Recent observations in marine ecosystems show that the presence of thiamine regulates primary production, but little is known about the ecological effect of thiamine in streams. We conducted nutrient enrichment experiments in four streams in the New York's Adirondack Mountains using nutrient diffusing substrates to evaluate the influence of thiamine (vitamin B1) on the growth of stream periphyton. Contrasting treatments in our study included nutrient additions of thiamine (C12H17ClN4OS·HCl), nitrogen (NH4Cl), and phosphorus (NaH2PO4). Thiamine limitation occurred in 12 of 14 experiments conducted from June through October in 2015–2017, nitrogen limitation occurred in eight experiments, and phosphorus limitation in one experiment. The magnitude of response of periphyton to thiamine enrichment varied among seasons, years, and streams. The growth‐enhancing effect on periphyton biomass from thiamine or nitrogen addition typically occurred between 8 and 32 d of incubation, though the periphyton accumulation rate declined after 16–24 d of incubation. Our results showed that thiamine is a limiting nutrient for primary producers in our study streams and its effect size is similar to that of nitrogen. These findings extend the prior recognition of thiamine's substantial influence on marine photosynthetic eukaryotes to having a similar role in freshwater ecosystems.

Thiamine (vitamin B1) and its precursor 2‐methyl‐4‐amino‐5‐hydroxymethylpyrimidine (HMP) have been shown to exert an important influence on primary production in marine environments, but little is known about their influence in fresh waters. We evaluated the ecological effects of thiamine and one of its precursors HMP in streams within the Qiyun catchment, a headwater mountain system of a Yangtze River tributary in Southern China, using nutrient‐diffusing substrates. Thiamine limitation of periphyton was observed in April 2019, and HMP limitation was observed in both April and May 2019. Co‐limitation by thiamine plus phosphorus and by HMP plus nitrogen occurred in May. The effect sizes of thiamine and HMP on growth of periphyton were similar to each other. Physiochemical features of the stream showed different potential influences on the effects of thiamine and HMP. Water temperature was negatively associated with the effect of thiamine, whereas ammonium showed a negative relationship with the effect of HMP. These findings extend our knowledge of nutrient limitation by thiamine and one thiamine precursor in freshwater environments.