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Shirodkar G, Uskaikar H, Naqvi SWA, Pratihary A, Hussain A, Shenoy DM, Gauns M, Manikandan B, Manjrekar S, Patil A. Seasonally varying biogeochemical regime around the coral habitats off central west coast of India. Mar Environ Res 2024; 196:106427. [PMID: 38479295 DOI: 10.1016/j.marenvres.2024.106427] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/18/2023] [Revised: 02/24/2024] [Accepted: 02/26/2024] [Indexed: 03/23/2024]
Abstract
The Western Indian Continental Shelf (WICS) experiences upwelling during the Southwest Monsoon (SWM), leading to deoxygenation and acidification of subsurface waters. The region has patchy growth of corals, e.g. in the Grande Island and Angria Bank. Measurements made during the late SWM of 2022 reveal that the shelf waters around the Grande Island were subject to varying environmental conditions, viz. lower temperature (21.3-26.1°C), oxygen (0-4.9 mL L-1) and pHT (7.506-7.927). Complete anoxia was associated with sulphide build-up to a maximum of 5.9 μmol L-1 at 17 m depth. An additional episodic condition (high temperature, low oxygen and pH) also occurred associated presumably with a plankton bloom in April 2017. Hence, unlike the offshore coral site Angria Bank, waters around the Grande Island experiences extreme changes in physico-chemical conditions (e.g. Ωarg ∼1.2-1.8 during October 2022) seasonally as reported here. The biogeochemical conditions are however not as intense (Ωarg = 0.6) as observed along the eastern boundary upwelling system of the Pacific Ocean.
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Affiliation(s)
- Gayatri Shirodkar
- CSIR-National Institute of Oceanography, Dona Paula, Goa-403004, India.
| | - Hema Uskaikar
- CSIR-National Institute of Oceanography, Dona Paula, Goa-403004, India
| | - S W A Naqvi
- CSIR-National Institute of Oceanography, Dona Paula, Goa-403004, India
| | - Anil Pratihary
- CSIR-National Institute of Oceanography, Dona Paula, Goa-403004, India
| | - Afreen Hussain
- CSIR-National Institute of Oceanography, Dona Paula, Goa-403004, India
| | - Damodar M Shenoy
- CSIR-National Institute of Oceanography, Dona Paula, Goa-403004, India
| | - Manguesh Gauns
- CSIR-National Institute of Oceanography, Dona Paula, Goa-403004, India
| | - B Manikandan
- CSIR-National Institute of Oceanography, Dona Paula, Goa-403004, India
| | | | - Anagha Patil
- CSIR-National Institute of Oceanography, Dona Paula, Goa-403004, India
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Shetye S, Pratihary A, Shenoy D, Kurian S, Gauns M, Uskaikar H, Naik B, Nandakumar K, Borker S. Rice husk as a potential source of silicate to oceanic phytoplankton. Sci Total Environ 2023; 879:162941. [PMID: 36934917 DOI: 10.1016/j.scitotenv.2023.162941] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/07/2022] [Revised: 03/10/2023] [Accepted: 03/14/2023] [Indexed: 05/17/2023]
Abstract
Global oceans are witnessing changes in the phytoplankton community composition due to various environmental stressors such as rising temperature, stratification, nutrient limitation, and ocean acidification. The Arabian Sea is undergoing changes in its phytoplankton community composition, especially during winter, with the diatoms being replaced by harmful algal blooms (HABs) of dinoflagellates. Recent studies have already highlighted dissolved silicate (DSi) limitation and change in Silicon (Si)/Nitrogen (N) ratios as the factors responsible for the observed changes in the phytoplankton community in the Arabian Sea. Our investigation also revealed Si/N < 1 in the northern Arabian Sea, indicating DSi limitation, especially during winter. Here, we demonstrate that rice husk with its phytoliths is an important source of bioavailable DSi for oceanic phytoplankton. Our experiment showed that a rice husk can release ∼12 μM of DSi in 15 days and can release DSi for ∼20 days. The DSi availability increased diatom abundance up to ∼9 times. The major benefitted diatom species from DSi enrichment were Nitzshia spp., Striatella spp., Navicula spp., Dactiliosolen spp., and Leptocylindrus spp. The increase in diatom abundance was accompanied by an increase in fucoxanthin and dimethyl sulphide (DMS), an anti-greenhouse gas. Thus, the rice husk with its buoyancy and slow DSi release has the potential to reduce HABs, and increase diatoms and fishery resources in addition to carbon dioxide (CO2) sequestration in DSi-limited oceanic regions such as the Arabian Sea. Rice husk if released at the formation site of the Subantarctic mode water in the Southern Ocean could supply DSi to the thermocline in the global oceans thereby increasing diatom blooms and consequently the biotic carbon sequestration potential of the entire ocean.
