Oceanologia No. 63 (3) / 21


Original research article

Short communication

Original research article

Microplankton size structure induced by a warm-core eddy in the western Bay of Bengal: Role of Trichodesmium abundance
Oceanologia 2021, 63(3), 283-300

Karnan Chinnadurai1,2, Jyothibabu Retnamma1,*, Arunpandi Nagarathinam1, Pandiyarajan Rethinam Subramanian1, Parthasarathi Singaram1, Santhikrishnan Shoba1
1CSIR – National Institute of Oceanography, Regional Centre, Kochi, India
2CSIR – National Institute of Oceanography, Dona Paula, Goa, India;
e-mail: rjyothibabu@nio.org
*corresponding author

keywords: Plankton, Size structure, Mesoscale eddy, Trichodesmium, Bay of Bengal, FlowCAM

Received 9 November 2020, Revised 12 February 2021, Accepted 15 February 2021, Available online 27 February 2021.


Mesoscale warm-core eddies are common in the Bay of Bengal (BoB), and this study in the western BoB during Pre-Southwest Monsoon (April 2015) presents how a prolonged warm-core core eddy could modify the microplankton biomass and size structure. To investigate this, field sampling and laboratory analyses were augmented with satellite data sets of sea surface temperature (SST), winds, mean sea level anomaly (MSLA), geostrophic currents and chlorophyll-a. High SST with positive MSLA (≥ 20 cm) and a clockwise circulation, represented the occurrence of a large warm-core eddy in the western BoB. Time series data evidenced that it was originated in the mid of March and persistent there till early June, which in turn caused a decrease in the surface nutrients and chlorophyll-a. The abundance and biomass of microplankton were negligible in the warm-core eddy region. FlowCAM data showed a significant decrease in the autotrophic microplankton parameters in the warm-core eddy (av. 13 ± 9 ind. L−1 and 0.1 ± 0.04 µgC L−1, respectively) as compared to the surrounding locations (av. 227 ± 143 ind. L−1 and 0.8 ± 0.5 µgC L−1, respectively). Low nutrients level in the warm core eddy region favoured high abundance of needle-shaped phytoplankton cells dominated by Trichodesmium cells. As a result, the size of micro-autotrophs in the warm-core eddy was larger (av. 91,760 ± 12,902 µm3 ind.−1) than its outside (av. 50,115 ± 21,578 µm3 ind.−1). This is a deviation from our belief that the oligotrophy decreases the phytoplankton size. We showed here that the above understanding might not be infallible in warm-core eddies in the northern Indian Ocean due to its inducing effect on the Trichodesmium abundance.
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Distribution and extent of benthic habitats in Puck Bay (Gulf of Gdańsk, southern Baltic Sea)
Oceanologia 2021, 63(3), 301-320

Adam Sokołowski1,* Emilia Jankowska2, Piotr Balazy2, Agnieszka Jędruch1
1University of Gdańsk, Faculty of Oceanography and Geography, Institute of Oceanography, Gdynia, Poland;
e-mail: adam.sokolowski@ug.edu.pl
*corresponding author
2Institute of Oceanology, Polish Academy of Sciences, Powstańców Warszawy 55, Sopot, Poland

keywords: Benthic habitats, Mapping, Spatial characteristics, Puck Bay, Southern Baltic Sea

Received 27 October 2020, Revised 18 February 2021, Accepted 11 March 2021, Available online 28 March 2021.


The majority of the southern Baltic Sea seabed encompasses homogenous soft-bottom sediments of limited productivity and low biological diversity, but shallow productive areas in the coastal zone such as wetlands, vegetated lagoons and sheltered bays show a high variety of benthic habitat types offering favourable biotopic conditions for benthic fauna. Within Polish marine areas, semi-enclosed Puck Bay (the western part of the Gulf of Gdańsk) features an exceptionally diverse environment covering a range of benthic habitats which underscores its unique biological value and aesthetic quality and providing an impetus for conservation and ecosystem-based development. Full-coverages maps on benthic habitats in this area are therefore a necessary foundation for maritime spatial planning and implementation of strategies for sustainable management and protection of the coastal environment. This study presents the first comprehensive description and distribution of benthic habitats in Puck Bay which were categorised using the revised EUNIS 2019 classification system. Typological analyses were carried out based on inventory datasets from 1995 to 2019 including scientific publications, satellite images, open databases, topographic and geological maps, reports, theses, information available on websites and unpublished data shared willingly by individual researchers and administrative institutions. Collating various spatial data sources, that were first georeferenced and then visualized using techniques available in ArcMap 10.4.1 software (Esri), resulted in the mapping of benthic habitats and sites of important and protected plant species, which can contribute to the high confidence in environmental assessments and monitoring activities.
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Kelvin-Helmholtz instabilities in the Colorado River Delta, Gulf of California
Oceanologia 2021, 63(3), 321-328

