Dissolved Organic Matter Dynamics in Natural and Human-Modified Watersheds II
Presiding: W H McDowell, University of New Hampshire; B A Pellerin, U.S. Geological Survey
B53A-01 13:30h
Seasonal and event-scale controls on dissolved organic carbon and nitrate flushing from catchments
To explore terrestrial and aquatic linkages controlling nutrient dynamics in forested catchments, we collected high-frequency samples from 2002 to 2004 at the Sleepers River Research Watershed in northeastern Vermont USA. We measured DOC (dissolved organic carbon), SUVA (specific UV absorbance), nitrate, and major ion concentrations over a wide range of flow conditions. In addition, weekly samples since 1991 provide a longer term record of stream nutrient fluxes. During events, DOC concentrations increased with flow consistent with the flushing of a large reservoir of mobile organic carbon from forest soils. Higher concentrations of DOC and SUVA in the growing versus dormant season illustrated seasonal variation in sources, characteristics (i.e. reactivity), availability, and controls on the flushing response of organic matter from the landscape to streams. In contrast, stream nitrate concentrations increased with flow but only when catchments "wetted-up" after baseflow periods. Growing season stream nitrate responses were dependent on short-term antecedent moisture conditions indicating rapid depletion of the soil nitrate reservoir when source areas became hydrologically connected to streams. While the different response patterns emphasized variable source and biogeochemical controls in relation to flow patterns, coupled carbon and nitrogen biogeochemical processes were also important controls on stream nutrient fluxes. In particular, leaf fall was a critical time when reactive DOC from freshly decomposing litter fueled in-stream consumption of nitrate leading to sharp declines of stream nitrate concentrations. Our measurements highlight the importance of "hot spots" and "hot moments" of biogeochemical and hydrological processes that control stream responses. Furthermore, our work illustrates how carbon, nitrogen, and water cycles are coupled in catchments, and provides a conceptual model for future work aimed at modeling forest stream hydrochemistry at the catchment scale.
B53A-02 13:45h
Bioavailability of Dissolved Organic Carbon and Nitrogen From Tropical Montane Rainforest Streams Across a Geologic age Gradient
Dissolved organic matter (DOM) is metabolically important in streams. Its bioavailability is influenced by organic matter sources to streams and inorganic nutrient availability. As forest canopies and soils develop over time, organic matter inputs to streams should switch from algal to watershed sources. Across this succession gradient, nutrient limitation should also change. This study examines how chemical composition and bioavailability of DOM from tropical montane rainforest streams on Hawaii change across a geologic age gradient from 4 ky to 150 ky. Dissolved organic C (DOC) and N (DON) concentrations, chemical characteristics, and bioavailability varied with site age. With increasing stream age, DOC and DON concentrations, DOM aromaticity, and the C:N of the stream DOM increased. Changes in stream DOM chemistry and inorganic nutrient availability affected DOM bioavailability. Fifty percent of the DOC from the 4 ky site was bioavailable, where little to none was bioavailable from the older streams. Inorganic nutrient availability did not affect DOC bioavailability. In contrast, DON bioavailability was similar (12%) across sites and was affected by inorganic nutrient availability. This study demonstrates that the chemistry and metabolism of streams draining forests change with ecosystem age and development.
B53A-03 14:00h
DOC and DON Dynamics along the Bagmati Drainage Network in Kathmandu Valley
We studied organic matter dynamics and inorganic chemistry of the Bagmati River in Kathmandu valley, Nepal, to understand the influence of human and geochemical processes on chemical loads along the drainage system. Population density appears to be the most fundamental control on the chemistry of surface waters within the Bagmati drainage system. DOC concentration increases 10-fold with distance downstream (from 2.38 to 23.95 mg/L) and shows a strong relationship with human population density. The composition of river water (nutrients, Cl) suggests that sewage effluent to the river has a major effect on water quality. Concentrations were highest during summer, and lowest during the winter monsoon season. In contrast to DOC, DON concentration shows surprisingly little variation, and tends to decrease in concentration with distance downstream. Ammonium contributes almost all nitrogen in the total dissolved nitrogen fraction and the concentration of nitrate is negligible, probably due to rapid denitrification within the stream channel under relatively low-oxygen conditions. Decreases in sulfate along the stream channel may also be due to the reduction of sulfate to sulfide due to the heavy organic matter loading. Water quality is unacceptable for any use and the whole ecosystem is severely affected within the urban areas. Based on a comparison of downstream and upstream water quality, it appears that human activities along the Bagmati, principally inputs of human sewage, are largely responsible for the changes in surface water chemistry within Kathmandu valley.
