The Arabian Sea as a high-nutrient, low-chlorophyll region during the late Southwest Monsoon

Abstract

Abstract.Extensive observations were made during the late Southwest Monsoon of 2004 over the Indian and Omani shelves, and along a transect that extended from the southern coast of Oman to the central west coast of India, tracking the southern leg of the US JGOFS expedition (1994)(1995) in the west. The data are used, in conjunction with satellite-derived data, to investigate long-term trends in chlorophyll and sea surface temperature, indicators of upwelling intensity, and to understand factors that control primary production (PP) in the Arabian Sea, focussing on the role of iron. Our results do not support an intensification of upwelling in the western Arabian Sea, reported to have been caused by the decline in the winter/spring Eurasian snow cover since 1997. We also noticed, for the first time, an unexpected development of high-nutrient, low-chlorophyll condition off the southern Omani coast. This feature, coupled with other characteristics of the system, such as a narrow shelf and relatively low iron concentrations in surface waters, suggest a close similarity between the Omani upwelling system and the Peruvian and California upwelling systems, where PP is limited by iron. 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 much wider Indian shelf, which experiences large-scale bottom water O 2 -depletion in summer, adequate iron supply from reCorrespondence to: S. W. A. Naqvi (naqvi@nio.org) ducing bottom-waters and sediments seems to support moderately high PP; however, such production is restricted to the thin, oxygenated surface layer, probably because of the unsuitability of the O 2 -depleted environment for the growth of oxygenic photosynthesizers.

Silicon cycle in Indian estuaries and its control by biogeochemical and anthropogenic processes

“…Diatom growth may affect DSi concentration in the estuaries through uptake (Admiraal et al, 1990;Hughes et al, 2011). In addition, anthropogenic activities such as damming may decrease DSi by favouring BSi retention in the reservoirs (Conley, 2002;Friedl et al, 2004;Laruelle et al, 2009;Hughes et al, 2012). In contrast, deforestation (Conley et al, 2008) and amplified erosion (Xiangbin et al, 2015) may increase the supply of both DSi and BSi to the coastal system.…”

“…On the other hand, the upper estuary gets dried up due to weak discharge, allowing the penetration of seawater during the dry period. The Indian subcontinent is bounded by the Arabian Sea on the west and the Bay of Bengal on the east and receives 0.3 × 10 12 m 3 freshwater influx from rivers to the former basin and 1.6 × 10 12 m 3 to the latter basin (Krishna et al, 2016). In addition to freshwater influx, the glacial and monsoonal rivers supply a enormous load of suspended matter of around 1.4 × 10 9 t to the Bay of Bengal and 0.2 × 10 9 t into the Arabian Sea respectively (Nair et al, 1989;Ittekkot et al, 1991).…”

“…Anderson (1986) stated that biological productivity is higher in the tidal freshwater reach because of reduced turbidity (and hence reduction of its limiting effect on light) as opposed to the turbidity maximum region along the increasing salinity gradient. Diatom growth may affect DSi concentration in the estuaries through uptake (Admiraal et al, 1990;Hughes et al, 2011). In addition, anthropogenic activities such as damming may decrease DSi by favouring BSi retention in the reservoirs (Conley, 2002;Friedl et al, 2004;Laruelle et al, 2009;Hughes et al, 2012).…”

“…In contrast, deforestation (Conley et al, 2008) and amplified erosion (Xiangbin et al, 2015) may increase the supply of both DSi and BSi to the coastal system. Moreover, heightened fertilizer usage (Li et al, 2007) results in a supply of excess N and P over Si leading to the development of non-diatom blooms favouring HAB (Harmful Algal Blooms) and eutrophication, major threats to the base of food chain Howarth et al, 2011).…”

“…The Indian subcontinent is rich with high mountain ranges, widespread plateaus and extensive plains traversed by large rivers. India has~220 major and minor estuaries with varying size and shape along the~7500 km coastline (Sarma et al, 2012;Rao et al, 2013). India experiences seasonally reversing monsoonal systems called southwest or summer monsoon (June to September) and northeast or winter monsoon (November to February).…”