Noel Carbajal1,*, José Tuxpan Vargas1, Juan Heberto Gaviño Rodríguez2, Yovani Montaño Ley3, David Alberto Salas de León3
1San Luis Potosí Institute of Scientific Research and Technology, Mexico;
e-mail: noelc@ipicyt.edu.mx
2Centro Universitario de Investigaciones Bibliotecológicas (CUIB), University of Colima, Mexico
3Institute of the Oceano Sciences and Limnology, The National Autonomous University of Mexico, Mexico
*corresponding author

keywords: Kelvin-Helmholtz instabilities, Colorado River Delta, Tides, Undulating topography, Suspended sediment

Received 17 December 2020, Revised 15 March 2021, Accepted 19 March 2021, Available online 1 April 2021.


In the Colorado River Delta, the interaction of tidal currents and sea-bottom sediment formed, in geological times, large-scale seabed patterns known as sandbanks. These patterns are oriented along the delta, almost parallel to the dominant tidal flow, with the bathymetry having an undulating character across the delta. Calculations and analysis showed that the interaction of tidal currents with the bathymetry causes velocity shears, faster flowing over the ridges than in the troughs. Kelvin-Helmholtz instabilities emerge from the velocity shear, and a large amount of suspended sediment makes the instabilities visible in satellite images. The physical and dynamic conditions allowed us to find an explanation for the existence of these Kelvin-Helmholtz instabilities. Since sandbanks have been observed in different seas such as the North Sea, The Gulf of Korea, the Gulf of Khambhat in India, the Jiangsu coast in China, the Persian Gulf, and Moreton Bay in Australia, the results suggest the existence of instabilities in these areas. Satellite images, intense tidal currents, undulating topography, and suspended sediment made it possible to explain the generation and identification of Kelvin-Helmholtz instabilities.
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Seasonal pattern of the chlorophyll-a in a coastal lagoon from the southern Baja California (Mexico), described with in situ observations and MODIS-Aqua imagery
Oceanologia 2021, 63(3), 329-342

María del Carmen Jiménez-Quiroz1,*, Raúl Martell-Dubois2,*, Rafael Cervantes-Duarte3, Sergio Cerdeira-Estrada2,*
1General Directorate for Fisheries Research in the Pacific, National Institute of Fisheries and Aquaculture, Del Carmen, Coyoacán, Mexico City, Mexico;
e-mail: carmen.jquiroz@inapesca.gob.mx
2National Commission for the Knowledge and Use of Biodiversity (CONABIO), Mexico City, Mexico;
e-mail: rmartell@conabio.gob.mx, scerdeira@conabio.gob.mx
3National Polytechnic Institute, Interdisciplinary Center of Marine Sciences, La Paz, Mexico
*corresponding author

keywords: Remote sensing, Coastal lagoons, Transitional zone

Received 7 December 2020, Revised 11 March 2021, Accepted 20 March 2021, Available online 5 April 2021.


This study aims to estimate, with a climatology perspective, the average seasonal pattern of phytoplankton biomass (SP-PBavg) and its distribution in Bahia Magdalena (Mexico) as a baseline to evaluate PB changes in future studies. This lagoon is in a semi-arid region, lacks river discharges, and channels with vegetation are limited at the north and south zones. SP-PBavg was estimated with chlorophyll-a (chl-a) data obtained in 21 sites on daily MODIS-Aqua imagery (2002–2013; n = 2,418) from a ready-to-use public database. The first step was to establish criteria to use imagery and validate with in situ observations taken in 14 sites (2002–2011; n = 312). MODIS-Aqua overestimated chl-a (mean ± confidence interval95: 5.09 ± 0.97 mg m−3; n = 225); with differences among sites. There were no differences near the inlet lagoon (p <0.05), where the water characteristics are Case-1 while values were significantly higher in the eastern shore and two or three times higher in the mouth of north and south channels, whose water characteristics are similar to Case-2. Multivariate statistical methods allow defining zones into the lagoon and describe their SP-PBavg with both in situ and MODIS Aqua data, but the former's sample size was small, and the patterns were only delineated. In the inlet surroundings, chl-a peaks from March/April to June/July. On the eastern shore, where MODIS Aqua and in situ data were correlated, despite concentration differences, chl-a is higher from March/April to October, with peaks in June and September. In the mouth of internal channels, chl-a was higher than other sites and during a longer period; however, the very high MODIS-Aqua values suggest that the satellite also detects organic matter supplied by phytoplankton and other vegetables, which explain the high lagoon's productivity. These results validate the use of MODIS Aqua imagery to describe the chl-a seasonal patterns in the sea's vicinity.
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Implications of an extensive salt water barrage on the distribution of black clam in a tropical estuarine system, Southwest coast of India
Oceanologia 2021, 63(3), 343-355

