We studied the cm to m scale spatial distribution of dissolved free amino acids (DFAA) in the upper epilimnion of oligomesotrophic Lake Zurich in 14 sampling campaigns over > 3 years and at various periods of the growing season. During each campaign, 10 sets of 10 simultaneously drawn samples (10 mL, 2 cm distance) were collected from 5 m depth. DFAA concentrations varied by one to > 3 orders of magnitude within sets, providing field evidence for DFAA release from macroscopic point sources and for substantial variability of the insitu growth conditions of bacterioplankton metacommunities. There was a tight relationship between the median DFAA concentration per sampling campaign and the compositional heterogeneity of the 15 most common AA: their composition was similar in samples from campaigns with high median DFAA concentrations, indicating that spatial distribution patterns were mainly a result of physical mixing. By contrast, AA composition was spatially variable in campaigns with low median DFAA concentrations, and serine, aspartate, and glycine were disproportionally high in the 10% samples with highest DFAA concentrations. We hypothesized that pelagic bacteria would preferably target pulses of such locally overrepresented AA. Short-term incubations with radiolabeled tracers revealed substantially higher microbial uptake of serine and, to a lesser extent, aspartate, than of two amino acids with consistently low in situ concentrations (leucine, isoleucine). This illustrates a "preparedness" of the bacterioplankton to preferably incorporate those AAs that are more available in DFAA hotspots.Dissolved organic carbon represents one of the largest exchangeable carbon reservoirs in aquatic systems (Carlson 2002). It overlaps with the pool of dissolved organic nitrogen in the subset of labile low-molecular-weight compounds that are readily available to microorganisms, such as dissolved free amino acids (DFAAs) (