Jennifer L. Tank

N uptake as a function of concentration in streams

Detailed studies of stream N uptake were conducted in a prairie reach and gallery forest reach of Kings Creek on the Konza Prairie Biological Station. Nutrient uptake rates were measured with multiple short-term enrichments of NO3− and NH4+ at constant addition rates in the spring and summer of 1998. NH4+ uptake was also measured with 15N-NH4+ tracer additions and short-term unlabeled NH4+ additions at 12 stream sites across North America. Concurrent addition of a conservative tracer was used to account for dilution in all experiments. NH4+ uptake rate per unit area (Ut) was positively correlated to nutrient concentration across all sites (r2 = 0.41, log–log relationship). Relationships between concentration and Ut were used to determine whether the uptake was nonlinear (i.e., kinetic uptake primarily limited by the biotic capacity of microorganisms to accumulate nutrients) or linear (e.g., limited by mass transport into stream biofilms). In all systems, Ut was lower at ambient concentrations than at elevated concentrations. Extrapolation from uptake measured from a series of increasing enrichments could be used to estimate ambient Ut. Linear extrapolation of Ut assuming the relationship passes through the origin and rates measured at 1 elevated nutrient concentration underestimated ambient Ut by ∼3-fold. Uptake rates were saturated under some but not all conditions of enrichment; in some cases there was no saturation up to 50 μmol/L. The absolute concentration at which Ut was saturated in Kings Creek varied among reaches and nutrients. Uptake rates of NH4+ at ambient concentrations in all streams were higher than would be expected, assuming Ut does not saturate with increasing concentrations. At ambient nutrient concentrations in unpolluted streams, Ut is probably limited to some degree by the kinetic uptake capacity of stream biota. Mass transfer velocity from the water column is generally greater than would be expected given typical diffusion rates, underscoring the importance of advective transport. Given the short-term spikes in nutrient concentrations that can occur in streams (e.g., in response to storm events), Ut may not saturate, even at high concentrations.

Effects of litter exclusion and wood removal on phosphorus and nitrogen retention in a forest stream

Many studies in the past have shown indirect evidence of the importance of terrestrial detritus in woodland streams, but recently WALLACE et al. (1997b) eliminated leaf and wood inputs to a small stream and direcdy demonstrated die importance of this material to stream food webs. Additionally, this whole-stream experiment has shown that terrestrial detritus is more than just food for invertebrates. TANK & WEBSTER (1998) found accelerated wood biofilm development and wood decomposition in the litter exclusion stream, and MEYER et al. (1998) used die litter exclusion experiment to estimate that leaves contribute approximately 30% of dissolved organic carbon exports. Previous studies have also suggested that leaf litter in streams is important to nutrient retention (MULHOLLAND et al. 1985, ELWOOD et al. 1988). The purpose of the current study was to examine the effects of litter exclusion and wood removal on retention of dissolved nutrients.

Organic matter dynamics along a stream-order and elevational gradient in a southern Appalachian stream

Predictions of the River Continuum Concept include changes in physical factors such as geomorphology and temperature along a stream-order gradient (VANNOTE et al. 1980). Changes in macroinvertebrate diversity (ALLAN 1975), biomass (GRUBAUGH et al. 1996), and functional feeding group composition (MlNSHALL et al. 1983) have been shown along such gradients. Changes in a variety of ecosystem processes along stream continua have also been demonstrated (MINSHALL et al. 1983, NAIMAN et al. 1987). HURYN & WALLACE (1987) and GRUBAUGH et al. (1996) found that macroinvertebrate habitat varies both longitudinally and locally and that sampling a single habitat (patch) may not be sufficient to reflect community composition throughout a stream continuum. Our objective was to investigate whether selected organic matter processes vary with patch type along a stream-order/elevational gradient.

The Lotic Intersite Nitrogen Experiments: an example of successful ecological research collaboration

Collaboration is an essential skill for modern ecologists because it brings together diverse expertise, viewpoints, and study systems. The Lotic Intersite Nitrogen eXperiments (LINX I and II), a 17-y research endeavor involving scores of early- to late-career stream ecologists, is an example of the benefits, challenges, and approaches of successful collaborative research in ecology. The scientific success of LINX reflected tangible attributes including clear scientific goals (hypothesis-driven research), coordinated research methods, a team of cooperative scientists, excellent leadership, extensive communication, and a philosophy of respect for input from all collaborators. Intangible aspects of the collaboration included camaraderie and strong team chemistry. LINX further benefited from being part of a discipline in which collaboration is a tradition, clear data-sharing and authorship guidelines, an approach that melded field experiments and modeling, and a shared collaborative goal in the form of a universal commitment to see the project and resulting data products through to completion.Abstract: Collaboration is an essential skill for modern ecologists because it brings together diverse expertise, viewpoints, and study systems. The Lotic Intersite Nitrogen eXperiments (LINX I and II), a 17-y research endeavor involving scores of early- to late-career stream ecologists, is an example of the benefits, challenges, and approaches of successful collaborative research in ecology. The scientific success of LINX reflected tangible attributes including clear scientific goals (hypothesis-driven research), coordinated research methods, a team of cooperative scientists, excellent leadership, extensive communication, and a philosophy of respect for input from all collaborators. Intangible aspects of the collaboration included camaraderie and strong team chemistry. LINX further benefited from being part of a discipline in which collaboration is a tradition, clear data-sharing and authorship guidelines, an approach that melded field experiments and modeling, and a shared collaborative goal in the form of a universal commitment to see the project and resulting data products through to completion.