Nitrogen Cycling in Freshwaters I
Presiding: P Mulholland, Oak Ridge National Laboratory; J Tank, University of Notre Dame
NB21D-01 08:30h
Detritivorous Fish Increase N-fixation in a Tropical Stream.
In nitrogen-(N) limited ecosystems, N-fixing cyanobacteria are an important source of primary production that is independent of N limitation. In an N-limited Venezuelan piedmont stream, we measured N-fixation in the presence and absence of the migratory benthic feeding fish, Prochilodus mariae. Prochilodus clear the sediment from the substratum when stream discharge is low, which may alleviate light limitation for attached cyanobacteria and increase N-fixation. We split a 230-m reach of stream longitudinally, where one side allowed movement of Prochilodus in and out of the reach, and the other side excluded the fish. The acetylene reduction method was used to measure N-fixation of the stream substrate. In the presence of Prochilodus, N-fixation was nearly 3 times higher than in the absence of this benthic feeding fish (9.6 mg of N/m2/day versus 3.4 mg of N/m2/day). This increase in N corresponds to one fourth of the daily ammonium uptake found in the split stream N budget. N-fixation is a substantial contributor to overall N cycling in this tropical stream, and Prochilodus facilitates the input of new N, potentially increasing the productivity of the river.
NB21D-02 08:45h
Effects of Landscape Age and Nutrients on N-fixation and Primary Production in Arctic Lakes
Primary production in most oligotrophic lakes in the Arctic is limited by nitrogen. Because water clarity is high and littoral zones are large, benthic production and N-fixation are important to whole-lake processes. Our objectives were to determine nutrient and landscape controls of benthic primary production and N-fixation in high-latitude arctic lakes at the arctic LTER site, Toolik Field Station, northern Alaska. A survey of benthic primary production (summers 1998 - 2002) and benthic N-fixation (summers 2000 and 2002) showed higher rates on younger substrates than on older substrates. Benthic fixation rates ranged from 0.1 to 1 mg N/m2/day respectively. Benthic primary production rates ranged from 70 - 300 mg C/m2/day respectively. Nutrient additions to intact mud cores from young surface lakes indicated that benthic primary production and N-fixation were not strongly phosphorus limited. This suggests that weathered material such as phosphorus from young landscapes may alleviate P limitation of N-fixation, causing overall lake production to increase and/or a change in nutrient limitation status. Variation across the landscape was much greater than variation caused by nutrients or other controls such as grazers, indicating that landscape-level controls may be more important than in-lake processes in determining lake productivity.
NB21D-03 INVITED 09:00h
Effects of Water Residence Time on Nitrate Removal in Flow-Regulated Backwater Lakes of the UMR Flood Plain
We examined in 2004 nitrate retention in five backwater lakes that are connected to the UMR via flow-controlled culverts. Nitrate loading and water residence time varied inversely as a function of culvert flow. Unlike engineered wetlands, we could not maximize source water nitrate concentration to increase loading. Under these constraints, maximum nitrate mass removal (42 percent) coincided with a water residence time of about 2 days. Removal decreased as flow and lake flushing rate increased. Hypothetical adjustments in flow and water residence time resulted in a 22 percent estimated load reduction. However, estimated removal could improve by 50 percent, due to increased contact time. Our results indicate that water residence time needs to be considered in connectivity design issues in order to maximize nitrate removal in backwater systems. This design goal is more difficult to achieve for natural backwater systems given the more complex interrelationships between flow, load, morphology, and water residence time.
NB21D-04 09:15h
Community and Ecosystem-Level Impacts of an Emergent Macrophyte on the Ventura River, California.
Ludwigia hexapetala is a pervasive, emergent vascular plant on the lower Ventura River. Presence of this plant appears to facilitate growth of shade-tolerant diatoms, while indirectly inhibiting filamentous green macroalgae. Four sites on the river were monitored during 2003; three downstream of a wastewater treatment plant, where Ludwigia is present, and one upstream site where it is absent. Filamentous algae occurred at all four sites, but declined rapidly at the below-treatment plant sites as growth and cover of vascular plants increased. By late summer, percent cover at these sites was dominated by Ludwigia, while the upstream site was consistently dominated by green macroalgae. Submerged plant parts provided substrate for diatom colonization, roughly doubling benthic diatom biomass (measured as chlorophyll a) at the downstream sites. Presence of the Ludwigia population also had strong ecosystem-level effects. The wastewater effluent produced typical stream water nitrate concentrations of 100-200 uM. Nitrate uptake rates downstream of the treatment plant inputs averaged 5 kg N/km/day, and direct uptake by Ludwigia could account for 20-40% of this nitrate drawdown. Further nitrate removal from the water column may be indirectly facilitated by the presence of Ludwigia through facilitation of diatom population growth.
NB21D-05 09:30h
Nutrient Availability and Stream Biofilm Responses: Contrast Among Streams Draining Catchments With Different Land Uses
Nutrient limitation has been shown to be common in pristine streams due to low availability or unbalanced stoichiometry. Humans have altered both the quantity and the ratios among nutrients in freshwater ecosystems. This effect highly depends on the type of activity. Within this context, we asked a) how nutrient inputs from human activity affected the potential limitation of stream biofilms, and b) how biofilm responded in front of two N sources (ammonium and nitrate). We conducted nutrient diffusing substrata experiments (6 treatments: control, nitrate, ammonium, phosphate, nitrate+phosphate, and ammonium+phosphate) in 3 streams located in the same catchment but subjected to distinct human pressures (pristine, agricultural, and urban) resulting in a gradient of N availability. On the last week of each experiment we performed a 12h 15NO3 addition. Chlorophyll a showed high variability within and between treatments and among streams, but was generally highest in the agricultural stream. N limitation was only observed in the pristine stream. Nitrate and ammonium treatments exerted no differential effect as potential limiting nutrient sources. All treatments were 15N labeled, but biofilm in control and phosphate treatments tended to show higher 15N values. These results suggest that nutrient availability affects both biofilm accrual and uptake capacity
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NB21D-06 INVITED 09:45h
Atmosphere-soil drivers and internal stream cycling of DON in forested watersheds
Dissolved organic nitrogen (DON) often dominates N loss from unpolluted watersheds but is not thought to vary with N inputs, capital, or demand. Here we show results from a multi-year survey of soils and streams in forested mountain watersheds spanning a wide atmospheric N deposition gradient (5 to 45 kg N ha -1yr-1) in the Appalachian Mts., USA. Stream DON and dissolved inorganic N (DIN) concentrations increased significantly and non-linearly with higher atmospheric deposition, shifting abruptly at a threshold deposition of ~ 9 kg N ha -1yr-1). Dissolved organic matter (DOM) C: N ratios declined with increased deposition, suggesting a shift in DOM quality. Soil C: N ratios showed steep declines with N deposition (from ~ 30 to 17). As a result, DON and DIN losses were negatively associated with soil C: N ratios. Temporal patterns in DON were not seasonal but rather related to antecedent precipitation. Yet, previous experiments showed rapid stream cycling of added monomeric DOM, suggesting an important role for labile DON in stream metabolism. Such forms, while low in abundance, may influence internal N demand in streams. At the landscape scale, atmospheric deposition, soil chemistry and hydrologic conditions are key drivers of watershed DON losses.