Phosphorus uptake by stream benthic biofilms: Empirical and experimental approaches to explaining variation

摘要

Elevated phosphorus (P) concentrations in streams are frequently linked with eutrophication and diminished water quality. Stream biofilms appear to play important roles in P assimilation thus representing a valuable transformation of nutrients in aquatic ecosystems. However, little work has identified parameters explaining variation in uptake rates, evaluated the effect of common disturbance techniques, addressed how increasing P-loads affect assimilative abilities, or estimated the influence of initial assimilatory processes on biofilm P dynamics. Therefore, there were four central approaches (chapters) to this dissertation: 1) perform an assessment of peer-reviewed literature reporting aquatic microbial P-uptake rates, 2) evaluate the effect of physical disturbance techniques commonly used in benthic biofilm metabolic studies, 3) measure P-uptake rates for benthic biofilms along an experimental and natural nutrient gradient, and 4) evaluate spatio-temporal P fluxes in biofilms. Regarding the first research approach, several ecological/experimental parameters were found significant in describing and explaining observed variation in published aquatic P-uptake rates: microbial group (benthic, planktonic), source (culture, wild), and sample time (long, short). This underscored the varied nature of microbial assimilatory kinetics and provided a quantitative synthesis of uptake rates thereby advancing nutrient dynamic models. The second chapter showed that common biofilm sampling techniques (physical disturbance) caused no differential effects on kinetic parameter estimates (t= 0.69, p= 0.492, df= 33), lending credence to numerous metabolic studies on benthic microbes post-abrasion and highlighting the potential for microbial uptake following scouring events. The third chapter identified the occurrence of P saturation in some stream biofilms and quantified its effect on the uptake of new P additions, and further concluded that nitrogen was a synergistic nutrient for resident benthic biofilms, particularly in streams of higher productivity (P legacy effect). Lastly, the fourth chapter demonstrated rapid P exchange processes occurring at early time periods (i.e., ≤ 5 minutes), the magnitude of which seems to diminish over longer periods (i.e., 15 - 30 minutes), further suggesting that experimental time periods scaled to hours or longer obscure such fundamental short-term responses. Overall, the studies conducted here employ both empirical and experimental techniques and help to explain ecological and biological variation in biofilm P-uptake rates.