Browsing by Author "Mulholland, P. J."

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    Endogenous and exogenous control of ecosystem function: N cycling in headwater streams
    Valett, H. M.; Thomas, S. A.; Mulholland, P. J.; Webster, Jackson R.; Dahm, C. N.; Fellows, C. S.; Crenshaw, C. L.; Peterson, C. G. (Ecological Society of America, 2008-12)
    Allochthonous inputs act as resource subsidies to many ecosystems, where they exert strong influences on metabolism and material cycling. At the same time, metabolic theory proposes endogenous thermal control independent of resource supply. To address the relative importance of exogenous and endogenous influences, we quantified spatial and temporal variation in ecosystem metabolism and nitrogen (N) uptake using seasonal releases of (15)N as nitrate in six streams differing in riparian-stream interaction and metabolic character. Nitrate removal was quantified using a nutrient spiraling approach based on measurements of downstream decline in (15)N flux. Respiration (R) and gross primary production (GPP) were measured with whole-stream diel oxygen budgets. Uptake and metabolism metrics were addressed as z scores relative to site means to assess temporal variation. In open-canopied streams, areal uptake (U; mu g N.m(-2).s(-1)) was closely related to GPP, metabolic rates increased with temperature, and R was accurately predicted by metabolic scaling relationships. In forested streams, N spiraling was not related to GPP; instead, uptake velocity (v(f); mm/s) was closely related to R. In contrast to open-canopied streams, N uptake and metabolic activity were negatively correlated to temperature and poorly described by scaling laws. We contend that streams differ along a gradient of exogenous and endogenous control that relates to the relative influences of resource subsidies and in-stream energetics as determinants of seasonal patterns of metabolism and N cycling. Our research suggests that temporal variation in the propagation of ecological influence between adjacent systems generates phases when ecosystems are alternatively characterized as endogenously and exogenously controlled.
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    Nitrogen cycling in a forest stream determined by a n-15 tracer addition
    Mulholland, P. J.; Tank, J. L.; Sanzone, D. M.; Wollheim, W. M.; Peterson, B. J.; Webster, Jackson R.; Meyer, J. L. (Ecological Society of America, 2000-08)
    Nitrogen uptake and cycling was examined using a six-week tracer addition of N-15-labeled ammonium in early spring in Walker Branch, a first-order deciduous forest stream in eastern Tennessee. Prior to the N-15 addition, standing stocks of N were determined for the major biomass compartments. During and after the addition, 15N was measured in water and in dominant biomass compartments upstream and at several locations downstream. Residence time of ammonium in stream water (5-6 min) and ammonium uptake lengths (23-27 m) were short and relatively constant during the addition. Uptake rates of NH4 were more variable, ranging from 22 to 37 mu g N.m(-2).min(-1) and varying directly with changes in streamwater ammonium concentration (2.7-6.7 mu g/L). The highest rates of ammonium uptake per unit area were by the liverwort Porella pinnata, decomposing leaves, and fine benthic organic matter (FBOM), although epilithon had the highest N uptake per unit biomass N. Nitrification rates and nitrate uptake lengths and rates were determined by fitting a nitrification/nitrate uptake model to the longitudinal profiles of N-15-NO3 flux. Nitrification was an important sink for ammonium in stream water, accounting for 19% of the total ammonium uptake rate. Nitrate production via coupled regeneration/nitrification of organic N was about one-half as large as nitrification of streamwater ammonium. Nitrate uptake lengths were longer and more variable than those for ammonium, ranging from 101 m to infinity. Nitrate uptake rate varied from 0 to 29 mu g.m(-2).min(-1) and was similar to 1.6 times greater than assimilatory ammonium uptake rate early in the tracer addition. A sixfold decline in instream gross primary production rate resulting from a sharp decline in light level with leaf emergence had little effect on ammonium uptake rate but reduced nitrate uptake rate by nearly 70%. At the end of the addition, 64-79% of added N-15 was accounted for, either in biomass within the 125-m stream reach (33-48%) or as export of N-15-NH4 (4%), N-15-NO3 (23%), and fine particulate organic matter (4%) from the reach, Much of the N-15 not accounted for was probably lost downstream as transport of particulate organic N during a storm midway through the experiment or as dissolved organic N produced within the reach. Turnover rates of a large portion of the N-15 taken up by biomass compartments were high (0.04-0.08 per day), although a substantial portion of the N-15 in Porella (34%), FBOM (21%), and decomposing wood (17%) at the end of the addition was retained 75 d later, indicating relatively long-term retention of some N taken up from water. In total, our results showed that ammonium retention and nitrification rates were high in Walker Branch, and that the downstream loss of N was primarily as nitrate and was controlled largely by nitrification, assimilatory demand for N, and availability of ammonium to meet that demand. Our results are consistent with recent N-15 tracer experiments in N-deficient forest soils that showed high rates of nitrification and the importance of nitrate uptake in regulating losses of N. Together these studies demonstrate the importance of N-15 tracer experiments for improving our understanding of the complex processes controlling N cycling and loss in ecosystems.
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    Phosphorus spiraling in a woodland stream: seasonal-variations
    Mulholland, P. J.; Newbold, J. D.; Elwood, J. W.; Ferren, L. A.; Webster, Jackson R. (Ecological Society of America, 1985)
    Four radiotracer releases were performed over an annual period in 1981-1982 to determine seasonal variation in indices and pathways of phosphorus spiralling in Walker Branch, a small woodland stream in eastern Tennessee, USA. Each release consisted of an addition of -370 MBq each of carrier-free 32PO4 and 3H2O over a 1-h period during baseflow. Concentrations of 32P and 3H dissolved in stream water were measured intensitively at several stations downstream for the radiotracer input during and immediately following each release. Activity of 32P in coarse particulate organic matter (CPOM), fine particulate organic matter (FPOM), and aufwuchs was measured 2-3 h after each release and at various intervals for 7 wk. Total biomass of CPOM, FPOm, and aufwuchs at the time of each release was also measured. Uptake of 32PO4 from the water was greatest in November and lowest in August, Uptake length (Sw) of phosphorus, defined as the average distance travelled by a PO4 ion dissolved in water, varied from 22 m in November to 97 m in August. Uptake of 32PO4 by CPOM was generally greatest, with -50% of total uptake, while that by aufwuchs was lowest, with <15% of the total. CPOM abundance was the major determinant of whole-steam PO4 uptake rate and Sw. Turnover length (Sp) of phosphorus, defined as the average distance traveled by an atom of P taken up by particulate material, was short compared to Sw, varying from 1 m in November to 3 m in January. Consequently total spiralling length (S) of P varied from 23 in November, just after peak autumn leaf fall, to 99 m in August, and reflected primarily the travel of P in the dissolved form. Our results indicate that the greatest increase in Sw, (and consequently in S) in Walker Branch occurs in late autumn or winter after storms reduce the abundance of CPOM in the lower portions of the stream bed. Although we calculate that Sp may increase by one of two orders of magnitude for short periods during storms, the greatest effect of storms on P spiralling over the long term is their impact on the quality of CPOM and FPOM in the stream bed after the return to baseflow. For most of the year, detrital organic carbon probably influences phosphorus spiralling more than phosphorus spiralling influences the processing of organic carbon in Walker Branch. Only during the fall and early winter periods, when CPOM abundance is high and Sw is short, does phosphorus spiralling exert strong control over biotic processes downstream.