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Affiliation(s)
- Suhas Shetye
- CSIR-National Institute of Oceanography, Dona Paula 403 004, Goa, India.
| | - Anil Pratihary
- CSIR-National Institute of Oceanography, Dona Paula 403 004, Goa, India
| | - Damodar Shenoy
- CSIR-National Institute of Oceanography, Dona Paula 403 004, Goa, India
| | - Siby Kurian
- CSIR-National Institute of Oceanography, Dona Paula 403 004, Goa, India
| | - Mangesh Gauns
- CSIR-National Institute of Oceanography, Dona Paula 403 004, Goa, India
| | - Hema Uskaikar
- CSIR-National Institute of Oceanography, Dona Paula 403 004, Goa, India
| | - Bhagyashri Naik
- CSIR-National Institute of Oceanography, Dona Paula 403 004, Goa, India
| | - K Nandakumar
- CSIR-National Institute of Oceanography, Dona Paula 403 004, Goa, India
| | - Sidhesh Borker
- CSIR-National Institute of Oceanography, Dona Paula 403 004, Goa, India
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Laglera LM, Uskaikar H, Klaas C, Naqvi SWA, Wolf-Gladrow DA, Tovar-Sánchez A. Dissolved and particulate iron redox speciation during the LOHAFEX fertilization experiment. Mar Pollut Bull 2022; 184:114161. [PMID: 36179387 DOI: 10.1016/j.marpolbul.2022.114161] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/14/2022] [Revised: 09/15/2022] [Accepted: 09/17/2022] [Indexed: 06/16/2023]
Abstract
The redox speciation of iron was determined during the iron fertilization LOHAFEX and for the first time, the chemiluminescence assay of filtered and unfiltered samples was systematically compared. We hypothesize that higher chemiluminescence in unfiltered samples was caused by Fe(II) adsorbed onto biological particles. Dissolved and particulate Fe(II) increased in the mixed layer steadily 6-fold during the first two weeks and decreased back to initial levels by the end of LOHAFEX. Both Fe(II) forms did not show diel cycles downplaying the role of photoreduction. The chemiluminescence of unfiltered samples across the patch boundaries showed strong gradients, correlated significantly to biomass and the photosynthetic efficiency and were higher at night, indicative of a biological control. At 150 m deep, a secondary maximum of dissolved Fe(II) was associated with maxima of nitrite and ammonium despite high oxygen concentrations. We hypothesize that during LOHAFEX, iron redox speciation was mostly regulated by trophic interactions.
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Affiliation(s)
- Luis M Laglera
- FI-TRACE, Departamento de Química, Universidad de las Islas Baleares, Palma, Balearic Islands 07122, Spain; Laboratori Interdisciplinari sobre Canvi Climàtic, Universidad de las Islas Baleares, Palma, Balearic Islands 07122, Spain.
