Nutrient Dynamics IV
Presiding: E Marti, Centre d'Estudis Avancats de Blanes (CSIC); J Beaulie, University of Notre Dame
NB41C-01 08:30h
Temporal Variation in Nutrient Uptake in Three Headwater Streams in the Upper Peninsula of Michigan.
We conducted short-term nutrient additions of ammonium (NH4+), nitrate (NO3-), and phosphate (PO4-3) to estimate whole-stream uptake in two reaches of three 2nd order streams in the Upper Peninsula of Michigan approximately monthly from May 2003-April 2004 (N = 96). Extensive replication permitted a unique opportunity for statistical analysis of seasonal variation in nutrient uptake within streams (between reaches) as well as variation among replicate streams. We expected seasonal variation in light, organic matter standing stocks, and temperature would regulate nutrient uptake via changes in autotrophic and heterotrophic biofilm activity. We initially predicted nutrient uptake velocities (vf) for all three solutes would be highest in spring and fall because of increased light availability for autotrophs (spring and fall) and microbial decomposition of leaves (fall). Among streams, NO3- and NH4+ uptake rates were significantly higher in spring and winter, lowest in fall, and rates in summer were variable, but PO4-3 uptake was not significantly different among seasons. We found no significant differences in nutrient uptake between reaches within streams. In general, there were significant differences in nutrient uptake among seasons for inorganic nitrogen forms, but not phosphorus, and nutrient demand was high in these streams despite low temperatures (0.5-2°C) in winter.
NB41C-02 08:45h
Dissolved O2, CO2, CH4, and N2O as indicators of biogeochemical processes in headwater streams.
The concentrations of carbon dioxide (CO2), oxygen (O2), methane (CH4), and nitrous oxide (N2O) dissolved in streamwater are determined by both in-stream biogeochemical transformations and chemical properties of the gases. Dissolved gas concentrations deviating from those expected based solely on atmospheric equilibrium indicate the influence of biogeochemical activity. To assess patterns in dissolved gas concentrations we sampled 23 headwater streams varying in watershed land-use. These streams were distributed across 8 biomes in the U.S. CH4 concentrations were variable ranging from below detection to >4000 times equilibrium concentrations (bd-10 Μmol CH4/L) and 87% of the streams were supersaturated, suggesting that methanogenesis is common in small streams. N2O concentrations were less variable, ranging from 0.4 to 10 times equilibrium concentrations (3.2 -86 ν mol N2O/L), and 86% of the streams were supersaturated with N2O, presumably due to denitrification and perhaps nitrification. Finally, all but two streams were supersaturated with CO2 (10.3-1335 Μmol CO2/L), which is consistent with the ubiquitous nature of aerobic heterotrophic respiration. These data demonstrate that headwater streams are often supersaturated with trace gases, likely due to both in-stream biogeochemical processes and inputs of groundwater with high dissolved gas concentrations.
NB41C-03 09:00h
A Stoichiometric Approach to Nutrient Retention in Stream Ecosystems
We examined patterns of variability in stream retention response among nutrients based on results from multiple nutrient additions conducted in 11 European streams covering a wide range of environmental conditions. Based on previous observations, we argued that streams were not equally efficient at retaining different nutrients (i.e., N and P) or different ionic forms of the same element (i.e., NO3 and NH4) and generally exhibited higher efficiency for the nutrient or ion that was available in lower proportion. In this study we focussed on mass transfer coefficients for NH4, NO3, and PO4 (i.e., stream nutrient demand, Vf in m/s) as the most appropriate stream retention metric for comparing among and within streams, and among different nutrient forms. Stream nutrient demand was significantly different depending on the nutrient under consideration. Ammonium demand was the largest, followed by phosphate demand, and nitrate was the lowest. Using ratios between stream demand for pairs of nutrients (VfNO3:VfNH4, VfNH4:VfPO4, and VFNO3:VFPO4), we found consistent patterns with chemical parameters as well as with parameters of ecosystem metabolism which supported our hypothesis. Consideration of different nutrients and their relative proportions proved useful to expand our understanding of stream ecosystems in terms of nutrient dynamics.
