Impacts of hydroclimatic variability on surface water and porewater dissolved organic matter in a semi-arid estuary

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Hydroclimatic disturbances, like flooding, may greatly impact dissolved organic matter (DOM) composition in coastal waters by altering hydraulic gradients, groundwater residence times, and seafloor recirculation. This study provides new evidence regarding seasonal molecular DOM changes, in a submarine groundwater discharge influenced semi-arid estuary, across different hydroclimatic conditions: from flood peak (summer) to flood recession (fall), to typical dry (winter) to semi-wet (spring) conditions. DOM molecular characterization of seasonal surface- and pore- water samples was conducted using PPL solid phase extraction and UPLC-Orbitrap Fusion mass spectrometry, in positive mode. The proportion of CHO compounds present was lowest during flooding (34%), increased during fall flood recession (39%), and was highest under semi-wet baseflow (spring) conditions (42%). While CHON compounds increased from the flood and flood recession events (27% and 22%, respectively) to dry and semi-wet conditions (35% and 34%, respectively), sulfur and phosphorous containing compounds decreased (flood ~34% and flood recession ~6%; dry <25% and semi-wet <3%, respectively). During the flood recession, DOM was considerably different from all other events, with over 1000 compounds significantly specific to this period. These significant changes were likely driven by submarine groundwater discharge, particularly as the compounds identified in surface- and pore- water were very similar, and surface water DOC and DON showed porewater influence. Baseflow characteristics were highly similar, with low saturation and high likelihood of rings or double bonds. It was evident that the influence of riverine inputs decreased within six months from the flood peak, which also showed the limited fresh riverine inputs outside extreme wet precipitation events when benthic fluxes became one of the major drivers of DOM characteristics. Thus, surface water DOM molecular characteristics showed significant responses to hydrologic changes, due to surface runoff inputs and benthic fluxes, which reflect changes in source and processing of OM within the estuary.