Located between the South and the East China Sea, the Taiwan Straits (TWS) are a marine shelf-channel area, with unique hydrological and geomorphological features affected by rivers inflow and with recent algal blooms with red tide events. This study aimed at assessing microbial distribution and function and their modulation in response to environmental gradients. Surface (0.5 m) water samples from 16 stations along five north to south transects were collected; total prokaryotic abundance by epifluorescence microscope and carbon substrate utilization patterns by Biolog Ecoplates were estimated. Spatially, a patchy microbial distribution was found, with the highest microbial metabolic levels and prokaryotic abundance in the TWS area between Minjiang River estuary and Pingtan Island, and progressive decreases towards offshore stations. Complex carbon sources and carbohydrates were preferentially metabolized. This study provides a snapshot of the microbial abundance and activity in TWS as a model site of aquatic ecosystems impacted from land inputs; obtained data highlights that microbial metabolism is more sensitive than abundance to environmental changes.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00248-022-02031-7.