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

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.