Oceanologia No. 64 (2 / 22)


Original research article

Short communication

Original research article

Validation of measurements of pine pollen grain concentrations in Baltic Sea waters
Oceanologia 2022, 64(2), 233-243

Magdalena M. Pawlik*, Dariusz Ficek
Institute of Biology and Earth Sciences, Pomeranian University in Słupsk, Słupsk, Poland;
e-mail: magdalena.pawlik@apsl.edu.pl
*corresponding author

keywords: Pine pollen grains, Particle size distribution, Suspended particulate, matter, The Baltic Sea, Remote sensing

Received 2 July 2021, Revised 16 November 2021, Accepted 17 November 2021, Available online 29 November 2021, Version of Record 3 May 2022.


Each spring, pine pollen coats considerable expanses of Baltic Sea surface waters. Measurements have shown that there are areas where its concentrations in this surface layer are so high that they are the dominant constituent of the suspended particulate matter (SPM) (Pawlik and Ficek, 2016). It then determines to a large extent the optical properties of the water surface, inter alia by modifying the sea colour. To date, however, the concentration of this constituent in the marine environment has rarely been studied, and its presence is not accounted for in the satellite algorithms used to define the composition and properties of sea water. This may well be the source of substantial errors in the remote sensing of the optical properties of the water and the measurement of concentrations of the optically important constituents it contains (chlorophyll a, TSM, CDOM). Measuring the concentration of pollen suspensions in Baltic Sea water, which often contains prodigious amounts of other SPM, is a daunting experimental challenge. Firstly, we characterized the pollen from pine trees growing near the southern shores of the Baltic Sea (northern Poland) using a microscope and two instruments routinely used in oceanography for measuring SPM size distributions: the LISST-100X and the Coulter counter. The measurements and analyses showed that a correct interpretation of the LISST-100X and Coulter measurements, is sufficient to count the number of pollen grains in distilled water alone. Furthermore, our laboratory analysis of the particle size distribution spectra enabled the fraction due only to pine pollen grains to be separated from the overall SPM. We then tested our method of analysing the SPM composition, which showed that the LISST-100x instrument is both a useful and an effective means for the in situ detection of the pine pollen that one sees in spring in Baltic waters.
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Size-fractionated primary production in the south-eastern Black Sea
Oceanologia 2022, 64(2), 244-266

Ertugrul Agirbas*, Mustafa Bakirci
Dept. of Marine Biology, Faculty of Fisheries, Recep Tayyip Erdogan University, Rize, Turkey;
e-mail: ertugrul.agirbas@erdogan.edu.tr
*corresponding author

keywords: C-14, Phytoplankton, Primary production, Size-fractionated, South-Eastern Black Sea

Received 18 August 2021, Revised 11 October 2021, Accepted 17 November 2021, Available online 10 December 2021, Version of Record 3 May 2022.


Size-fractionated primary production (PP) and chlorophyll-a (Chl-a) with phytoplankton abundance and nutrients were investigated in the south-eastern Black Sea from November 2014 to August 2015. A 14C radio-tracing technique was used to estimate phytoplankton primary production. C-14 experiments revealed that total PP ranged from 295 mgC m−2 d−1 to 5931 mgC m−2 d−1 along the study area. Size-fractionated PP varied from 84 to 1848 mgC m−2 d−1, from 96 to 3156 mgC m−2 d−1 and from 56 to 3363 mgC m−2 d−1 for pico-, nano- and microphytoplankton, respectively. Overall, winter (4163 mgC m−2 d−1) and spring (5931 mgC m−2 d−1) were the most productive seasons, which coincided with high phytoplankton abundance. Contributions of microphytoplankton and nanophytoplankton were prominent in spring with maximum PP values. Winter was the second productive season with high contributions of nano- and microphytoplankton PP. Summer and autumn were less productive seasons, which were characterised by a high contribution of pico- and nanophytoplankton PP. Dinoflagellates were represented with the highest species richness (68 species, 53.54%) and diatoms were the second group along the area. Diatoms and other phytoplankton species (mainly Emiliania huxleyi) were the most abundant groups in terms of quantitative contribution. The results show that microphytoplankton along the study area are responsible for the majority of PP. However, the measured high Chl-a against low size-fractionated PP clearly indicates that smaller groups (i.e., pico- and nanophytoplankton) were dominant during these periods. Hence, the quantification of size-fractionated PP rates together with ecological indicators will allow for a more comprehensive assessment of the Black Sea ecosystem.
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Multivariate approach to evaluate the factors controlling the phytoplankton abundance and diversity along the coastal waters of Diu, northeastern Arabian Sea
Oceanologia 2022, 64(2), 267-275