Arunpandi Nagarathinam1, Jyothibabu Retnamma1,*, Jagadeesan Loganathan1,2, Parthasarathi Singaram1, Savitha Mohanan Kannampally Madam1, Albin Konnakkamannil Jose1, Pandiyarajan Rethinam Subramanian1
1CSIR – National Institute of Oceanography, Regional Centre, Kochi, India;
e-mail: rjyothibabu@nio.org
2CSIR – National Institute of Oceanography, Regional Centre, Visakhapatnam, India;
*corresponding author

keywords: Black clam, Larvae, spawning, Saltwater barrage, KBW

Received 6 November 2020, Revised 26 December 2020, Accepted 30 December 2020, Available online 14 January 2021.


Based on a monthly field sampling over a year in the Kochi backwaters (KBW), this study presents the larval ecology of black clam and discusses how an extensive saltwater barrage [Thannermukkom barrage (TB)] impacted the natural black clam resource distribution. Spatial variations in salinity were found minimal during the Southwest Monsoon (June–September) due to the predominance of the freshwater associated with heavy monsoonal rainfall. Conversely, significant spatial changes in salinity were evident during the Pre-Southwest Monsoon (March–May) and Post-Southwest Monsoon (October–February). Monthly sampling exercises revealed that the black clam stock in the KBW breeds throughout the year, as their larvae were found (8 indiv. m–3–494 indiv. m–3) in all the locations. This observation is the modification of the traditional belief that black clam in the KBW breeds only twice a year. Mesohaline condition (salinity 5–18) is the most conducive for peak spawning and larval production. There were two peaks of larval production in the KBW over a year, mainly associated with the prevalence of the optimum salinity conditions on different spatial scales. The closing of the TB after the Southwest Monsoon (September) causes shrinkage of the area of the oligohaline and mesohaline conditions, the most conducive environment for the peak spawning and larval production of black clam in the KBW. This study presents a clear case of how human alterations of the natural environment impact valuable biological resources, which may apply to many similar aquatic ecosystems across the globe.
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Winter upwelling in the Gulf of Finland, Baltic Sea
Oceanologia 2021, 63(3), 356-369

Ülo Suursaar
University of Tartu, Estonian Marine Institute, Tallinn, Estonia;
e-mail: ulo.suursaar@ut.ee

keywords: Aerosol, Lidars measurements, Atmospheric boundary layer

Received 27 January 2021, Revised 6 April 2021, Accepted 12 April 2021, Available online 27 April 2021.


Traditionally, upwelling-related studies in the Baltic Sea have been limited to the period from May to September. Based on wintertime in situ measurements at two nearshore locations in the Gulf of Finland, clear evidence of winter “warm” upwelling events was detected and analysed. The process was very common. At a 10 m deep location, upwelling caused water temperature (T) to switch from 0–1 to 4–5°C and salinity (S) to switch from 4.5 to 6 PSU; at 20 m depth it caused a switch in T between 1 and 2–4°C and in S between 5.5 and 6.8 PSU. Differently from summer upwelling, T and S variations were positively correlated to each other. Salinity variations remained roughly the same throughout the winter, whereas T differences were higher in winter onset, then decreased to ca. 1°C, and increased again after the process reversed to summer-type upwelling in April–May. Based on analysis of SatBaltyk (January to March) sea surface temperature and salinity product imagery, winter upwelling occurrence along the North Estonian coast was 21–28% over 2010–2021, and slightly less along the Finnish coast. Regarding S variations, winter upwelling occurred with roughly similar frequencies and impacts in the northern and southern parts of the gulf. However, the impacts on T and sea ice conditions were highly asymmetrical. Upwelling kept the Estonian coast ice-free longer and water temperatures slightly higher than at the Finnish coast. Winter upwelling as a phenomenon has long been ignored and therefore probably underestimated.
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Benthic diffusive fluxes of organic and inorganic carbon, ammonium and phosphates from deep water sediments of the Baltic Sea
Oceanologia 2021, 63(3), 370-384

Monika Lengier1, Beata Szymczycha1, Aleksandra Brodecka-Goluch2, Żaneta Kłostowska1, Karol Kuliński1,*
1Institute of Oceanology, Polish Academy of Sciences, Sopot, Poland;
e-mail: kroll@iopan.pl
2Institute of Oceanography, University of Gdańsk, Gdynia, Poland
* corresponding author

keywords: Biogeochemistry, Benthic diffusive fluxes, Organic matter remineralization, Nutrients, Oxygen availability

Received 27 January 2021, Revised 6 April 2021, Accepted 12 April 2021, Available online 27 April 2021.


