COS 59-6
Consumer biomass and production in five detritus-based stream ecosystems: Response to xperimental gradients in dissolved N:P ratios

Wednesday, August 7, 2013: 9:50 AM
L100G, Minneapolis Convention Center
Lee M. Demi, Biological Sciences, University of Alabama, Tuscaloosa, AL
Jonathan P. Benstead, Biological Sciences, University of Alabama, Tuscaloosa, AL
Amy D. Rosemond, Odum School of Ecology, University of Georgia, Athens, GA
John C. Maerz, Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA
Background/Question/Methods

Ecological stoichiometric theory predicts that somatic growth and biomass production of consumers are maximized when nutrient demand is stoichiometrically balanced by nutrient supply (resource C:N:P). However, many primary consumers ingest resources that are depleted in N and P relative to their own tissues. This imbalance between consumer and resource stoichiometry may be particularly important in detritus-based food webs, because vascular plant detritus is typically characterized by very high C:nutrient ratios. We tested consumer response to microbially modulated changes in detrital stoichiometry by continuously adding N and P to five detritus-based headwater streams (beginning July 2011) at the Coweeta Hydrologic Laboratory (North Carolina, USA) along a steep gradient of dissolved N:P ratios (2:1, 8:1, 16:1, 32:1, 128:1).

Results/Conclusions

Preliminary data indicate reduced leaf litter standing stocks during year 1 of nutrient enrichment (mean C:P~3500, CV=0.51)  relative to a pre-enrichment year (mean C:P~6000, CV=0.39) across all 5 N:P treatments. There was an apparent increase in mean macroinvertebrate biomass during February-May (6%-130%) in all five streams during year 1 of nutrient enrichment relative to the same period during the pre-enrichment year, though these increases were not statistically significant (p≥0.16).   Biomass within the shredder functional feeding group appeared to increase (39%-160%), though not significantly, during the same period in 4 of 5 streams (-4% in N:P=8 stream).  There was no relationship between the percent change in total macroinvertebrate biomass (p=0.44), or shredder biomass (p=0.53), and target dissolved N:P ratios, indicating little effect of N:P ratio on macroinvertebrate response to N and P enrichment. Incorporation of additional monthly biomass data (including a second year of enrichment) and secondary production estimates may provide greater insight into the role of detrital stoichiometry in regulating the productivity of detritus-based food webs.