P. Sathish Kumar1,2,*, S. Venkatnarayanan1, Vikas Pandey1, Krupa Ratnam1, Dilip Kumar Jha1, S. Rajaguru1, G. Dharani1,*, R. Arthur James2,*, M.A. Atmanand1
1National Institute of Ocean Technology (NIOT), Ministry of Earth Sciences (Govt. of India), Chennai, Tamil Nadu, India;
e-mail: marinesathis@gmail.com, dhara@niot.res.in
2Department of Marine Science, Bharathidasan University, Tiruchirappalli, Tamil Nadu, India;
e-mail: james.msbdu@gmail.com
*corresponding author

keywords: TSS, Phytoplankton, Dinoflagellate, Chlorophyll a, Nutrient, Monsoon

Received 30 June 2021, Revised 15 November 2021, Accepted 24 November 2021, Available online 9 December 2021, Version of Record 3 May 2022.


The present study investigated the phytoplankton assemblage and diversity with physicochemical parameters of Diu coastal waters in different seasons during 2018–19. During the study period, 61 phytoplankton species comprising diatoms (50 sp.), dinoflagellates (8 sp.), and cyanophyceae (3 sp.) were recorded. Diatom was found to be a major community and contributed 79 to 99% of total phytoplankton abundance. Reduction in dinoflagellate and dominance of pennate-diatoms were observed during the monsoon. Chlorophyll-a concentration also showed a similar trend and decreased during the monsoon. However, the phytoplankton abundance was low particularly during the monsoon which might be due to the elevated total suspended solids (TSS) load. Canonical correspondence analysis revealed that diatoms were able to survive in high TSS with the support of high nutrients; while dinoflagellates were limited due to those conditions. Overall, the reduction in phytoplankton abundance, diversity, and biomass was recorded due to the elevated TSS input along the coastal waters of Diu.
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Ecological assessment of heavy metals accumulation in sediments and leaves of Avicennia marina along the Diu coast of the northeast Arabian Sea
Oceanologia 2022, 64(2), 276-286

Varsha Patale, Jigna G. Tank*
Department of Biosciences, Saurashtra University, Gujarat, India;
e-mail: jignagtank@gmail.com

keywords: Ecological assessment, Heavy metals, Sediments, Avicennia marina, Diu coast

Received 6 May 2021, Revised 22 November 2021, Accepted 3 December 2021, Available online 17 December 2021, Version of Record 3 May 2022.


The Coastal region of Diu is the natural habitat dominated by Avicennia marina mangrove species at the southeast coast of Saurashtra in Gujarat state of India. However, Diu being a famous industrial and tourism place survival of these mangrove species is threatened due to anthropogenic activities. In present studies, sediment and leaf samples of A. marina were collected from the Diu coast to evaluate the ecological threat of heavy metals accumulation in the marine habitat. There was remarkable presence of heavy metals such as copper, nickel, cadmium, chromium and lead in sediments and leaf samples of A. marina. The values of Biological concentration factors (BCFs) of heavy metals in leaf samples were high for cadmium, chromium and lead which suggest chelation of these heavy metals with biomolecules. The geo-accumulation index suggested that Site-4 and Site-5 were heavily contaminated with copper and nickel. The ecological risk index suggested that there is no significant effect of heavy metals on growth of plants in the mangrove ecosystem. Principal component analysis revealed that the samples collected from the natural habitats (Site-4 and Site-5) near the fishing and industrial areas were the main sources of heavy metal contamination. Hence, it was concluded that the concentration of heavy metals in the studied ecosystem had limited impact on growth of plants at Site-1, Site-2 and Site-3. However, growth of plants at Site-4 and Site-5 were threatened due to the toxic effect of copper and nickel present in its sediments.
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Distribution and ecological risk evaluation of bioavailable phosphorus in sediments of El Temsah Lake, Suez Canal
Oceanologia 2022, 64(2), 287-298

Alaa M. Younis1, Naglaa F. Soliman2,3,*, Eman M. Elkady4, Lamiaa I. Mohamedein4
1Department of Aquatic Environment, Faculty of Fish Resources, Suez University, Suez, Egypt
2Department of Environmental Studies, Institute of Graduate Studies & Research – Alexandria University, Egypt
3Basic and Applied Science Institute, Egypt-Japan University of Science and Technology (E-JUST), Alexandria, Egypt;
e-mail: naglaa_farag2007@yahoo.com
4National Institute of Oceanography & Fisheries (NIOF), Suez, Egypt
*corresponding author

keywords: Sediments, Bioavailable phosphorus, Chemical extraction methods, Phosphorus ecological risk, El Temsah Lake, Egypty

Received 23 March 2021, Revised 23 November 2021, Accepted 3 December 2021, Available online 17 December 2021, Version of Record 3 May 2022.


