability. The Arabian Sea becomes productive in summer not only along the coastal regions of Somalia, Arabia and southern parts of the west coast of India due to coastal upwelling but also in the open waters of the central region. The open waters in the north are fertilized by a combination of divergence driven by cyclonic wind stress curl to the north of the Findlater Jet and lateral advection of nutrient-rich upwelled waters from Arabia. Productivity in the southern part of the central Arabian Sea, on the other hand, is driven by advection from the Somalia upwelling. Surface cooling and convection resulting from reduced solar radiation and increased evaporation make the northern region productive in winter. During both spring and fall inter-monsoons, this sea remains warm and stratified with low production as surface waters are oligotrophic. Inter-annual variability in physical forcing during winter resulted in one-and-a-half times higher production in 1997 than in 1995.
Physical forcing of biological productivity in the Northern Arabian Sea during the Northeast Monsoon
Deep-sea Research Part Ii-topical Studies in Oceanography, 2001
Time-series observations at a nominally "xed location in the northern Arabian Sea (213N, 643E) during the Northeast Monsoon (winter, February) of l997 showed the prevalence of cold sea-surface temperatures (SST) and deep mixed layers resulting from winter cooling and convection. The covariation of nitrate concentrations in the surface layers and concentrations of chlorophyll a and primary production in the euphotic zone with mixed-layer depth (MLD) and wind suggests that carbon "xation was controlled primarily by physical forcing. Cooler waters during winter 1997 relative to winter 1995 were associated with deeper MLDs, higher nitrate concentrations, elevated primary productivity, and higher chlorophyll a concentrations, leading to the inference that even a 13C decrease in SST could lead to signi"cantly higher primary productivity. Satellite data on sea surface temperature (advanced very high-resolution radiometer; AVHRR) and TOPEX/POSEl-DON altimeter data suggest that this interannual variation is of basin-wide spatial scale. After the termination of winter cooling and subsequent warming during the Spring Intermonsoon, the Arabian Sea has low primary production. During the latter period, micro-organisms, i.e. heterotrophic bacteria and microzooplankton)-proliferate, a feeding mode through the microbial loop that appears to be inherent to mesozooplankton for sustaining their biomass throughout the year in this region.
Primary productivity in the Arabian Sea: A synthesis of JGOFS data
Progress in Oceanography, 2005
Temporal and spatial variations in phytoplankton biomass exist in the Arabian Sea at all scales from the diurnal to the seasonal, and from fine to large scale. Phytoplankton physiological rate parameters and productivity measurements suggest that phytoplankton are not strongly limited by either irradiance or nutrient supply. Grazing activities, in most cases, match phytoplankton net photosynthetic rates and growth. The variations in biomass are therefore explained from variations in mixing at diurnal to seasonal scales, and spatial variations caused by upwelling near the Omani coast and the presence of mesoscale eddies. Vertical mixing regulates the supply of irradiance and nutrients, but vertical mixing is never deep enough to limit phytoplankton productivity, and nitrogen does not appear to be a factor limiting phytoplankton growth. Vertical mixing, however, also affects grazing by diluting micro-grazers along with phytoplankton. It is argued here that mixed layer deepening acts as a natural Ôdilution experimentÕ that allows phytoplankton to escape grazing losses and grow, and thereby create the observed variability in phytoplankton biomass.
Interannual changes of the Arabian Sea productivity
Marine Biology Research, 2012
Inter-annual changes in temperature and chlorophyll a across the Arabian Sea (subdivided into 61 2-degree regions) were analysed. For each 2-degree region, from appropriate databases, remotely sensed chlorophyll a, sea surface temperature, and wind speed time series were retrieved. Spatial and temporal trend analysis showed physicalÁbiological oscillations with dominant periods of 12 and 6 months (reflecting the seasonality of monsoonal winds) with a globally warming trend, but no overall increase in chlorophyll during the period 1997Á2009. Variation coefficients of the inter-annual time series of chlorophyll a implied high variability in western regions of the sea in comparison to eastern regions. The basin-wide maps of chlorophyll distribution did not show the enlargement of the productive area over time and overall, not only did the Arabian Sea not get more productive, but several regions in its eastern basin showed a decline in chlorophyll a concentration.
Production and respiration rates in the Arabian Sea during the 1995 Northeast and Southwest Monsoons
2001
In this paper we examine the relationships among oxygen, carbon and nitrogen production and respiration rate measurements made in the Arabian Sea during the 1995 Northeast (NEM) and Southwest (SWM) Monsoons. Increased biological production characterized the SWM, with rates 12}53% higher than the NEM. In most cases, we found remarkable similarity in production rates during the two monsoons and an absence of strong spatial gradients in production between nearshore and o!shore waters, especially during the SWM. Daily C and total N production underestimated gross C production, and at the majority of stations C and total N production were either the same as net C production or between gross and net C production. Moreover, new production (NO ), scaled to carbon, was substantially less than net C production. Approximately 50% of the POC was metabolized during the photoperiod, with smaller losses (7}11%) overnight. The simplest explanation for the discrepancy between gross and total N production and between net C and new production was the loss of N-labeled particulate matter as dissolved organic matter. Partitioning of metabolized gross C production into respiratory and dissolved pools showed distinct onshore}o!shore distributions that appeared to be related to the composition of the phytoplankton assemblage and probably re#ected the trophodynamics of the ecosystem. The percentage of gross C production released as dissolved organic carbon (DOC) was highest in the nearshore waters where diatoms dominated the phytoplankton assemblage, while community respiration was a more important fate for production further o!shore where picoplankton prevailed. In general, stations that retained more gross C production as net production (i.e., high net C/gross C ratios) had higher rates of DOC production relative to community respiration. Locations where community respiration exceeded DOC production were characterized by low rates of net C production and had low net C/gross C ratios. In those ecosystems, less net C production was retained because higher metabolic losses reduced gross C production to a greater extent than at the more productive sites.
