Effects of Water Column P-availability and Litter Microbial Mediated Processes and Stoichiometry in Aquatic Systems

Anthropogenic nutrient enrichment of aquatic ecosystems is prevalent and can have detrimental effects on biological condition. Many of these effects have been attributed directly or indirectly to changes in autotrophic processes. However, in detrital-based streams changes in course detrital stoichiometry and decomposition have been related to changes community structure and production at higher trophic levels highlighting the importance of considering the heterotrophic microbial responses to enrichment. Leaf litter stoichiometry, which is determined in part by the tree species source and the microbial communities that comprise the associated biofilm, is likely related to decomposition rates, but few studies examine these variables in combination across a nutrient concentration gradient. Dissolved inorganic phosphorus (DIP) (0, 10, 25, 50, 75, 100, 250, 500, 1000, 2000 microg/L DIP) and litter type (sugar maple and post oak) were manipulated in a 210 day laboratory microcosm study to determine their effect on litter C:P and decomposition rates and to examine their effects on short duration measures of respiration and alkaline phosphatase activity (APA) taken on several dates spanning the decomposition process. I found that C:P declined and decomposition rates increased in a saturating relationship along the DIP gradient indicating P-limitation of decomposition at low DIP concentrations. Saturation values for maple and oak C:P and decomposition rates were 500 and 830 and 0.0085 + 0.0005 and 0.0029 + 0.0002, respectively. Litter C:P saturation DIP concentrations saturated at approximately 80 microg/L for both litter types, while decomposition rates saturated at less than 25 microg/L for both litter types. APA and respiration rates were not correlated, but positive correlations were found between C:P and APA on several days (range in r = 0.568-0.670, p<0.008). Positive correlations between respiration rates and decomposition across several days but explained little variation in decomposition rate (r<0.5) Maple tended to have greater APA and respiration relative to oak litter, though not always statistically. These results demonstrate that litter type and DIP concentration are both important factors affecting litter stoichiometry and quantity in detrital systems, but do not support the use of APA and respiration as instantaneous measures of limitation of decomposition in streams. I provide insight into the complexity of the dynamics of nutrient enrichment in stream, educating potential impacts of nutrient loading on aquatic ecosystems.