Phosphorus reactivity and bioavailability in lake sediments is determined by diverse fractions of phosphorus (P) and their distribution. To gain deeper insights into P dynamics in Lakes, sediments from El Temsah Lake were investigated for water soluble P (WSP), readily desorbable P (RDP), algal available P (AAP) and Olsen-P using different chemical extraction methods. Total P (TP), organic P (OP), inorganic P (IP) contents, were also investigated. The TP, OP and IP concentrations in the sediments were 598.39 µg/g, 199.76 µg/g and 398.63 µg/g, correspondingly. Concentrations of the bioavailable P in the sediments followed the order AAP (48.42 µg/g)>WSP (14.60 µg/g)>RDP (1.82 µg/g)>Olsen-P (1.50 µg/g). Pearson correlation analysis exposed that there were significant correlations among the bioavailable P fractions concentrations and the TP concentrations (r=0.83; p>0.01, r=0.94; p>0.01, r=0.62; p>0.05); for WSP, AAP, and Olsen-P respectively. Moreover, there were no obvious associations amongst total P and N, Al, Ca, Fe, Mg, Mn, and OM in the sediments. The outcomes of phosphorus ecological risk assessment in sediments by single pollution standard index method revealed that the standard index of TP varied from 0.19 to 1.85. It demonstrated that the ecological pollution risks of phosphorus in El Temsah Lake sediments was comparatively low.
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Diurnal variation of cloud cover over the Baltic Sea
Oceanologia 2022, 64(2), 299-311

Marcin Paszkuta1,*, Tomasz Zapadka2*, Adam Krężel1*
1Institute of Oceanography, University of Gdańsk, Gdynia, Poland;
e-mail: marcin.paszkuta@ug.edu.pl
2Institute of Physics, Pomeranian University in Słupsk, Poland

keywords: Clouds, Baltic Sea, Environmental assessment

Received 3 January 2021, Revised 12 December 2021, Accepted 13 December 2021, Available online 29 December 2021, Version of Record 3 May 2022.


Instantaneous cloud cover over the Baltic Sea, estimated from satellite information, may differ by as much as several dozen percent between the day and night. This difference may result from both weather conditions and different algorithms used for the day and night. The diurnal differences in cloudiness measured by proprietary and operational systems were analysed as part of research on marine environmental assessment and monitoring. An optimised algorithm for 2017 was presented and supplemented with information from radiation modelling. The study showed that, in general, the average values of daily changes in cloud cover over the sea depend on the season, which generally corresponds to the length of the day and contrasts with the amount of cloudiness. The results were compared with available online data that met the night and day detection criteria, the climate model, and the climate index. The averaged analysis of seasonal changes showed that similar values of the satellite estimates are higher than those obtained from the climate model and the lidar estimation. The satellite estimates from SatBaltic showed the lowest uncertainty. The diurnal cycle was confirmed by all analysed systems. These results may indicate common physical mechanisms and a methodological reason for the uncertainty of satellite-based data. The results clearly showed the existing diurnal difference in the amount of cloud cover over the Baltic Sea and indicated that this difference is not always explained by the physical properties of the atmosphere. The probable causes for these uncertainties were identified and diagnosed.
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Regime shift in sea-ice characteristics and impact on the spring bloom in the Baltic Sea
Oceanologia 2022, 64(2), 312-326

Ove Pärn1,*, René Friedland2, Jevgeni Rjazin3, Adolf Stips1
1European Commission, Joint Research Centre, Ispra, Varese, Italy;
e-mail: ove.parn@ext.ec.europa.eu
2Leibniz-Institute for Baltic Sea Research Warnemünde, Rostock, Germany
3Hereditas, Tartu, Estonia
* corresponding author

keywords: Climate change, Ecosystem, Biogeochemical model, Sea surface temperature, Chlorophyll-a concentration

Received 31 May 2021, Revised 13 December 2021, Accepted 14 December 2021, Available online 29 December 2021, Version of Record 3 May 2022.


