SS3.16 Lentic-Lotic Linkages in Freshwaters: Comparisons from Different Ecosystems
Date: Thursday, June 13, 2002
Location: Poster Session - VCC
 
MarcarelliAM, Utah State University, Logan, UT, USA, amym@cc.usu.edu
Wurtsbaugh, W, A, Utah State University, Logan, UT, USA, wurts@cc.usu.edu
 
MACRONUTRIENT LIMITATION OF CYANOBACTERIAL NITROGEN FIXATION IN SUB-ALPINE, OLIGOTROPHIC STREAMS OF THE SAWTOOTH MOUNTAINS, IDAHO
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Algal biomass in streams and lakes of the Western United States is often limited by nitrogen or co-limited by nitrogen and phosphorus. Although nitrogen-fixing cyanobacteria should have an advantage under nitrogen limiting conditions, the factors that control them, and their ability to make up nitrogen deficits in a watershed have never been studied. To test whether nutrients controlled nitrogen fixation in sub-alpine streams, we deployed nutrient-diffusing substrates above and below four lakes along two watersheds of the Sawtooth Mountains. Chlorophyll levels and nitrogen fixation were measured in four treatments: controls, +nitrogen, +phosphorus, and nitrogen +phosphorus. Periphyton biomass in these streams was most often co-limited by nitrogen and phosphorus. Nitrogen fixation in the lower watershed sites was stimulated 200-800% by phosphorus and suppressed as much as 99% by nitrogen or nitrogen +phosphorus additions. Nitrogen fixation rates were 1.5 to 10 times higher in the outflows than the inflows of three of four lakes, but responses to nutrients were similar. Our results indicate that nitrogen fixation can be controlled by available concentrations of nitrogen and phosphorus in subalpine, oligotrophic watersheds.