Mixotrophic bacterivory by small phytoflagellates (SP) can equal that of  heterotrophic flagellates (HF) in marine euphotic waters. Due to their differing nutritional requirements from prey, SP and HF bacterivory are thought to differentially impact nutrient and carbon cycling through microbial food webs. However, little is known on how the relative dominance of SP and HF feeding shifts along environmental gradients, especially over short spatio-temporal scales. Here, SP and HF bacterivory rates were compared over 5 days at 3 depths in the upper water column of a dynamic coastal zone, through the use of surrogate prey. SP and HF bacterivory strongly fluctuated over both spatial (< 5 meters) and temporal (day to day) scales despite relatively stable abundance, with a respective 7- and 10-fold difference between minimum and maximum bacterivory rates. Irradiance regulated SP community bacterivory, leading to a grouping into high rates under light limitation, mid-level rates under optimal irradiance levels for growth and low rates at very high irradiance levels. SP bacterivory only dominated over that of HF at irradiance ranges generally considered optimal for photosynthetic growth (~ 40 - 300 µmol photons m-2 s-1). SP bacterivory, but not that of HF, was positively correlated to total bacterial abundance, with HF potentially selectively targeting actively growing bacteria. Overall, the present study indicates that meteorological conditions affecting irradiance levels could strongly impact microbial food web dynamics and the cycling of carbon and nutrients through coastal marine systems.