We evaluated the temporal and spatial trends of the hydrological (temperature and sea ice) and biochemical (chlorophyll-a concentration) characteristics in springtime in the Baltic Sea. Both are strongly affected by climate change, resulting in a decrease in the duration of sea-ice melting in the previous decade. A new regime of sea ice began in 2008 and in all basins of the Baltic Sea, a rapid warming during spring could be detected. Using satellite data, the temporal and spatial variations in spring bloom were analysed during severe and warmer winters. Using a coupled hydrodynamic-biogeochemical model, we tested the response of spring bloom to the changing ice conditions. The results of the modelling indicated that the presence of ice significantly influences the predicted chlorophyll-a concentration values in the Baltic Sea. Therefore, it is necessary that any coupled model system has a realistic ice model to ensure the best simulation results for the lower trophic food web as well.
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Numerical simulation of tidal hydrodynamics in the Arabian Gulf
Oceanologia 2022, 64(2), 327-345

Fawaz Madah1,*, Sameer H. Gharbi1,2
1Department of Marine Physics, Faculty of Marine Sciences, King Abdul-Aziz University, Jeddah, Saudi Arabia;
e-mail: fmaddah@kau.edu.sa
2Department of Physical Oceanography, Faculty of Marine Sciences and Environment, Hodeidah University, Hodeidah, Yemen
*corresponding author

keywords: Arabian Gulf, Delft3D, Strait of Hormuz, Amphidromic point, Co-tidal charts

Received 28 August 2021, Revised 10 January 2022, Accepted 13 January 2022, Available online 31 January 2022, Version of Record 3 May 2022.


A vertically 2-D numerical model based on the Delft3D modelling system is set up, calibrated, and validated to simulate the tidal hydrodynamics in the Arabian Gulf. The model is a barotropic solution, controlled by 13 tidal components at open boundaries. The performance of the numerical model was evaluated using the hourly water level observations and the TOPEX/Poseidon altimetry data. Statistical analysis showed a good agreement between the simulated and observed water levels. RMS error was found to be ranged from 0.07 to 0.23 m, with maximum discrepancies observed at Ras Tanura and Mina Sulman stations. However, the IOA between the simulated and observed water levels was significant (0.95–0.99). On average, the errors for the tidal constituents considered in the analysis are in the order of <0.02 m (4%). The M2, S2, K1 and O1 tidal waves represent the largest among other constituents, where the amplitude of S2 represents almost 30% of the M2, and the O1 tidal wave represents about 50% of the K1 tide. The co-tidal charts of the semidiurnal tides show the existence of two anticlockwise amphidromic systems in the north and south ends (centred around 28.25° and 24.5°N respectively) close to the western side, while the diurnal constituents form only a single amphidromic point in the central part, centred around 26.8°N (North Bahrain). On the other hand, the velocity amplitudes of the U and V components of the numerical model were compared with a previous observational study and found to be agreed well.
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Seasonal carbonate system vis-à-vis pH and Salinity in selected tropical estuaries: Implications on polychaete diversity and composition towards predicting ecological health
Oceanologia 2022, 64(2), 346-362

Palanivel Partha Sarathy1,*, Veeraiyan Bharathidasan1, Perumal Murugesan1, Palanichamy Selvaraj2, Rengasamy Punniyamoorthy1
1Centre of Advanced Study in Marine Biology, Faculty of Marine Sciences, Annamalai University, Tamil Nadu, India;
e-mail: partha7915@gmail.com
2Department of Biotechnology, Sri Kaliswari College (Autonomous), Sivakasi -626130
*corresponding author

keywords: Salinity, pH, Estuaries, Polychaetes, Ecological status

Received 5 December 2020, Revised 9 January 2022, Accepted 13 January 2022, Available online 31 January 2022, Version of Record 3 May 2022.


