Microbial activity associated with seston in headwater streams: effects of nitrogen, phosphorus and temperature

SUMMARY 1. The influences of temperature and dissolved nitrates and phosphates on microbial activity associated with suspended fine particulate organic matter (seston) were evaluated in four headwater streams in the southern Appalachian Mountains. 2. Temperature manipulations of ± 5°C always induced significant changes in [14C] glucose mineralization (ANOVA; P<0.05) and [3H]thymidine incorporation (ANOVA; P<0.05). 3. Nutrient amendments of 1.0 mg NO3 I−1 and 0.05 mg PO4I−1 induced no significant alterations in bacterial mineralization of [14C]glucose (ANOVA; P>0.05) or incorporation of [3H]thymidine (ANOVA; P>0.05) in short‐term (i.e. 3 h) experiments. 4. Microorganisms attached to refractory particulate organic matter do not appear to be limited by nitrogen or phosphorus even in streams with ambient nutrient concentrations as low as 0.06 mg NO3 I−1 and <0.03 mg PO4 I−1. 5. Our results indicate that variations in water temperature resulting from diurnal and seasonal temperature fluctuations, forest clear‐cutting, and catchment elevation and aspect can have marked effects upon microbial activity and production, while short‐term alterations in nutrient regime appear to have no significant effect on microbial activity associated with seston.