| | - Hema Uskaikar
- National Institute of Oceanography, Dona Paula, Goa, India
| | - Christine Klaas
- Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany
| | | | - Dieter A Wolf-Gladrow
- Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany
| | - Antonio Tovar-Sánchez
- Department of Ecology and Coastal Management, Andalusian Institute for Marine Science, ICMAN (CSIC), Campus Universitario Río San Pedro, Puerto Real, Cádiz, Spain
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Shenoy DM, Kurian S, Shirodkar G, Uskaikar H, Gauns M, Naqvi SWA. Impact of physical processes on oxygen loss and production of hydrogen sulphide and methane in a tropical freshwater reservoir. Environ Sci Pollut Res Int 2021; 28:39655-39667. [PMID: 33761073 DOI: 10.1007/s11356-021-13472-x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/01/2020] [Accepted: 03/11/2021] [Indexed: 06/12/2023]
Abstract
Carbon neutrality of tropical reservoirs is a big concern in recent years as some estimates project high methane emission from these reservoirs. While there are studies available on the impact of physical processes (stratification and mixing) on the biogeochemistry of tropical reservoirs, not much information is available on the inter-annual variability in the low-oxygen conditions and production/accumulation of hydrogen sulphide (H2S) and methane (CH4) during summer. This paper presents time series data based on monthly in situ observations from a tropical reservoir (Tillari, Maharashtra) situated in the Western Ghats in India. Sampling was carried out for temperature, dissolved oxygen (DO), H2S, and CH4 at a fixed location from March 2010 until June 2014. The reservoir experiences stable stratification during summer (March to June) with complete loss of oxygen and production of H2S (max. ~ 9 μM) and CH4 (max. ~ 185 μM) in the profundal zone. During the summer stratification, the hypolimnion acted as a pool of CH4 with integrated values ranging between 3502 and 41,632 mg m-2. However, the intensity and duration of anoxia varied during different years, influencing H2S and CH4 production. Mixing in the reservoir was observed between July and September in association with the monsoonal runoff, which increased the DO concentrations in the sub-surface layers. Besides, complete mixing was observed between December and February due to winter convection. This, however, was found to play an important role, as weaker mixing in the preceding year was associated with severe oxygen loss in the profundal zone during the following summer with a production of H2S and CH4. In contrast, more robust mixing during winter led to moderate low-oxygen conditions with less production of these gases in the subsequent summer. Based on our observations and considering a large number of reservoirs in the tropics, we hypothesise that with the present trends of global warming and less cold winters, low-oxygen conditions in the profundal zone may become more severe in the future with positive feedback on H2S and CH4 production during summer.
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Affiliation(s)
- Damodar M Shenoy
- Chemical Oceanography Division, CSIR-National Institute of Oceanography, Dona Paula, Goa, 403004, India.
| | - Siby Kurian
- Chemical Oceanography Division, CSIR-National Institute of Oceanography, Dona Paula, Goa, 403004, India
| | - Gayatri Shirodkar
- Chemical Oceanography Division, CSIR-National Institute of Oceanography, Dona Paula, Goa, 403004, India
| | - Hema Uskaikar
- Chemical Oceanography Division, CSIR-National Institute of Oceanography, Dona Paula, Goa, 403004, India
| | - Mangesh Gauns
- Chemical Oceanography Division, CSIR-National Institute of Oceanography, Dona Paula, Goa, 403004, India
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Kurian S, Chndrasekhararao AV, Vidya PJ, Shenoy DM, Gauns M, Uskaikar H, Aparna SG. Role of oceanic fronts in enhancing phytoplankton biomass in the eastern Arabian Sea during an oligotrophic period. Mar Environ Res 2020; 160:105023. [PMID: 32907734 DOI: 10.1016/j.marenvres.2020.105023] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/13/2020] [Revised: 05/04/2020] [Accepted: 05/19/2020] [Indexed: 06/11/2023]
Abstract
In the present study, using in-situ and satellite observations, we investigate the influence of physical processes on the enhancement of phytoplankton biomass in the eastern Arabian Sea (EAS). Water column measurements were carried out from 9⁰N to 21⁰N (stations II-2 to II-14) along 68⁰E transect in the EAS during the beginning of fall intermonsoon (FIM) of 2014. Both in-situ and satellite-derived chlorophyll a (Chl a) showed higher biomass at 15⁰N (station II-8) compared to northern and southern stations. We explored the possible physical processes which can lead to high biological productivity at this station. Our study shows that nearly two times enhancement in Chl a at station II-8 was contributed by an open-ocean front, which occurred two days before the measurement. Based on phytoplankton marker pigments, it was evident that haptophytes were abundant at II-8 with a minor contribution from diatoms and dinoflagellates. This condition also led to a high concentration (4.9 nM) of dimethylsulphide (DMS), an anti-green house gas with a net flux of 3.76 μmol m-2d-1 at this site. Among the picophytoplankton, Synechococcus were abundant at this station, however Prochlorococcus were absent as confirmed by both marker pigment and flow cytometric counts. The case study presented here demonstrates the dynamic nature of open ocean fronts and their overall contribution to the productivity of the eastern Arabian Sea during the oligotrophic inter-monsoon period.
