Nutrient Dynamics II
Presiding: S Earl, US EPA; L Taylor, University of Notre Dame
NB14B-01 15:30h
The effect of land use on DOC and DON uptake in headwater streams in the Kalamazoo River basin, Michigan
Research on the effect of land use on stream ecosystems has largely focused on impacts of elevated inorganic nutrients such as nitrogen. However, anthropogenic activities can potentially alter the dynamics of dissolved organic carbon (DOC) and nitrogen (DON). We selected 18 streams draining 6 land-use types: forested, urban, distal urban, agriculture, distal agriculture, and buffered agriculture. We conducted seasonal short-term nutrient releases of acetate (representing labile DOC) and glycine (representing labile DON), and compared them with short-term releases of inorganic N (NH4+). We predicted that the increase in inorganic nitrogen associated with land use would lead to increased DOC uptake and decreased demand for DON. Preliminary results indicate that demand for DOC (as uptake velocity) was highest in the urban stream during both autumn and spring. Uptake velocity of NH4+ was variable in all streams in autumn and highest in the agricultural stream in spring. In fall, DOC uptake velocity was generally higher than for NH4+, but this pattern was not seen in spring. DOC uptake velocity increased with an increase in water-column NH4+ (r2=0.387, p=0.041) indicating that elevated inorganic N concentrations associated with land use can potentially increase DOC uptake, further altering stream nutrient processing and export.
NB14B-02 15:45h
Do Upstream Lakes Affect the Quantity and Absorbance of Dissolved Organic Matter in Streams?
We hypothesized that the presence of upstream lakes would affect dissolved organic matter (DOM) quantity and absorbance in streams from northern Michigan (USA). We sampled DOM concentration and UV absorbance in 21 streams flowing out of lakes and in 19 streams without upstream lakes during May and August, 2004. We also determined watershed land cover and morphology to assess whether other landscape variables accounted for DOM differences between the two stream types. The concentration of DOM, its UVB absorbance (280-330 nm), and its UVB molar absorbtivity were all significantly lower in lake-outflow streams than in streams without upstream lakes. From the landscape characterization, we found that stream DOM concentration and chemistry were also strongly related to the proportion of wetlands among watershed. However, the strength of the relationships between DOM properties and wetlands differed between stream categories and sampling dates. Long water residence times of lakes appears to lead to lower quantity and greater transparency of DOM moving into streams of this region. The presence of upstream lakes thus represents an additional landscape factor that warrants consideration in future studies of the control of DOM concentration and chemistry in stream ecosystems.
NB14B-03 16:00h
Dynamics of Dissolved Organic Carbon (DOC) in a Stream During a Quarter Century of Forest Succession
A five-year study of DOC concentration in a stream draining a clear-cut catchment at Coweeta Hydrologic Laboratory reported lower DOC concentration and export than in a reference stream and predicted that DOC concentration would increase until it was indistinguishable from the reference stream. We now have a 25-year record of DOC concentrations to determine if that prediction was correct. Both streams exhibit a consistent seasonal pattern in DOC concentration with concentrations highest in autumn, declining through March, and increasing through the summer. Average DOC concentrations are higher in the reference stream in all months; the difference in concentration between the two streams is greatest in the growing season and least in the dormant season. The differences in DOC concentrations in the two streams were greatest in dry years and have increased as forest succession proceeded. Hence the prediction based on only five years of data was not correct. Mean annual DOC concentration changes as a function of amount of leachable organic matter in both the stream benthos and its catchment. There has been a depletion of these DOC sources during the first quarter century of forest recovery from clear-cutting resulting in lower DOC concentrations in the stream.
NB14B-04 16:15h
Carbon Dioxide and Methane Emissions from Streams in the Boreal Forest of Interior Alaska
The boreal forest accounts for only 8% of the land surface yet stores nearly 40% of the world's reactive soil carbon. Much of the boreal forest is underlain with discontinuous permafrost, which is actively degrading with climatic warming. As permafrost thaws, soil organic matter is exposed and potentially degraded by microorganisms. Using measurements of carbon dioxide and methane concentrations in stream water, in conjunction with experimental injections of conservative solute and volatile gas tracers, we examined CO2 and CH4 emissions from streams of varying size in interior Alaska. Both CO2 and CH4 were supersaturated with average partial pressures in headwater streams ranging from 10 - 24 ppmv for CH4 and 680 - 1043 ppmv for CO2. In mid-order streams, the partial pressure of CH4 was higher ranging from 44 - 85 ppmv, but CO2 was not elevated relative (573 - 675 ppmv) to headwaters. However, in thermokarst streams, which form due to rapid degradation of permafrost, both gases were greatly elevated with partial pressures from 1140 - 4519 ppmv for CO2 and 985 - 3400 ppmv for CH4. These results suggest that as soil organic matter is liberated by permafrost degradation, a sizeable proportion of carbon will be lost to the atmosphere as methane.
NB14B-05 16:30h
Characterization of Organic Carbon Released from Different Wetland Habitats in the Sacramento-San Joaquin Delta
The Sacramento-San Joaquin Delta is a source of drinking water for over 22 million people in California as well as a source of carbon for the aquatic foodweb in San Francisco Bay. To improve the ecological health of the Bay-Delta system, large areas of the Delta may be restored to wetlands. We investigated the potential impact of these changes on the compositional quality of dissolved organic material (DOM) in the Delta by examining alterations of DOM concentration and composition through interactions with tidal and non-tidal wetlands, and agricultural water use. Fourteen sites were sampled for two years and analyses were conducted on both whole water and XAD isolated material. Our comprehensive chemical characterization of DOM included measurement of drinking water disinfection byproduct formation potential, bioavailability, optical properties, isotopic ratios (C, N, S), carbohydrate content and lignin-phenolic content. The isotopic and molecular tracer data can help us elucidate if the DOM is derived from wetland plants, peat soils, or in-channel algal productivity, which vary seasonally and by site. The impact of restored wetlands on drinking water quality and the aquatic foodweb will depend on DOM loads as well as composition.
NB14B-06 16:45h
Hyporheic Respiration and Metazoan Dynamics Using an In-situ Flow-through Mesocosm.
We developed an in situ flow-through mesocosm to study the limits to metazoan production and sources of carbon in alluvial groundwater on the Middle Fork, Flathead River (MT). Eight 440-liter mesocosms were situated in pairs, representing lateral hyporheic positions (HP): infiltration, midgradient and exfiltration zones. We used local sediments simulating a bimodal gravel formation, typical of alluvial aquifers except that particulate carbon was removed. Whole mesocosm respiration was approximately 6.8 ug O2 per liter sediment per hr, 50 per cent lower than previous lab-based estimates, with highest rates at the infiltration site near the river. Metazoans, dominated by copepods and other meiofauna, reached mean densities of 5, 4 and 3 per liter, and richness of 6, 5 and 4 taxa for respective HPs, similar to results from well pumping. DOC based microbial production, estimated at 0.464 ugC/lhr, was sufficient to explain estimated metazoan production (0.041ugC/lhr). Evidence from well sampling indicates that high levels of invertebrate production may be associated with buried wood jams. We are currently monitoring the response to an experimental amendment of debris jam sediment and organic matter, and we expect an increase in community respiration and metazoan biomass downgradient of the amendment.