Contrasting patterns in pH variability in the Arabian Sea and Bay of Bengal.

Continuous understanding of the ongoing ocean acidification (OA) is essential for predicting the future impact of OA on marine ecosystems. Here we report the results of open ocean time-series measurements (19 cruises) of seawater pH in total hydrogen ion scale (pHT) and associated parameters in the Arabian Sea (AS) and the Bay of Bengal (BoB). During southwest monsoon (SWM), the pHT within the 30 to 100 m water column shows the maximum difference between the two basins with BoB pHT being lower (up to ~0.39 units) than AS which could be due to freshwater influx from rivers, mixed layer dynamics, and cold-core eddies. However, during Spring inter-monsoon (SIM), the pHT of BoB follows the trend of AS. A contrasting finding is that the lowest pHT occurs at 350 to 500 m in the BoB while it is ~1000 m in the AS. The pHT within the 150 to 1500 m layer of these two basins shows lower values by 0.03 (±0.02) in the BoB as compared to the AS. The possible reasons for the low pHT within the BoB oxygen minimum zone (OMZ) could be due to intrusion of western Pacific water in the BoB, freshwater influx from rivers, variations in OMZ of the two basins, higher temperature (~2°C) within the OMZ of the AS, and denitrification in the AS. The pHT in both the basins (500 to 1000 m) is lower than in the North Atlantic and higher than in the North Pacific waters; however, the pHT in the 200 to 500 m is lower in the BoB than in all these basins. This study highlights the under-saturation of calcium carbonate at very shallow depths (~ 100 m) in the BoB, indicating that the plankton in the BoB are facing a major risk from OA compared to the AS and need further investigation.

A study on microzooplankton community from the coastal waters of eastern Arabian Sea: emphasis on the dominance of heterotrophic dinoflagellates.

We studied the community composition of microzooplankton (MZP) from the surface waters off Candolim, Goa. The MZP communities were examined for the year 2013, covering different seasons and four stations (Near-shore: G3 & G4, offshore: G5 & G6). A total of 30 species belonging to 24 genera were recorded, which include loricate ciliates (LC: 14 species of 13 genera), aloricate ciliates (ALC: 5 species of 3 genera), heterotrophic dinoflagellates (HDS: 11 species of 8 genera), and copepod nauplii. The MZP abundance in the coastal waters varied spatially irrespective of different seasons, with higher abundance in the offshore stations (G5 & G6, 38-127 cells L-1) and lower abundance in the near-shore stations (G3 & G4, 20-97 cells L-1). The MZP community composition showed the dominance of HDS (16-85%) in the near-shore stations during most of the seasons and inferiority during NEM (16-18%). Moreover, all the coastal waters (near and offshore) were dominated by HDS (58-85%) during spring inter-monsoon. The dominant species of HDS were Dinophysis apicata, Dinophysis caudata, Prorocentrum micans, Protoperidinium breve, Protoperidinium latistriatum, and Protoperidinium granii. The statistical analysis (Canonical correspondence analysis and Spearman's rank correlation) depicts that the MZP abundance and community composition were mainly controlled by salinity (r = 0.4-0.7). Whereas the dominance of HDS in the coastal waters could be the reason for its mixotrophic nature and diverse feeding mechanism. Thus, a strong positive correlation between the HDS and LC (r = 0.73-0.92) showed the feeding ability of HDS in their relative community.

Recurrence of Gonyaulax polygramma bloom in the southeastern Arabian Sea.

Gonyaulax polygramma, a bloom-forming dinoflagellate, has been repeatedly observed along the southeastern Arabian Sea in recent years. During our study in October 2021, a patch of reddish-brown water was observed in the nearshore waters off Kannur (southwest coast of India) and later identified as Gonyaulax polygramma using scanning electron microscopy (SEM) and HPLC-based phytoplankton marker pigments. Gonyaulax polygramma accounted for 99.4% of the phytoplankton abundance at the bloom location, with high concentrations of peridinin and chlorophyll-a at the study site. High concentration of SiO42- was observed at the bloom site, while other nutrients were lower than the previously reported values. The bloom of Gonyaulax polygramma also resulted in high concentrations of dimethylsulfide, an anti-greenhouse gas, at the bloom site. In addition to onsite observation, Sentinel-3 satellite data was also used in the detection and validation of the observed bloom using the NDCI index. From the satellite image, it was evident that the bloom persisted at the mouth of the rivers during the study period. Since the red tide of Gonyaulax polygramma has been observed recurrently in the southeastern Arabian Sea, it is proposed to use satellites to detect and monitor the bloom on a routine basis.

