Short-term impacts of cold front passage on coastal water quality and material transport

Amid growing attention to climate change, most coastal oceanographic studies have focused on long-term trends, with comparatively less emphasis on the impacts of short-lived synoptic weather systems, such as cold fronts, on coastal water quality. These transient events can drive rapid shifts in biogeochemical processes with significant ecological consequences. This study investigated the short-term variability of water quality parameters in the Louisiana Continental Shelf in response to wind-driven changes in coastal currents following a cold front. A vessel-based survey conducted in March 2021 involved repeated high-frequency sampling along a 30 km transect over a 36-hour period. Measurements included salinity, temperature, chlorophyll-a (chl-a), pH, dissolved oxygen (DO), colored dissolved organic matter (CDOM), and turbidity from CTD casts; concentrations of dissolved organic carbon (DOC), nitrate (NO₃^-), nitrite (NO₂^-), ammonium (NH₄⁺), and phosphate (PO₄³⁻) from discrete water samples; and continuous velocity profiles from an onboard acoustic Doppler current profiler (ADCP). Results showed pronounced increases in salinity and temperature gradients following the cold front, accompanied by more than a threefold change in coastal current velocities associated with a transiting high-pressure system. Westward DOC transport peaked at 132 kg/s, equivalent to approximately 11,379 metric tons per day. Southward transport reached 68.7 kg/s, or roughly 5,940 metric tons per day, indicating significant along- and cross-shelf carbon transport. NH₄⁺ transport reached a maximum of 18.3 kg/s, while NO₃⁻ transport peaked at 18 kg/s, both reflecting strong nutrient pulses. Westward chl-a transport was highest at 0.554 kg/s during the first pass along the transect and declined thereafter, mirroring post-frontal shifts in primary productivity. CDOM transport decreased from 0.0386 kg/s to 0.0177 kg/s over the course of the survey, suggesting offshore dilution and mixing. These findings highlight the substantial influence of short-term atmospheric forcing on biogeochemical fluxes, revealing how compound hydrometeorological events, characterized by abrupt wind shifts, thermal gradients, and freshwater inputs, can drive rapid and nonlinear changes in coastal water quality. The results demonstrate the tight coupling between meteorological dynamics and near-shore biogeochemical processes on the LCS, and, when considered alongside evidence from other river-influenced coastal systems worldwide, underscore the broader global relevance of transient atmospheric events in shaping coastal circulation, nutrient transport, and ecosystem variability.