Salinity and pH play a fundamental role in structuring spatial patterns of physical properties, biota, and biogeochemical processes in the estuarine ecosystem. In this study, the influence of salinity-pH gradient and carbonate system on polychaete diversity in Ennore, Uppanar, Vellar, and Kaduvaiyar estuaries was investigated. Water and sediment samples were collected from September 2017 to August 2018. Univariate and multivariate statistical analyses were employed to define ecological status. Temperature, salinity, pH, and partial pressure of carbon-di-oxide varied between 21 and 30°C; 29 and 39 ppt; 7.4 and 8.3; and 89.216 and 1702.558 µatm, respectively. PCA and CCA results revealed that DO, chlorophyll, carbonate species, and sediment TOC have a higher influence on polychaete community structure. Forty-two species such as Ancistrosyllis parva, Cossura coasta, Eunice pennata, Euclymene annandalei, Lumbrineris albidentata, Capitella capitata, Prionospio cirrifera, P. pinnata, P. cirrobranchiata, and Notomastus sp. were found dominantly in all estuaries. Shannon index values ranged between 1.619 (UE-1) and 3.376 (VE-2). Based on these findings, high levels of carbonate species and low pH have a greater impact on polychaete diversity and richness values. The results of the AMBI Index revealed that stations UE-1, UE-2, UE-3 in Uppanar, EC-1, EC-2 in Ennore indicate “moderately disturbed”, while other stations are under the “slightly disturbed” category. This trend was quite evident in M-AMBI as well.
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Coastal upwelling by wind-driven forcing in the Caspian Sea: A numerical analysis
Oceanologia 2022, 64(2), 363-375

Fatemeh Fallah*, Dariush Mansoury*
Faculty of Natural Resources and Marine Sciences, Tarbiat Modares University, Noor, Iran;
e-mail: fa.fallah1374@gmail.com, mansoury@modares.ac.ir
*corresponding author

keywords: Upwelling, Vertical velocity, Wind field, Bottom topography, Hydrodynamic modelling, Caspian Sea

Received 25 March 2021, Revised 16 January 2022, Accepted 18 January 2022, Available online 31 January 2022, Version of Record 3 May 2022.


In this study, wind-driven coastal upwelling in the Caspian Sea was investigated using a developed three-dimensional hydrodynamic numerical model based on the Princeton Ocean Model (POM). The model was forced with wind fields and atmospheric fluxes from the ECMWF database and it considers freshwater inflows from the Volga, Kura and Ural Rivers. This model was implemented for 10 years (2008–2018). Findings indicated that the upwelling in the Caspian Sea was due to effects of wind and bottom topography, often occurring from May to September. In June and July, in the eastern part of the middle and sometimes southern basins, up to 3°C water temperature difference occurs between coastal and offshore areas. The vertical temperature gradient in the middle basin was larger than that in the southern basin. Upwelling in August in the eastern coasts of the middle basin within 25 km of coast from the depth of 15 m to the surface was shown, which was due to the effects of wind and bottom topography. In the middle basin, the highest vertical velocities caused by upwelling in June, July and August were 12, 13.82, and 10.36 m/day, respectively.
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Investigation of tidal asymmetry in the Shatt Al-Arab river estuary, Northwest of Arabian Gulf
Oceanologia 2022, 64(2), 376-386

Ali Abdulridha Lafta*
Department of Marine Physics, Marine Science Center, University of Basrah, Iraq;
e-mail: ali.lafta@uobasrah.edu.iq
*corresponding author

keywords: Tidal asymmetry, Harmonic analysis, Tidal skewness, Arabian Gulf, Shatt Al-Arab river

Received 20 July 2021, Revised 19 January 2022, Accepted 21 January 2022, Available online 3 February 2022, Version of Record 3 May 2022.


Approximately one-year water level records were utilized for examining the tidal dynamics and tidal asymmetry at the Shatt Al-Arab river estuary. The harmonic and the tidal skewness, two traditional methods in quantifying tidal asymmetry in tidal systems, were used. The water level measurements revealed a presence of a tidal wave attenuation when propagating further towards the inland direction, with notable reductions in the tidal range. The results of the harmonic analysis indicated that the diurnal and semi-diurnal constituents experience considerable damping towards the upstream direction. The largest constituent was M2, followed by K1, O1, and S2. The largest shallow water constituent was MK3, followed by M4, MS4, MN4, and M6. The tidal form number ranged from 0.68 to 0.7 along the estuary; then, mixed, mainly semi-diurnal tidal nature was observed. However, six possible combinations of tidal constituents were used to quantify the tidal asymmetry, involving the interactions between astronomical constituents alone as well as with the higher harmonics. According to the harmonic method, the relative phase difference of M2 and M4 constituents was in the range of 63 to 87.06, suggesting a flood dominance behavior of tidal wave along the estuary. Positive values of the tidal skewness were observed at all stations, with a pronounced increase towards the inland direction. The M2 and M4 interaction was the main contributor to tidal asymmetry, followed by M2-K1-O1, M2-S2-MS4, M2-M4-M6, K1-M2-MK3, and M2-N2-MN4 interactions.
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Does topography of rocky intertidal habitat affect aggregation of cerithiid gastropods and co-occurring macroinvertebrates?
Oceanologia 2022, 64(2), 387-395

