The effects of nutrient limitation and stream discharge on the epilithic microbial community in an oligotrophic Arctic stream

Abstract

Nutrient limitation of epilithic microbial activity is modified by stream discharge and drainage from the tundra surrounding the Kuparuk River, Alaska, USA. During 1984, after three weeks of whole stream enrichment with phosphorus, autotrophic activity per unit biomass had increased in the enriched section of the stream suggesting that phosphorus availability was limiting productivity. In contrast, after three weeks of phosphorus enrichment during 1985, heterotrophic and autotrophic activity was similar in the control and enriched sections of the stream. However, when ammonia or nitrate and phosphorus were added to an in situ bioassay chamber for two weeks, higher community biomass and heterotrophic activity resulted. Ten days later biomass significantly dropped in the unenriched section. Nitrate levels over this period increased four fold concomitantly with decreased stream discharge. Apparently during 1985, nitrogen was limiting epilithic microbial community in the phosphorus enriched section of the Kuparuk River. The significant negative relationship between nitrate concentration and stream discharge observed during 1984 supported the trends seen in 1985. These data suggest that nutrient concentrations which limit epilithic microbial activity and biomass are regulated by the stream discharge and drainage from the surrounding tundra.