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

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 O-18 = 0:0843 x S - 2.1; where freshwater end member is designated as -2.1%; similar to the composition recorded for the rain water. We have estimated percentage of monthly freshwater Cuxes 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 O-18 of mollusc shell growth layers varied over a range between similar to 4.3% and similar to 2.1%. We interpreted this as seasonal signal assuming the growth rate from the culture experiment. The delta O-18 of estuary water and observed temperature are used to simulate the isotopic composition of seasonal growth bands. The lighter delta O-18 value of similar to 4.3% precipitated during the month of July 2010, which coincides with the time of low productivity ( delta C-13 = -3.5%). While the heavier delta O-18 (similar to 2.1%) is recorded in the growth layer generated during November 2010 deBning the period of post-monsoon growth.