The subannual variability of total alkalinity (TA) distributions in the northeastern Gulf of Mexico was examined through the use of TA data from ship-based water sampling, historical records of riverine TA, and contemporaneous model output of surface currents and salinity. TA variability was restricted to the upper 150 m of the water column, where relationships between salinity and TA were controlled primarily by subannual variations in the extent of mixing between seawater and river water. A transition in TA distribution patterns between the river-dominated northern margin (near the Mississippi-Atchafalaya River System) and the ocean current-dominated eastern margin (West Florida Shelf) was observed. An index for riverine alkalinity input was formulated to provide insights about riverine alkalinity contributions in the upper water column. Spatial and temporal variations of total alkalinity in the northeastern Gulf of Mexico are primarily controlled by riverine TA inputs and ocean currents.