Assessing stream biofilm nutrient status across natural and experimental nutrient gradients

Open Access
Author:
May, Melissa
Graduate Program:
Wildlife and Fisheries Science
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
January 21, 2011
Committee Members:
  • Hunter Carrick, Thesis Advisor
Keywords:
  • biofilm
  • periphyton
  • nitrogen
  • phosphorus
Abstract:
Stream biofilms can provide important information on stream ecosystem nutrient status and productivity which cannot be gained from water column nutrient concentrations alone. I analyzed primary producer biomass (chlorophyll-a), biofilm nitrogen: phosphorus (N:P) ratio, alkaline phosphatase activity (APA), and nitrate reductase (NR) as measures of biofilm community nutrient status across two types of nutrient gradients (experimental and empirical). Nutrient status indicators were first studied using an in-situ enrichment experiment deployed at a low nutrient limestone spring (Big Spring, Bellefonte, PA), in order to calibrate them against known nutrient loads. Nutrient diffusing substrata leached nitrate and phosphate in a factorial design of nine PxNx treatments (zero, 0.045M, and 0.50M nitrate and phosphate). Increasing P treatment led to significantly higher biomass and suppression of APA response, indicating P limitation at the site. However, when enriched with P, biomass also increased with additional N, providing evidence for secondary nitrogen limitation. N:P ratios varied with N and P enrichment, but did not correlate to biomass. NR activity increased in the highest N treatment, demonstrating that biofilm NR responds positively when nitrate is the dominant N source. Chl-a, N:P, and APA were also measured seasonally on biofilms grown on incubated natural substrates in four subwatersheds of Spring Creek, Centre County, PA. Significant spatial and temporal differences were found between subwatersheds, with temporal variation in Chl-a and APA overshadowing between site variation. Biofilm N:P ratio was strongly correlated to water column DIN:SRP, but did not reflect the same patterns of seasonal P enrichment that were observed in the inverse relationship of Chl-a and APA. Alkaline phosphatase activity appears to be a more sensitive indicator of nutrient status than water column nutrient concentrations or N:P stoichiometry alone. This increased sensitivity likely reflects the difference between nutrient concentrations in streams and actual nutrient loads. Despite primary P limitation and low water column PO4-P at the study sites, biofilms in Spring Creek exhibited a range in biomass and physiology that indicated varying levels of P were influencing the system – possibly pointing toward episodic nutrient loading events. Overall, biofilm nutrient status is complex, and ambient nutrient concentrations are not adequate to fully understand the extent of seasonal and spatial variation. Measurements of biofilm physiology are needed in order to get a complete picture of stream nutrient status.