Applications of Machine Learning in Chemical and Biological Oceanography.

Machine learning (ML) refers to computer algorithms that predict a meaningful output or categorize complex systems based on a large amount of data. ML is applied in various areas including natural science, engineering, space exploration, and even gaming development. This review focuses on the use of machine learning in the field of chemical and biological oceanography. In the prediction of global fixed nitrogen levels, partial carbon dioxide pressure, and other chemical properties, the application of ML is a promising tool. Machine learning is also utilized in the field of biological oceanography to detect planktonic forms from various images (i.e., microscopy, FlowCAM, and video recorders), spectrometers, and other signal processing techniques. Moreover, ML successfully classified the mammals using their acoustics, detecting endangered mammalian and fish species in a specific environment. Most importantly, using environmental data, the ML proved to be an effective method for predicting hypoxic conditions and harmful algal bloom events, an essential measurement in terms of environmental monitoring. Furthermore, machine learning was used to construct a number of databases for various species that will be useful to other researchers, and the creation of new algorithms will help the marine research community better comprehend the chemistry and biology of the ocean.

Rice husk as a potential source of silicate to oceanic phytoplankton.

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.

Light stress induced DMS(P) production in Skeletonema costatum: An experimental approach and field observation.

Dimethylsulphide is a dominant biogenic sulphur anti-greenhouse gas produced by marine phytoplankton. A non-axenic culture of Skeletonema costatum was studied to comprehend the effects of different growth stages and light stress on DMSP/DMS production. The intracellular DMSP concentration increased during late exponential to mid-stationary phase and attained a maximum (0.59 pg S cell-1) during the stationary phase, indicating more contribution from actively dividing smaller cells. Likewise, exposure to first light after a 12-hour dark phase caused stress, invariably leading to elevated levels of DMS (~9 fold). These observations were upheld by additional laboratory and field experiments, and a field time-series observation, which recorded higher DMS concentrations during exposure to first light after a dark cycle and during early mornings, respectively. While our study depicts the variable DMSP and DMS concentrations during different growth stages of S. costatum, it gives new information on the effect of light stress on DMS production.

Malformation in coccolithophores in low pH waters: evidences from the eastern Arabian Sea.

Oceanic calcifying plankton such as coccolithophores is expected to exhibit sensitivity to climate change stressors such as warming and acidification. Observational studies on coccolithophore communities along with carbonate chemistry provide important perceptions of possible adaptations of these organisms to ocean acidification. However, this phytoplankton group remains one of the least studied in the northern Indian Ocean. In 2017, the biogeochemistry group at the Council for Scientific and Industrial Research-National Institute of Oceanography (CSIR-NIO) initiated a coccolithophore monitoring study in the eastern Arabian Sea (EAS). Here, we document for the first time a detailed spatial and seasonal distribution of coccolithophores and their controlling factors from the EAS, which is a well-known source of CO2 to the atmosphere. To infer the seasonality, data collected at three transects (Goa, Mangalore, and Kochi) during the Southwest Monsoon (SWM) of 2018 was compared with that of the late SWM of 2017. Apart from this, the abundance of coccolithophores was studied at the Candolim Time Series (CaTS) transect, off Goa during the Northeast Monsoon (NEM). The most abundant coccolithophore species found in the study region was Gephyrocapsa oceanica. A high abundance of G. oceanica (1800 × 103cells L-1) was observed at the Mangalore transect during the late SWM despite experiencing low pH and can be linked to nitrogen availability. The high abundance of G. oceanica at Mangalore was associated with high dimethylsulphide (DMS). Particulate inorganic carbon (PIC) and scattering coefficient retrieved from satellites also indicated a high abundance of coccolithophores off Mangalore during the late SWM of 2017. Interestingly, G. oceanica showed malformation during the late SWM in low pH waters. Malformation in coccolithophores could have a far-reaching impact on the settling fluxes of organic matter and also on the emissions of climatically important gases such as DMS and CO2, thus influencing atmospheric chemistry. The satellite data for PIC in the EAS indicates a high abundance of coccolithophore in recent years, especially during the warm El Nino years (2015 and 2018). This warrants the need for a better assessment of the fate of coccolithophores in high-CO2 and warmer oceans.

