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Microplankton size structure induced by a warm-core eddy in the western Bay of Bengal : Role of Trichodesmium abundance

Chinnadurai Karnan, Retnamma Jyothibabu, Nagarathinam Arunpandi, Subramanian Pandiyarajan Rethinam, Singaram Parthasarathi, Shoba Santhikrishnan

Oceanologia

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2021

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No. 63 (3)

283--300

EN

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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Implications of an extensive salt water barrage on the distribution of black clam in a tropical estuarine system, Southwest coast of India

Nagarathinam Arunpandi, Retnamma Jyothibabu, Loganathan Jagadeesan, Singaram Parthasarathi, Madam Savitha Mohanan Kannampally, Jose Albin Konnakkamannil, Subramanian Pandiyarajan Rethinam

Oceanologia

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2021

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No. 63 (3)

343--355

EN

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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Impact of human-altered hydrographical setting on the Copepod community structure in an extensive tropical estuary along the southwest coast of India

Nagarathinam Arunpandi, Retnamma Jyothibabu, Loganathan Jagadeesan, Singaram Parthasarathi, Arayillath Anjusha, Jose Albin Konnakkamannil

Oceanologia

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2021

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No. 63 (1)

115--132

EN

This study presents how human-altered hydrographical settings (flow restrictions) impacts the natural distribution and community structure of copepods in the Kochi Backwaters (KBW), the largest monsoonal estuary along the southwest coast of India. This study is primarily based on an extensive seasonal sampling in the KBW and their comparison with a historical data set. Thannermukkom Barrage (TB) was built in the southern section of the KBW in the 1970s to prevent saline water intrusion to the upstream during the non-monsoon periods. Thirteen locations (1-4 in the downstream, 5-9 in the midstream, and 10-13 in the upstream) were sampled in this study over the entire stretch of the KBW during the Pre-Southwest Monsoon (PRM), Southwest Monsoon (SWM), and Post-Southwest Monsoon (PSWM). The overall effect of TB in the KBW is a seaward push of mesohaline conditions during all seasons with varying intensities. In response to the seaward push of mesohaline conditions, copepods Acartiella keralensis, Acartia plumosa, Acartia sp., Pseudodiaptomus annandalei, Pseudodiaptomus serricaudatus, Euterpina acutifrons and Oithona brevicornis showed a corresponding spatial shift for their highest abundance and diversity from midstream during PRM to the downstream during the SWM/PSWM. Multivariate and IndVal analysis demarcated many indicator species of copepods of different hydrographical settings in the KBW. A comparison with the historical data set showed that there is an apparent long-term change in hydrography, copepod composition and community structure in the upstream of the KBW due to TB.