In this study, Baltic Sea sediments, as a source of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), ammonium (NH4+), and phosphates (PO43−), were investigated based on samples obtained in 2017 and 2018, shortly after a sequence of inflows from the North Sea that occurred between 2014 and 2017. Two different data sets (I and II) were used to assess benthic diffusive fluxes and thus elucidate both the temporal conditions at the time of sampling (data set I) and the diffusion potential of the sediments (data set II). The estimated fluxes were characterized by a high spatial variability within the whole Baltic Sea and ranged between −0.01 and 3.33 mmol m−2 d−1 for DIC, −0.02 and 0.44 mmol m−2 d−1 for DOC, −40.5 and 1370.1 µmol m−2 d−1 for NH4+, and −5.9 and 60.9 µmol m−2 d−1 for PO43−. The estimated benthic diffusive fluxes indicated a high potential for DIC, DOC, NH4+, and PO43− release from Baltic Sea sediments. The high O2 concentrations in the water column of the Gulf of Bothnia together with major Baltic inflows (MBIs) bringing oxygenated seawater to the Baltic Proper and to some extent the Eastern Gotland Basin regulate the amounts of chemicals released from the sediment. Our study showed that a sequence of inflows has greater impact on the diminution of diffusive fluxes than does a single MBI and that the sediments of the Baltic Proper, even under the influence of inflows, are an important source of C, N, and P (159 kt yr−1 for DIC+DOC, 6.3 kt yr−1 for N-NH4+ and 3.7 kt yr−1 for P-PO43−) that should be considered in regional budget estimations.
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Short communication

Note on estimating bed shear stress caused by breaking random waves
Oceanologia 2021, 63(3), 385-390

Dag Myrhaug1,*, Muk Chen Ong2
1Department of Marine Technology, Norwegian University of Science and Technology (NTNU), Trondheim, Norway;
e-mail: dag.myrhaug@ntnu.no
2Department of Mechanical and Structural Engineering and Materials Science, University of Stavanger, Stavanger, Norway
*corresponding author

keywords: Bed shear stress, Breaking random waves, Surf similarity parameter, Wave height, Individual waves, Joint distributions, Surf and swash zones

Received 9 October 2020, Revised 17 March 2021, Accepted 26 March 2021, Available online 11 April 2021.


This note presents a method of how the bed shear stress caused by breaking random waves on slopes can be estimated. This is obtained by adopting the Sumer et al. (2013) bed shear stress formula due to spilling and plunging breaking waves on hydraulically smooth slopes combined with the Myrhaug and Fouques (2012) joint distribution of surf similarity parameter and wave height for individual random waves in deep water. The conditional mean value of the maxima of mean bed shear stress during wave runup given wave height in deep water is provided including an example for spilling and plunging breaking random waves corresponding to typical field conditions. Another example compares the present results with one case from Thornton and Guza (1983) estimating the wave energy dissipation caused by bed shear stress beneath breaking random waves.
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First report of Protoperidinium steinii (Dinophyceae) bloom from the coastal marine ecosystem – an observation from tropical Indian waters
Oceanologia 2021, 63(3), 391-402

Rengasamy Subramaniyan Sathishkumar1,*, Gouri Sahu2, Ajit Kumar Mohanty2,*, Kantha Deivi Arunachalam1,*, Rajagopal Venkatesan2
1Center for Environmental Nuclear Research, SRM Institute of Science and Technology, Kattankulathur, Chennai, India;
e-mail: rssathishphd@gmail.com, kanthad.arunachalam@gmail.com
2Radiological and Environmental Safety Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, India;
e-mail: ajit@igcar.gov.in
*corresponding author

keywords: Protoperidinium steinii, Red tide, Non-toxic HAB, Eutrophication, Backwaters, Bay of Bengal

Received 22 August 2020, Revised 29 March 2021, Accepted 12 April 2021, Available online 27 April 2021.


A dense bloom of Protoperidinium steinii was observed in the backwaters adjoining the western Bay of Bengal, Kalpakkam coast, which might be the first report for the world oceans. The brownish-red bloom appeared on 2 October 2019, and it was monitored on alternate days up to 14 October. Surface water temperature was about 27.5°C and salinity was <17 PSU during the bloom. Dissolved inorganic nutrients like nitrate, ammonia, silicate, and phosphate were extremely high compared to that of the coastal waters. The chlorophyll-a maxima (20.95 mg m−3) coincided with the highest Protoperidinium density (113.9 × 104 cells l−1). The contribution of P. steinii ranged from 17−93% of the total phytoplankton population. Since P. steinii is a heterotroph and voracious grazer, low autotroph density was observed during the bloom. No mass mortality of fish or other organisms was observed, thereby indicating the non-toxic nature of the bloom.
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