Soniya Jethva1, Kiran Liversage2,*, Rahul Kundu1
1Department of Biosciences, Saurashtra University, Gujarat State, India
2Estonian Marine Institute, University of Tartu, Tallinn, Estonia;
e-mail: kiran.liversage@gmail.com
*corresponding author

keywords: Habitat heterogeneity, Cerithiidae, Intertidal snails, Patchiness, Over-dispersed distribution, Intertidal ecology

Received 17 May 2021, Revised 18 January 2022, Accepted 21 January 2022, Available online 3 February 2022, Version of Record 3 May 2022.


Species aggregation has important implications for population survivorship and ecological functioning in many habitats, including rocky intertidal shores, which have been valuable to research for understanding ecological patterns and process. Intertidal gastropods of the family Cerithiidae often form extensive aggregations for which the driving mechanism may be positive thigmotaxis, i.e. movement occurring until an obstructing surface is contacted, then cessation of movement. However, it is unknown if thigmotaxis may occur by cerithiids contacting and aggregating around uneven surfaces of the rock topography, or by contacting other conspecific individuals. We quantified aggregation patterns in invertebrate assemblages and topographic complexity at intertidal rock platforms in NW India with extensive cerithiid populations. The cerithiids Clypeomorus moniliferus and Cerithium caeruleum were the most common species. Distribution analysis confirmed significant over-dispersion indicative of aggregation (densities were often around zero but occasionally reached up to 680 dm−2). Multivariate correlation analyses showed that topographic complexity contributes to overall species assemblage variability, but there was no evidence that topographic complexity correlates with cerithiid abundances or was likely to affect their aggregation. Thus the thigmotaxis producing cerithiid aggregation is probably associated with individuals contacting each other rather than any feature of the rock surface such as crevices or raised areas. Overall, while some components of species assemblages were associated with complex topography, regarding the abundant cerithiids, potential population benefits from aggregation (e.g. reduced desiccation and temperature) may be expected on rocky shores with any level of topographic complexity.
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Short communication

Algal bloom, hypoxia, and mass fish kill events in the backwaters of Puducherry, Southeast coast of India
Oceanologia 2022, 64(2), 396-403

Pravakar Mishra1, Subrat Naik1,*, P. Vipin Babu2, Umakanta Pradhan1, Mehmuna Begum1, Thanamegam Kaviarasan1, Athan Vashi1, Debasmita Bandyopadhyay1, P. Ezhilarasan1, Uma Sankar Panda1, M.V. Ramana Murthy1
1National Centre for Coastal Research (NCCR), Ministry of Earth Sciences, NIOT Campus, Pallikaranai, Chennai, India;
e-mail: subrat@nccr.gov.in, naiksubrat@gmail.com
2Puducherry Pollution Control Committee (PPCC), Puducherry, India
*corresponding author

keywords: Algal bloom, Hypoxia, Fish kill, Water quality, Puducherry, India

Received 13 March 2021, Revised 7 October 2021, Accepted 22 November 2021, Available online 5 December 2021, Version of Record 3 May 2022.


The Chunnambar backwater of Puducherry experienced changes in water quality over a period. The most significant impact was the sudden mass fish kill event coincided with the Pseudo-nitzschia bloom. On 25th September 2019, a mass fish kill event was reported, i.e. about a 0.25 metric ton (MT) floating on the water surface. On 29th September 2019, a much larger (∼1 MT) than the earlier incident had occurred. Sampling was carried out to assess the causes thereof. The results indicate that high organic matter and bacterial loads accumulated in the water and sediment due to the closure of the river mouth for an extended period. High ammonia (61.4 µM) and phosphate (6.2 µM) levels attributed to eutrophic conditions in the water column and hypoxemia due to low dissolved oxygen (1.62 mg/L at St.1 and 2.4 mg/L at St.5) and algal bloom were the proximate cause of these sudden fish kills. We recommend periodic dredging to facilitate proper water exchange between the backwater and the sea.
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