Spatial variation of phytoplankton pigments along the southwest coast of India

Phytoplankton composition and abundance were studied along the southwestern Indian coast toward the end of the upwelling season in October 2004. Phytoplankton pigment analyses, complemented by limited microscopic counts, were carried out to determine the community structure. Chlorophyll a was the most abundant of all pigments, followed by fucoxanthin. Zeaxanthin was abundantly found in the southern part of the study region (off Trivandrum), whereas fucoxanthin was the dominant marker pigment in the north (off Goa). The inferred shift in the community structure from a dominant picoplankton fraction and Prymnesiophytes to diatom-dominated microplankton toward the north is ascribed to differences in the physico-chemical environment

Benthic fluxes in a tropical Estuary and their role in the ecosystem

In-situ measurements of benthic fluxes of oxygen and nutrients were made in the subtidal region of the Mandovi estuary during premonsoon and monsoon seasons to understand the role of sediment-water exchange processes in the estuarine ecosystem. The Mandovi estuary is a shallow, highly dynamic, macrotidal estuary which experiences marine condition in the premonsoon season and nearly fresh water condition in the monsoon season. The benthic flux of nutrients exhibited strong seasonality, being higher in the premonsoon compared to the monsoon season which explains the higher ecosystem productivity in the dry season in spite of negligible riverine nutrient input. NH4+ was the major form of released N comprising 70-100% of DIN flux. The benthic respiration rate varied from -98.91 to -35.13 mmol m-2 d-1, NH4+ flux from 5.15 to 0.836 mmol m-2 d-1, NO3- + NO2- from 0.06 to -1.06 mmol m-2 d-1, DIP from 0.12 to 0.23 mmol m-2 d-1 and SiO44- from 5.78 to 0.41 mmol m-2 d-1 between premonsoon to monsoon period. The estuarine sediment acted as a net source of DIN in the premonsoon season, but changed to a net sink in the monsoon season. Variation in salinity seemed to control NH4+ flux considerably. Macrofaunal activities, especially bioturbation, enhanced the fluxes 2-25 times. The estuarine sediment was observed to be a huge reservoir of NH4+, PO43- and SiO44- and acted as a net sink of combined N because of the high rate of benthic denitrification as it could remove 22% of riverine DIN influx thereby protecting the eco system from eutrophication and consequent degradation. The estuarine sediment was responsible for ~30-50% of the total community respiration in the estuary. The benthic supply of DIN, PO43- and SiO44- can potentially meet 49%, 25% and 55% of algal N, P and Si demand, respectively, in the estuary. Based on these observations we hypothesize that it is mainly benthic NH4+ efflux that sustains high estuarine productivity in the NO3- depleted dry season

Coastal versus open-ocean denitrification in the Arabian Sea

International audienceThe Arabian Sea contains one of the three major open-ocean denitrification zones in the world. In addition, pelagic denitrification also occurs over the inner and mid-shelf off the west coast of India. The major differences between the two environments are highlighted using the available data. The perennial open-ocean system occupies two orders of magnitude larger volume than the seasonal coastal system, however, the latter offers more extreme conditions (greater nitrate consumption leading to complete anoxia). Unlike the open-ocean system, the coastal system seems to have undergone a change (i.e., it has intensified) over the past few decades presumably due to enhanced nutrient loading from land. The two systems also differ from each other with regard to the modes of nitrous oxide (N2O) production: In the open-ocean suboxic zone, an accumulation of secondary nitrite (NO2?) is invariably accompanied by depletion of N2O whereas in the coastal suboxic zone high NO2? and very high N2O concentrations frequently co-occur, indicating, respectively, net consumption and net production of N2O by denitrifiers. The extents of heavier isotope enrichment in the combined nitrate and nitrite (NO3?+NO2?) pool and in N2O in reducing waters appear to be considerably smaller in the coastal region, reflecting more varied sources/sinks and/or different isotopic fractionation factors

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

© The Authors, 2010. This article is distributed under the terms of the Creative Commons Attribution 3.0 License. The definitive version was published in Biogeosciences 7 (2010): 2091-2100, doi:10.5194/bg-7-2091-2010.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 (O2) depletion in the region. Over the much wider Indian shelf, which experiences large-scale bottom water O2-depletion in summer, adequate iron supply from reducing 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 O2-depleted environment for the growth of oxygenic photosynthesizers.Financial support was provided by CSIR through the Network Project CMM0009 to SWAN and by NSF through OCE-0327227S to JWM