Organic carbon dynamics in the continental shelf waters of the eastern Arabian Sea.

The seasonal and spatial distribution of total organic carbon (TOC) is presented for the coastal waters of the eastern Arabian Sea, which experiences seasonal suboxia during the late southwest monsoon (SWM). This study reveals that high TOC was observed off Kochi as compared to Goa and Mangalore transects, and may be attributed to stronger upwelling along the Kerala coast. This is also supported by the excess carbon due to upwelling during the late SWM that varied from 37 µM (Goa), 39 µM (Mangalore), to 51 µM (Kochi). Our seasonal data from 2014 to 2020 at the Goa transect indicates that high TOC is seen during late SWM to fall inter monsoon (FIM) and between the late northeast monsoon (NEM) to the early spring inter monsoon (SIM). The high TOC concentrations and C/N ratios observed during the FIM are a combination of high primary production, the buildup of remnant organic matter from the previous season (due to prevailing low oxygen conditions), accumulation of refractory organic carbon, and release from diatoms (especially Chaetoceros sp.). Inter-annual variations indicate that phytoplankton blooms resulted in higher TOC concentrations, especially during the year 2020. Based on a comparison with an Elnino-Southern Oscillation (ENSO) year (2015), we can infer that the partitioning of carbon may increase from particulate to dissolved phase in future warming scenarios.

Impact of physical processes on oxygen loss and production of hydrogen sulphide and methane in a tropical freshwater reservoir.

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.

Sea foam-associated pathogenic bacteria along the west coast of India.

Anthropogenic activities release effluents containing nutrients and pathogenic bacteria that change the characteristics of coastal ecosystems. An important type of marine pollution which has occurred in 3 different states in India during 2019 is sea foam. Sea foam was found on Hole beach, Goa (Lat: 15.404° N, Long: 73.787° E), where nutrients (NO3- = 137 µM and organic nitrogen = 121 µM) from a garbage dumpyard are released directly via streams/gutters to coastal waters. This resulted in a bloom of the diatom Thalassiosira pseudonana, associated with high concentration of total organic carbon and fucoxanthin. Decay of this bloom along with strong agitation due to rocks and wave action resulted in sea foam. We isolated foam-associated bacteria and identified pathogenic bacteria including Enterobacter cancerogenus through 16S rRNA gene sequencing. Such foam-associated pathogenic bacteria, could be antibiotic resistant, and may have adverse effects on human health. This can also hamper the tourism industry of a small state like Goa that relies heavily on tourism.

Total organic carbon and its role in oxygen utilization in the eastern Arabian Sea.

We report seasonal and temporal variation of total organic carbon (TOC) in the eastern Arabian Sea (AS). In comparison to the deep, TOC in the top 100 m showed spatial variation with higher concentrations towards northern AS during North east monsoon (NEM) and South west monsoon (SWM). A comparison with the US-JGOFS data (1995) shows warmer temperatures, enhanced TOC and low chlorophyll in the recent years. High TOC is associated with Arabian Sea high saline waters (ASHSW), advected from the Arabian Gulf, might have resulted in an enhancement of TOC in the eastern AS. This excess TOC supports a high abundance of bacteria despite the low primary productivity. TOC oxidation accounted for 14.3% and 22.5% of oxygen consumption for waters with potential density between 24.5 and 27.3 kg/m3. This study attains great significance considering the missing links with respect to the role of transport processes in ocean deoxygenation under ongoing warming scenarios.

Role of oceanic fronts in enhancing phytoplankton biomass in the eastern Arabian Sea during an oligotrophic period.

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 9°N to 21°N (stations II-2 to II-14) along 68°E 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 15°N (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.

Variation in phytoplankton community and its implication to dimethylsulphide production at a coastal station off Goa, India.

