ESTIMATING SUBMARINE GROUNDWATER DISCHARGE IN THE CILICIAN BASIN BY RADIOACTIVE ISOTOPE TRACERS AND HYDRODYNAMIC MODELING

Date

2023-7-26

Author

Kuyumcu, Burak

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The Northeastern Levantine Sea has an oligotrophic nature, however, rising agricultural and industrial activities along its coasts pose a major eutrophication risk. There is a urgent necessity to identify and assess coastal water and nutrient inputs in order to have effective ecosystem management. In this study, submarine groundwater discharge rates and associated nutrient fluxes into the Cilician Basin were estimated for the first time, utilizing a 228-Radium mass-balance approach and numerical modeling. Two basin-wide cruises were conducted (April 2022 & September 2022) in addition to two catchment surveys (March 2022 & September 2022) to obtain Radium and nutrient samples for seawater (#207 samples both), rivers (#28 samples both), and groundwater (#33 samples both) in 2022. A box model was established, defining each source and sink of 228-Radium activities measured by gamma spectrometry. Nutrient loads were determined by integrating estimated fluxes with the median nutrient concentrations in groundwater samples. Moreover, a Lagrangian particle tracking model was utilized to estimate the residence time of the basin, which then used in the offshore water exchange calculations in the mass balance equation. The outcome of the model indicated there was substantial seasonal and interannual variation in the residence time, particularly for the surface layer, 0-150 m depth. The highest variation was found between the wet and dry season of 2019 with 24 and 74 days, respectively, for the surface layer. The gamma spectrometry results yielded approximately 24.97 x 10^13 dpm 228- Radium inventory in the defined box for the wet season, and the inventory of İskenderun bay was found as 5.57 x 10^12 dpm. There was significant 228-Radium enrichment in the basin, especially in the İskenderun bay and near the western boundary. The mass balance revealed that the submarine groundwater discharge may have contributed at least 121 km³ per year to the water budget the Cilician Basin where the total annual riverine discharge was approximately 13 km³. A large range of groundwater endmember 228-Radium activity was determined using both the literature data and groundwater activities measured within this study. The highest endmember activity was used for the most conservative submarine groundwater discharge rate and the lowest activity was used to calculate maximum discharge estimates. Correspondingly, submarine groundwater discharge associated nutrient loads were 0.27, 37.73, and 50.92 kT/year of PO4-P, DIN-N, and SiO4-Si, respectively, in the most conservative scenario. Comparison with riverine nutrient loads calculated by one wet season and one dry season sample for each river indicated the submarine groundwater discharge corresponded to at least 66%, 106%, and 96% of riverine P-PO4, DIN-N, and SiO4-Si inputs, respectively, using a conservative approach. Furthermore, the elevated dissolved inorganic nitrogen to phosphate ratio of 139 found for the groundwater suggested that it might be a contributing factor to the severe phosphorus limitation in the area. Therefore, it may have an effect on primary production by changing the structure of phytoplankton communities and the interaction between trophic levels. The study emphasized that the submarine groundwater discharge might have a fundamental role in the water and nutrient budgets of the Cilician Basin despite the caveats regarding temporal variations and the wide range of groundwater end-member activities, as no saline groundwater sample was sampled in the study. Further investigation is required to have a better understanding on the spatiotemporal variations in submarine groundwater discharge and its potential effects on primary production as significant fluctuations might occur due to the variations in residence time and nutrient concentrations.

Subject Keywords

Cilician Basin, Land-sea interactions, Radium, Residence time, Submarine groundwater discharge

URI

https://hdl.handle.net/11511/104884

Collections

Graduate School of Marine Sciences, Thesis