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16 October 2012 Divergent responses of biomass and enzyme activities suggest differential nutrient limitation in stream periphyton
David A. Lang, Ryan S. King
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We used in situ nutrient-enrichment experiments at sites in 4 Texas streams spanning a steep nutrient gradient to test the hypothesis that periphyton photoautotrophs and heterotrophs are differentially limited by N and P, respectively. We contrasted the responses of total periphyton biomass (ash-free dry mass [AFDM]), photoautotrophic biomass (chlorophyll a [CHLA]), % CHLA of AFDM [% CHLA]), alkaline phosphatase activity (APA), and nitrogenase activity (NA) to ambient N and P concentrations among sites and among nutrient-diffusing substrata (NDS) (control, N, P, and N P) within sites. Periphyton biomass (AFDM and CHLA) increased steadily with increasing ambient N and P among sites. Both APA and NA were relatively high on the control treatments at the lowest nutrient site and decreased with increasing ambient N and P until falling to negligible levels at the highest nutrient site. Within sites, P enrichment decreased APA but did not increase % CHLA or CHLA/cm2. P enrichment did increase AFDM/cm2 at one of the low nutrient sites. N enrichment reduced NA and increased % CHLA and CHLA/cm2 without increasing AFDM/cm2. N P enrichment resulted in the highest % CHLA and CHLA/cm2 at the 3 lowest nutrient sites but did not increase AFDM/cm2. Combined, these findings suggest that photoautotrophs in periphyton communities may be more limited than heterotrophs by N or become more competitive for P under elevated N. The high levels of APA and NA on the controls coupled with minimal difference in AFDM among nutrient treatments at most sites despite increases in % CHLA on N and N P treatments provide experimental evidence that differential nutrient limitation probably occurs in periphyton communities. Our results support the view that the single-limiting-nutrient paradigm may lead to overly simplistic interpretations of periphyton responses to nutrient enrichment.

The Society for Freshwater Science
David A. Lang and Ryan S. King "Divergent responses of biomass and enzyme activities suggest differential nutrient limitation in stream periphyton," Freshwater Science 31(4), 1096-1104, (16 October 2012).
Received: 24 February 2012; Accepted: 1 July 2012; Published: 16 October 2012

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extracellular enzymes
nitrogen fixation
subtropical limestone streams
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