B53A-04 14:15h
The Effects of Land Use on the Dynamics of Organic Carbon, Nitrogen, and Phosphorus Exported from Coastal Watersheds
We investigated the relationship between changes in land use and the abundance and metabolism of organic C, N, and P in coastal streams of the Chesapeake Bay watershed. Concentrations of inorganic N and P declined along a gradient of urbanization whereas concentrations of organic C, N, and P increased in streams. Storm sewers appeared to be a substantial contributor of inorganic nutrients to streams, but showed a lower relative contribution of organic nutrients suggesting the importance of in-stream generation in addition to external sources. Laboratory incubations indicated seasonal changes in the bioavailability of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON), but temporal patterns in the metabolism of both DOC and DON differed based on land use. The bioavailability of DOC peaked sharply during spring months in forested watersheds whereas it peaked later during summer months in suburban and urban watersheds. Bioavailability of DON in streams increased with urbanization, and concentrations of bioavailable DON in developed areas sometimes exceeded concentrations of inorganic nitrogen in undeveloped forested streams. Across gradients of urbanization, shifts in the relative abundance of organic nutrients versus inorganic nutrients coincided with variations in the enzymatic activities of heterotrophic microbial communities in stream sediments. Changes in land use may lead to large increases in the amount and reactivity of DOC, DON, and DOP exported from watersheds and organic nutrients may need to be regulated for the maintenance of water quality.
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B53A-05 14:30h
Regional processes affecting dissolved organic material in the Sacramento-San Joaquin River system
The Central Valley of California, USA, is drained primarily by the Sacramento and San Joaquin River System into a delta that interacts tidally with the San Francisco Bay estuary. We use historical data along with molecular and isotopic tracers to determine the impact of land use, water impoundments, and diversions on dissolved organic material (DOM) concentration and quality. River-borne DOM supports two thirds of the heterotrophic demand of the estuary, lowers the quality of drinking water diversions from the delta, and affects the transport and methylation of mercury. DOM concentration in the rivers and delta varies by over a factor of 6 throughout the year, with a peak in early spring. Our previous results indicated that the delta DOM contribution to the estuary varies seasonally, supplying from 10 percent to 50 percent of the DOM exported by the river system into the estuary, with the greatest contribution occurring during winter and spring. Recent results using molecular source indicators suggest the DOM is largely added by local aquatic production rather than by terrestrial inputs, and is substantially altered by the heterotrophic microbial community. The molecular and isotopic results suggest that water management and land use significantly impact the timing and composition of DOM.
B53A-06 14:45h
Potential Impacts of Organic Wastes on Small Stream Water Quality
We monitored concentrations of dissolved organic carbon (DOC), dissolved oxygen (DO) and other parameters in 17 small streams of the South Fork Broad River (SFBR) watershed on a monthly basis for 15 months. The subwatersheds were chosen to reflect a range of land uses including forested, pasture, mixed, and developed. The SFBR watershed is heavily impacted by organic wastes, primarily from its large poultry industry, but also from its rapidly growing human population. The poultry litter is primarily disposed of by application to pastures. Our monthly monitoring results showed a strong inverse relationship between mean DOC and mean DO and suggested that concentrations of total nitrogen (TN), DOC, and the trace gases nitrous oxide, methane and carbon dioxide are impacted by organic wastes and/or nutrients from animal manure applied to the land and/or human wastes from wastewater treatment plants or septic tanks in these watersheds. Here we estimate the organic waste loads of these watersheds and evaluate the impact of organic wastes on stream DOC and alkalinity concentrations, electrical conductivity, sediment potential denitrification rate and plant stable nitrogen isotope ratios. All of these water quality parameters are significantly correlated with watershed waste loading. DOC is most strongly correlated with total watershed waste loading whereas conductivity, alkalinity, potential denitrification rate and plant stable nitrogen isotope ratio are most strongly correlated with watershed human waste loading. These results suggest that more direct inputs (e.g., wastewater treatment plant effluents, near-stream septic tanks) have a greater relative impact on stream water quality than more dispersed inputs (land applied poultry litter, septic tanks far from streams) in the SFBR watershed. Conductivity, which is generally elevated in organic wastes, is also significantly correlated with total watershed waste loading suggesting it may be a useful indicator of overall watershed waste loading. Although this work was reviewed by EPA and approved for publication, it may not necessarily reflect official Agency policy.