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Affiliation(s)
- Siby Kurian
- CSIR-National Institute of Oceanography, Dona Paula, Goa, 403 004, India.
| | - A V Chndrasekhararao
- CSIR-National Institute of Oceanography, Dona Paula, Goa, 403 004, India; Bharathidasan University, Tiruchirappalli, Tamil Nadu, 620 024, India
| | - P J Vidya
- CSIR-National Institute of Oceanography, Dona Paula, Goa, 403 004, India
| | - Damodar M Shenoy
- CSIR-National Institute of Oceanography, Dona Paula, Goa, 403 004, India
| | - Mangesh Gauns
- CSIR-National Institute of Oceanography, Dona Paula, Goa, 403 004, India
| | - Hema Uskaikar
- CSIR-National Institute of Oceanography, Dona Paula, Goa, 403 004, India
| | - S G Aparna
- CSIR-National Institute of Oceanography, Dona Paula, Goa, 403 004, India
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Naik BR, Gauns M, Bepari K, Uskaikar H, Shenoy DM. Variation in phytoplankton community and its implication to dimethylsulphide production at a coastal station off Goa, India. Mar Environ Res 2020; 157:104926. [PMID: 32275508 DOI: 10.1016/j.marenvres.2020.104926] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/16/2019] [Revised: 02/04/2020] [Accepted: 02/23/2020] [Indexed: 06/11/2023]
Abstract
Seasonal hypoxia/suboxia (at times anoxia) towards the end of Southwest monsoon (SWM; June to September) at the coastal time series site off Goa, West coast of India was found to influence the dynamics of phytoplankton biomass, community structure and production of climatically active gas, dimethylsulphide (DMS). In this diatom dominated study region, high DMS production in the subsurface waters during late SWM might possible be attributed to the stress experienced by micro- and macro-algae from the prevailing low oxygen subsurface waters through different pathways specifically believed to be via methylation pathway (see Schafer et al., 2010). Based on laboratory experiments, we hypothesize presence of floating seaweeds mostly Sargassum species washed from the shore to the study site to contribute sizably to DMS production in the water column as they sink and degrade during the senescence phase. However, we are yet to address its loss/emission processes across the oxic-hypoxic boundary of seasonal (and permanent) oxygen minimum zone of the northern Indian Ocean, which is important from the viewpoint of global climate change.
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Affiliation(s)
- Bhagyashri R Naik
- CSIR-National Institute of Oceanography, Dona Paula, Goa, India; School of Earth, Ocean and Atmospheric Sciences, Goa University, Taleigao Plateau, Goa, India
| | - Mangesh Gauns
- CSIR-National Institute of Oceanography, Dona Paula, Goa, India.
| | - Kausar Bepari
- CSIR-National Institute of Oceanography, Dona Paula, Goa, India; Centre for Marine Living Resources and Ecology, Kochi, Kerala, India
| | - Hema Uskaikar
- CSIR-National Institute of Oceanography, Dona Paula, Goa, India
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