Investigating Fine Particulate Matter in Streams Uncovers Peak Bacterial Activity in Agricultural Streams and on the Finest Particles

The processing of organic matter (OM) in stream ecosystems is a key component of the fluvial carbon cycle. While research has largely focused on the cycling of dissolved OM (DOM), suspended particles are increasingly recognized as active sites of OM cycling. Anthropogenic land use change can strongly modify stream biogeochemistry, yet its influence on particulate OM processing remains poorly constrained. It is unclear how land use affects the properties and processing of fine particles (<1 mm) and how bacterial activity varies across fine particle size classes, especially <100 μm. This study investigated the bacterial activity and biochemical particle composition along a fine particle size continuum in five forested mountain and five agricultural lowland streams. Suspended particles were separated into five size classes from 500 to 0.7 μm. Volumetric bacterial production rates were 20 times higher in agricultural streams than in mountain streams. The particle composition did not differ between stream types, implying that environmental characteristics can control fine particle processing. Across size classes, bacterial production rates were highest on the finest particles (<15 μm), which accounted for >98% of the total particle-associated bacterial activity in both stream types. Additionally, the fatty acid composition and C/N ratios indicated an increased bacterial biomass on the finest particles. Thus, suspended fine particles <15 μm are overlooked hotspots of microbial OM processing, highlighting the importance of land use and size-specific processes for understanding fluvial carbon cycling across the aquatic OM size continuum.