Photosynthetic physiology and physicochemical forcing in the Arabian Sea, 1995
Deep Sea Research Part I: Oceanographic Research Papers, 2002
The Arabian Sea, characterized by strong seasonal monsoonal forcing as well as prolific mesoscale features, is a complex hydrographic environment for phytoplankton photosynthesis. Previous studies have demonstrated seasonal cycles in both primary production and photosynthetic biomass that may be related to patterns of photosynthetic efficiency, but the magnitude of water column efficiency is high throughout the year suggesting that other mechanisms may be operating. To uncover the patterns and regulation of photosynthetic efficiency on the synoptic scale (basinwide, seasonal dynamics), here we focus on the relationship between physicochemical forcing such as nitrate concentrations and mixed layer depth and photophysiology. These observations, which are from a transect from the coast of Oman to about 1400 km offshore in the Arabian Sea at 65E, were made on five US JGOFS Arabian Sea Process cruises conducted between March and December 1995. Parameters of photosynthesis as quantified by pigmentation, photosynthesis-irradiance curves and spectral absorption properties were spatially and temporally variable, but demonstrate several marked trends. During the early Southwest Monsoon, when nitrate concentrations were elevated, maximum quantum yield of carbon uptake (f C;max ; 0.05170.024 mol C mol Q À1 ) and light-utilization coefficients (a B ; 0.04070.016 mg C mg Chl a À1 h À1 (mmol Q m À2 s À1 ) À1 ) were increased and non-photosynthetic pigment indices (NPPi, 0.33970.089 g g À1 ) and chlorophyll a-specific mean absorption (s Chl a ; 0.03270.012 m 2 mg Chl a À1 ) were typically reduced compared to the Spring and Fall intermonsoon seasons (f C;max ; 0.02070.009; a B ; 0.02570.011; NPPi, 1.06570.173; s Chl a ; 0.05870.018). These patterns are in part driven by some significant correlations between mixed layer depth and nitrate concentrations and NPPi, f C;max ; a B and s Chl a at the synoptic scale. However, other factors not measured here such as time-lagged processes may also be important to the overall variability of photosynthetic physiology. In general, photosynthetic efficiency as measured by f C;max suggests phytoplankton populations that are exceptionally photosynthetically efficient during the SW Monsoon period, while at other times of *Corresponding author. Massachusetts Institute of Technology, 48-336A MIT, : S 0 9 6 7 -0 6 3 7 ( 0 1 ) 0 0 0 6 8 -1 the year efficiency may be a contributing factor to the limitation of primary production. r
The Arabian Sea as a high-nutrient, low-chlorophyll region during the late Southwest Monsoon
Biogeosciences Discussions, 2010
Extensive observations during the late Southwest Monsoon of 2004 over the Indian and Omani shelves, and along an east-west transect reveal a mosaic of biogeochemical provinces including an unexpected high-nutrient, low-chlorophyll condition off the southern Omani coast. This feature, coupled with other characteristics of the system, 5 suggest a close similarity between the Omani upwelling system and the Peruvian and California upwelling systems, where primary production (PP) is limited by iron. An intensification of upwelling, reported to have been caused by the decline in the winter/spring Eurasian snow cover since 1997, is not supported by in situ hydrographic and chlorophyll measurements as well as a reanalysis of ocean colour data extending 10 to 2009. Iron limitation of PP may complicate simple relationship between upwelling and PP assumed by previous workers, and contribute to the anomalous offshore occurrence of the most severe oxygen (O 2 ) depletion in the region. Over the Indian shelf, affected by very shallow O 2 -deficient zone, high PP is restricted to a thin, oxygenated surface layer probably due to unsuitability of the O 2 -depleted environment for 15
Variability in primary production as observed from moored sensors in the central Arabian Sea in 1995
Deep Sea Research Part II: Topical Studies in Oceanography, 1998
Carbon assimilation was calculated using surface irradiance and fluorescence data collected from moored sensors located in the Arabian Sea (15°30N, 61°30E), beginning the twelvemonth period in October 1994. The calculation uses an assumed quantum efficiency and independently estimated phytoplankton absorption coefficients. Fluorescence was calibrated to chlorophyll a. Estimated primary production (C assimilation) varied seasonally and was roughly correlated with chlorophyll a biomass. Variations in integral primary production estimated from the moored observations as a function of integral chlorophyll a are interpreted in terms of the variations in mixed layer depth and possible losses of chlorophyll a biomass. Deep mixed layers suggest lower chlorophyll a-specific production, and variations in chlorophyll a may indicate grazing losses. Seasonal variability in a measure of primary production is useful for establishing the relationship with environmental forcing in the Arabian Sea and in understanding the export of production to the deep sea.
1997
Seasonal variation of chlorophyll has been of considerable interest on account of the effect of photosynthesis on ocean-atmosphere carbon exchange. It can be predicted by a dynamical system model of the marine ecosystem coupled with a physical oceanographic model. There is however a major difficulty in the calibration of contemporary ecosystem models on account of sparse data and a large number of model parameters. This paper reports a new approach of macrocalibration in which values of six parameters axe determined by examining in detail the seasonal variation of chlorophyll and primary productivity keeping in view the observations of two Indian JGOFS cruises. Both switching and nonswitching versions of grazing functions are used in a 7-component FDM model. Detailed simulations are reported for one station (16~ 65~