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.

Variability of organic nitrogen and its role in regulating phytoplankton in the eastern Arabian Sea.

The relative importance of organic nitrogen (ON) and inorganic nitrogen (IN) as nitrogen sources for the phytoplankton communities were studied through a seasonal time series of nitrogen species in the eastern Arabian Sea. Seasonal data on nitrogen species showed that ON dominates the system throughout the year and account for >90% of the total nitrogen (TN) during north east monsoon (NEM). The average ON concentration in the study region was 11.5 µM during NEM as compared to 7.2 µM during south west monsoon (SWM). The contribution of picoplankton to the total phytoplankton pool (fpico) increased from 19% during SWM to 36% in NEM. Along with cyanobacteria, the diatoms Thalassionema nitzchoides and Thalassiosira sp. and among the dinoflagellates Pronoctiluca sp. were found to proliferate in ON rich waters. We suggest atmospheric deposition, riverine-input, resuspension of bottom sediment and ground water intrusion as the possible sources of ON in the study region.

A checklist of tintinnids (loricate ciliates) from the coastal zone of India.

We report here a checklist of tintinnids (loricate ciliates) for the coastal zone of India. Based on available literature (1978 to 2017), a total of 25 stations were studied from 5 distinct areas of Indian coastal waters. A total of 151 species of 33 genera belonging to 14 families of tintinnids were documented. Diversity patterns of tintinnids were recorded higher along the west coast (30 genera and 106 species) than the east coast of India (20 genera and 93 species). Among tintinnids families, the Codonellidae (2 genera and 52 species, 20%) and the Tintinnidae (8 genera and 21 species, 19%) are the major contributors to the total tintinnids diversity in the Indian coastal zone.

Seasonal variations and relationships between environmental parameters and heavy metal concentrations in tissues of Crassostrea species and in its ambience from the tropical estuaries.

This study aimed to evaluate the relationship between physicochemical parameters and heavy metal (Cu, Ni, Pb, and Cd) concentrations from sediment, seawater, and its accumulation in tissues of oyster species (Crassostrea madrasensis and C. gryphoides) from the three sites (Chicalim Bay (CB), Nerul Creek (NC), Chapora Bay (ChB)) along the Goa coast (India). Results showed enrichment of Cu and Ni in sediment exceeding the effect range low (ERL) level. The higher concentrations of Cu and Ni in sediments and in suspended particulate matter (SPM) from all the study sites are indicative of severe contamination of estuarine and associated habitats. Moreover, particulate Ni (at all the sites), Cu (at NC and ChB), Pb (at NC), and Cd (at CB and NC) concentrations were recorded more than its total loadings in surface sediment. Concentration of Cu and Cd in oyster tissue was several folds higher than its concentration in ambience. Further, this study showed that the levels of metal in oysters and their ambient environment were higher during the monsoon season. Hence, the consumption of oysters needs to be considered carefully with respect to the health hazards posed by the elevated levels of metal contaminants in certain seasons. The present study concludes that metals associated with the particulate matter in water column are the main source of metal accumulation in oyster. It is also suggested that concentration of metal pollutants in coastal and estuarine water bodies should be monitored regularly to ensure the acceptable limits of metal concentrations.

Methane stimulates massive nitrogen loss from freshwater reservoirs in India.

The fate of the enormous amount of reactive nitrogen released to the environment by human activities in India is unknown. Here we show occurrence of seasonal stratification and generally low concentrations of dissolved inorganic combined nitrogen, and high molecular nitrogen (N2) to argon ratio, thus suggesting seasonal loss to N2 in anoxic hypolimnia of several dam-reservoirs. However, 15N-experiments yielded low rates of denitrification, anaerobic ammonium oxidation and dissimilatory nitrate reduction to ammonium-except in the presence of methane (CH4) that caused ~12-fold increase in denitrification. While nitrite-dependent anaerobic methanotrophs belonging to the NC10 phylum were present, previously considered aerobic methanotrophs were far more abundant (up to 13.9%) in anoxic hypolimnion. Methane accumulation in anoxic freshwater systems seems to facilitate rapid loss of reactive nitrogen, with generally low production of nitrous oxide (N2O), through widespread coupling between methanotrophy and denitrification, potentially mitigating eutrophication and emissions of CH4 and N2O to the atmosphere.