Massive Nitrogen Loss Over the Western Indian Continental Shelf During Seasonal Anoxia: Evidence From Isotope Pairing Technique

The western Indian continental shelf houses the world's largest naturally formed coastal low-oxygen zone that develops seasonally during the summer monsoon. We investigated multiple reductive nitrogen transformation pathways and quantified their rates in this system through anaerobic incubations with additions of N-15-labeled substrates during the anoxic period for three consecutive years (2008-2010). Addition of N-15 labeled ammonium ((NH4+)-N-15) resulted in low to moderate anaerobic ammonia oxidation (Anammox) rates in about half of our incubations from the oxygen depleted waters. In contrast, incubations with labeled nitrite ((NO2-)-N-15) led to large production of N-30(2) over N-29(2) in all incubation experiments, indicating denitrification to be the dominant N-loss pathway. Rates of dissimilatory nitrate/nitrite reduction to ammonium (DNRA) were found to be highly variable and were lower by an order of magnitude than the denitrification rates. Extrapolation of average rates over the sampling periods and volume of anoxic waters showed large nitrogen removal (3.70-11.1 Tg year(-1)) which is about three times as high as the previously reported estimate (1.3-3.8 Tg year(-1)). Despite the small area it occupies, this shallow seasonal anoxic zone may account for as much as 20-60% the of the total annual fixed nitrogen loss in the perennial oxygen minimum zone of the Arabian Sea

Water column characteristics following the September 2004 stench event off southern Malabar coast

199-205An unusual stench emanated from the coastal waters of southwest coast India during September 2004 associated with an algal bloom. Water samples collected from 14 stations from the affected area after three weeks of the event, revealed that phytoplankton were dominated by holococcolithophorids (max 27 x 10⁶ cells l⁻¹) in the coastal stations in comparison with the offshore stations (max 8000 cells l⁻¹, which contained mainly diatoms. Slightly low oxygen concentration (81-191 µM) associated with relatively lower temperature and high nutrients indicated the prevalence of weak upwelling in the region. Chlorophyll a to phaeopigments ratio at the coastal stations indicated that the bloom was in the degrading phase. Resultant microbial activities perhaps led to higher ammonia concentration in the study region. The holococcolithophorids seem to be uncommon to this region, but upon availability of right conditions, presumably temperature and nutrients formed massive bloom and consequently a stench affecting coastal population

Variations in some environmental characteristics including C and N stable isotopic composition of suspended organic matter in the Mandovi estuary

Chemical and isotopic (δ13C and δ15N) investigation of the Mandovi estuary along the Indian west coast affected strongly by the seasonal monsoon cycle was carried out. The Mandovi estuary is a major waterway for Goa and extensively used for transportation of iron and manganese ore. In addition, with large population centers as well as agricultural fields located on its shores, the estuary is assumed to have been influenced by human activities. Measurements of chemical and isotopic parameters made in the lower part of the estuary during the southwest (SW) monsoon and post-monsoon seasons reveal distinct changes, and it is observed that despite considerable enrichment of macronutrients during the SW monsoon, productivity of the estuary (phytoplankton biomass), as inferred from the chlorophyll-a content, is not as high as expected. This is due to occurrences of high turbidity and cloud cover that limits photosynthetic productivity. The isotopic characterization (C and N isotopes) of suspended organic matter produced/transported during the monsoon and post-monsoon seasons of year 2007 provides a baseline dataset for future isotopic studies in such type of tropical estuaries

Response of phytoplankton to nutrient enrichment with high growth rates in a tropical monsoonal estuary - Zuari estuary, India

314-325Present study is about the impact of macronutrient enrichment on phytoplankton biomass and community structure in a tropical monsoonal estuary. In situ experiments carried out during the pre-monsoon period (February-March 2006), showed that the response time of phytoplankton to nutrient enhancement was 24-32 h. Phytoplankton biomass increased sizably, indicating nitrate and silicate limitation for phytoplankton growth. An increase in 23µg chl a l-1 resulted in an uptake of 10µM nitrate, 0.6µM phosphate and 17µM silicate. Phytoplankton showed high growth rates with an average value of 1.36µg chl l-1 d-1.This phytoplankton community was largely dominated by diatoms (>96%), particularly chain forming species. Relative preference index (RPI) value for nitrate was >1, suggesting that, irrespective of the ambient ammonium concentration, estuarine autotrophs preferred nitrate. Few species like Skeletonema costatum and Thalassionema nitzschioides exhibited the ability to withstand hypoxic condition. </span