Dissolved organic matter as P source for biofilms in two contrasting low-order streams

Abstract

Forest streams receive large amounts of leaves whose leachates are an important source of dissolved organicmatter (DOM), providing not only carbon but also organic nutrients to the microbial communities in streams.I carried out a field study to evaluate the effect of different DOM concentrations on the biofilm structure and functionaltraits in two similar forest streams belonging to the same catchment. I compared biofilm biomass and nutrientcontent throughout one year, algal species composition, and biofilm community-level physiological profilesin two streams with different DOM concentration and aromaticity. Dissolved nutrient concentrations were higherin the stream with higher DOM concentration, with a concomitant higher biofilm biomass, and there was also atemporal pattern, with higher values during the autumn. Phosphorus content in biofilms was also higher in the highDOM stream, coincidently with a higher capacity of the community to utilize organic P source (glucose-1-P) as asubstrate. In contrast, the biofilms from the stream with lower DOM concentrations preferentially used N-organicsubstrates (amino acids and amines). These results reveal that the biofilms of forest streams make use of organicmatter nutrients, so that streams with different DOM loads may differ in biofilm biomass due to changes in bothbacterial and autotrophic biomass. In addition, biofilm dynamics may be related to forest phenology, as the highestOM input in this deciduous forest is represented by tree leaves, which supply DOM through leachates, and inparticular, with P-rich leachates. In conclusion, different DOM concentrations in two nearby streams led to differencesin the community-level physiological profile, as has been previously demonstrated at larger spatial scales inoceans, lakes and along larger rivers.