Antioxidant responses in gills and digestive gland of oyster Crassostrea madrasensis (Preston) under lead exposure.

Crassostrea are ecologically and economically important bivalves and provide a good livelihood for coastal regions of many countries, including India. This study aims at evaluating the response of the antioxidant defense system in oyster Crassostrea madrasensis against lead (Pb) exposure under laboratory conditions. Antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), and glutathione-s-transferase (GST) and oxidative damage parameter lipid peroxidation (LPO) were measured in the gills and digestive glands of oysters exposed to 1-50µg/l of Pb (NO3)2 over a period of 8 days. LPO index increased progressively with increase in Pb concentration (1, 10, 25 and 50µg/l) in both tissues, gills and digestive gland. Although CAT and SOD activities induced together in the initial phase (upto 6th day), their activities decreased at a later stage of the experiment. However, GST activity increased on 8th day in both the tissues at concentration 10, 25 and 50µg/l indicates the compensatory defense mechanism against oxidative stress. The induced antioxidant responses recorded at 25 and 50µg/l of Pb concentrations suggest the presence of Pb-induced oxidative stress at these concentrations. The results of this work also indicate that LPO, SOD, and GST could be used as biomarkers to assess the impact of Pb on the C. madrasensis. This study concludes that any high level of dissolved Pb concentration (>10µg/l) in surrounding seawater could be harmful to the physiology of the C. madrasensis.

Concentrations of Heavy Metals in Commercially Important Oysters from Goa, Central-West Coast of India.

The major beds of oyster along the central-west coast of India are exposed to different anthropogenic activities and are severely exploited for human consumption. In this viewpoint, tissues of oyster Crassostrea madrasensis, C. gryphoides and Saccostrea cucullata were analyzed for Cu, Ni, Cd and Pb concentrations (dry weight) from Chicalim Bay, Nerul Creek and Chapora Bay in pre-monsoon, monsoon and post-monsoon seasons. A higher concentration of Cu (134.4-2167.9 mg kg-1) and Cd (7.1-88.5 mg kg-1) was found, which is greater than the recommended limits in all the three species (and sites). Moreover, significant (p < 0.05) variations were observed for all the metals concentrations among the species, seasons and sites. The high concentrations of Cd and Cu in tissues of edible oyster pose a threat to human health. Therefore, continuous monitoring, people awareness and a stringent government policy should be implemented to mitigate the metal pollution along the studied sites.

Biogeochemistry and ecology of Pyrosoma spinosum from the Central Arabian Sea.

BACKGROUND: A swarm of pelagic tunicate (Pyrosoma spinosum) was found in the surface open waters of the Arabian Sea during late southwest monsoon (September 2007). The swarm site was characterized by moderate southwesterly wind (approximately 7 m s-1), relatively low sea-surface temperature (approximately 26°C), shallow mixed layer (approximately 50 m), and substantial macro-nutrient concentrations (surface values: 2.5 µM nitrate, 0.3 µM phosphate, 0.9 µM silicate, and 5.0 µM ammonium). Despite adequate macronutrient availability, the swarm site was characterized by low diversity of phytoplankton (>5 µm) and mesozooplankton in the upper 200 m. Low chlorophyll a concentration (27.3 mg/m2 in the upper 120 m) at the swarm site was dominated (90% to 95% in the upper 40 m) by the Synechococcus (20 × 106 /ml). RESULTS: Silicate deficiency in surface waters upwelled or entrained from the thermocline may be a key factor for the dominance of smaller autotrophs (flagellates and cyanobacteria) that seems to offer favorable conditions for episodic occurrence of swarms of these filter feeders. Low carbon content (37% of total dry weight) and the lower molar(carbon-to-nitrogen) ratio (5) in P.spinosum suggestgrowth of these organisms is carbon-limited. CONCLUSIONS: Wedescribe various physicochemical and biological conditions at the P.spinosum swarmlocation and at two other nearby sites not affected by it. The biological factors predominantly high densities of Synechococcus and flagellates were best suited conditions for the proliferation of pyrosome biomass in the central Arabian Sea.