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NB41C-04 09:15h
Factors influencing the uptake of nutrients in streams within the New York City water-supply source areas.
The uptake of nutrients was measured in each of ten streams within the water supply source areas for New York City, once each year between 2000 and 2002. Uptake lengths were estimated from the conservative-tracer-corrected downstream attenuation of short-term (1-2 h) nutrient releases. Uptake lengths correlated with stream size and were converted to uptake velocities (Vf) for further analysis. Vf of phosphate, with a mean of 0.018 mm/s, fit Michaelis-Menten uptake kinetics with a half-saturation of 7 Îœg/L background phosphate. Vf of ammonium, with a mean of 0.58 mm/s, did not correlate with background ammonium concentration, but fit an uptake curve that used total dissolved nitrogen as the substrate, with a half-saturation of 1 mg/L. Vf of glucose and arabinose were not related to background concentrations. Vf for all four nutrients correlated with community respiration (CR) from diel oxygen variation. For phosphorus uptake, however, CR was collinear with background phosphorus. Vf for ammonium correlated with the macroinvertebrate-based Water Quality Score and Vf for both ammonium and phosphate correlated with some molecular tracers of anthropogenic sources. These results point to nutrient uptake as a sensitive integrator of water quality, ecosystem metabolism, and community structure.
NB41C-05 09:30h
The Effects of Nutrient Stoichiometry on Bacterial Community Composition in Streams
Bacterial biofilm community composition in streams may be affected by the nutrient stoichiometry of the surrounding water. Specifically, varying nitrogen to phosphorus (N:P) molar ratios potentially can select for or against different taxa, such as various subclasses of Proteobacteria, and thus alter community structure. In this study, bacterial communities at three sites along the Mahoning River (Ohio) with different inorganic nutrient concentrations were compared. Bacteria in biofilms on cobbles were enumerated using fluorescent in situ hybridization (FISH) to determine the abundance of alpha-, beta-, and gamma-Proteobacteria, and the Cytophaga-Flavobacterium-cluster. Nitrate, ammonia, and soluble reactive phosphate (SRP) concentrations in the water ranged from undetectable to 0.05 g/L of SRP and 0.3 g/L of ammonia. Beta-Proteobacteria appeared to be the most affected by N:P (ranging from 11 to 150) showing a positive correlation between their abundance and the N:P ratio. The Cytophaga-Flavobacterium showed effects that were nearly opposite of the beta-Proteobacteria. These findings provide evidence that limitation by single nutrients may not be as good a predictor of bacterial community structure as the molar ratios of these nutrients. Also, the nutrient stoichiometry could have a bottom up effect on stream ecosystems because of the central role that microbes play in stream food webs.
NB41C-06 09:45h
Nutrient Uptake in Mountain Lake Outflow Streams: Evidence for the Importance of Inflow Hydrodynamics and In-Lake Processes
In oligotrophic, high mountain lakes, cold plunging inflows may send nutrients directly to deep chlorophyll layers in the metalimnia. Compared to surface inflows, plunging inflows create a larger mixing volume for inflowing water and may allow phytoplankton in deep chlorophyll layers to take up nutrients before they can be exported from the lake. We hypothesized that inflow hydrodynamics interact with biological processes to influence availability of nutrients to primary producers in the outflow. We added water-mass (bromide) and biologically active (15N-ammonia) tracers to two oligotrophic, high mountain lakes via either a plunging stream water inflow or a surface addition. The average hydraulic residence time was three-times higher following the plunging inflow addition, compared to the surface addition. The nutrient retention index, however, was an order of magnitude higher, suggesting the interaction between inflow hydrodynamics and biological processes leads to a greater effect on nutrient export than on hydrologic export. Thus, if inflow streams are warm and mix directly into the epilimnion (as in our surface addition) nutrients will pass through the system faster and be more readily available to primary producers in outflow streams.