• LEENA RAMDAS

      Articles written in Journal of Earth System Science

    • Seasonal freshwater flux estimation using mollusc from the tropical Mandovi Zuari estuary, Goa, India

      PROSENJIT GHOSH LEENA RAMDAS YOGARAJ BANERJEE THAMIZHARASAN S SANCHITA BANERJEE

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      Seawater incursion and freshwater discharges into the tropical Mandovi Zuari (MZ) estuary is investigated here using stable oxygen isotope data on monthly water samples from locations spatially separated within the estuary. Surface water samples were analysed for $\delta^{18}$O and salinity relationship,$\delta^{18}$O=0.0843$\times$S−2.1, where freshwater end member is designated as −2.1 per thousand; similar to the composition recorded for the rain water. We have estimated percentage of monthly freshwater fluxes into the estuary using mass balance equation adopting an isotopic value of seawater end member. Our estimates showed freshwater contribution to the estuary water during monsoon time was 63%, while in the post-monsoon season the fraction drops to 34.7%. The contribution of the freshwater registered a minimum value of 6.8% during the pre-monsoon season. Further, we analysed the seasonal growth band secreted by the mollusc from the same estuary to understand the potential of mollusc as a recorder of seasonal water composition. The $\delta^{18}$O of mollusc shell growth layers varied over a range between −4.3 per thousand and −2.1 per thousand. We interpreted this as seasonal signal assuming the growth rate from the culture experiment. The $\delta^{18}$O of estuary water and observed temperature are used to simulate the isotopic composition of seasonal growth bands. The lighter $\delta^{18}$O value of −4.3 per thousand precipitated during the month of July 2010, which coincides with the time of low productivity ($\delta^{13}$C = −3.5 per thousand). While the heavier $\delta^{18}$O (−2.1 per thousand) is recorded in the growth layer generated during November 2010 defining the period of post-monsoon growth.

      $\bf{Highlights}$

      $\bullet$ Here we presented our observation on the spatiotemporal variability of salinity and $\delta^{18}$O$_{water}$ across a tropical estuary, Mandovi Zuary located at the western coast of India and fed by rivers originating from the peninsular region which receives rainfall during period of Indian summer monsoon.

      $\bullet$ We designed a two-component mixing model for the estimation of freshwater fluxes at monthly time intervals and demonstrated its efficacy with the tide gauge data.

      $\bullet$ The study highlighted the scope of isotope mass balance approach in estimating freshwater fluxes at seasonal time domain.

      $\bullet$ Further, we demonstrated that the isotopic signature preserved in the growth bands of the bivalve shell can be useful proxy for the reconstruction of freshwater fluxes in an estuary and thus add a new tool for reconstruction of seasonal runoff.

      $\bullet$ The isotope approach used here can also be extended to other estuaries in the peninsular India and also in other tropical continental settings where hydrological process is driven by seasonal reversal of wind direction.

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