Nutrient Dynamics I
Presiding: A Steinman, Annis Water Resources Institute, Grand Valley State University; S Earl, US EPA
NB13B-01 13:30h
Extrapolating Ambient Nutrient Uptake From Multiple Nutrient Amendments: A Method Evaluation
Nutrient dynamics in streams are often quantified with short-term nutrient amendments that elevate stream water nutrient concentration. However, elevating nutrient concentration can alter nutrient uptake from ambient conditions. Previous studies have documented that the relationship between nutrient concentration and uptake follows Michaelis-Menten kinetics and that ambient uptake length can be estimated by extrapolation from a series of increasing nutrient amendments. We tested this approach in five streams spanning a gradient of nitrate concentration. We used an isotope tracer (15NO3-N) to quantify the ambient uptake length (Sw). We compared Sw to the uptake length determined by a single nitrate amendment and to the uptake length extrapolated from a series of amendments in which nitrate concentration was incrementally elevated. Extrapolated ambient uptake length was a better predictor of Sw than single amendment-derived uptake length in three streams. Extrapolated ambient uptake length was negative in two streams with high background nitrate concentrations. Our data suggest that nitrogen limitation was weak in these two streams and that the relationship between nitrate concentration and uptake deviated from Michaelis-Menten kinetics. The extrapolation technique proved to be a useful method; however, this approach may be less effective in streams where the studied nutrient is not limited.
NB13B-02 13:45h
Temperature and Nutrients Interact to Control Nitrogen Fixation in a Subalpine Stream: An Experimental Examination
To test the importance of factors controlling N-fixation in subalpine streams, I conducted a stream-side mesocosm experiment with epilithic communities and nutrient diffusing substrates (NDS) to test how temperature and nutrients interact to influence algal communities. Within two days, warm temperature (18°C) stimulated N-fixation by Calothrix in the epilithic community 2X above cold temperature (13°C), indicating a strong physiological response. Community responses measured on NDS indicated that cold-water diatoms dominated by day 45 in the cold treatment, while diatoms containing N-fixing endosymbionts dominated only in warm treatments with added phosphorus. There was a significant interaction between nutrient supply and temperature on N-fixation rates in the experiment. On nutrient controls, warm temperature boosted fixation 2X above cold temperature, but when P was added, temperature increased fixation 20X. This study indicates that N-fixation is stimulated both by temperature and nutrients in this stream, but the magnitude of response to phosphorus was much greater than to temperature. Furthermore, our results support the hypothesis that biological characteristics in streams, including community structure and biogeochemical processes, can be altered in complex ways by disturbances like grazing and logging that alter multiple controlling factors simultaneously.
NB13B-03 14:00h
Longitudinal Patterns of Metabolic Nutrients in a Large Midwestern River
Small streams can be more efficient at processing nutrients per unit length than large rivers, which are assumed to transport nutrients downstream without substantial transformation or retention. However, recent studies have suggested that processing in large channels may be substantial. Two longitudinal surveys were taken over a 36-mile section of the Wisconsin River to assess the degree of large-river nutrient processing. Samples were analyzed for DOC, specific UV absorbance (SUVA), chl-a, NH4, NO3, and SRP. NH4 flux decreased logarithmically from 15g/s to 5g/s over 36 miles of river (R2=0.62, p<.01) and nitrate flux increased linearly from 30g/s to 65g/s over the same distance (R2=0.75, p<.01). NH4 reduction is likely due to biological uptake and nitrification. DOC quality did not change with distance (R2=0.12, p=0.13), although overall DOC flux increased downstream (R2=0.99, p<.01), as did SRP flux (R2=0.45, p<.01). If the DOC increase were due to autochthonous (algal) sources, a change in DOC quality (SUVA) would be expected; however, no such change was observed. Therefore, there must be alternate sources of DOC along the study reach. We found evidence of significant in-channel carbon and nitrogen transformation in the Wisconsin River, indicating that large rivers may transform substantial quantities of nutrients.
NB13B-04 14:15h
Factors Influencing Internal Phosphorus Loading in Spring Lake, Michigan
Spring Lake is a eutrophic, drowned river mouth lake that drains into the Grand River about 1 km upstream from Lake Michigan. In 2003, we determined that internal P loading accounted for approximately 65% of the total annual P load to this lake, and that an alum concentration of 24 mg/L effectively inactivated P release in experimental sediment core tubes. In 2004, we studied the influence of alum concentration, sediment resuspension, and bioturbation on P release rates from the sediments. Based on laboratory incubations, we determined that P release rates were no different at alum concentrations of 15 mg alum/L than at concentrations of 25 mg/L. Resuspension of sediments substantially increased TP concentrations, even at high alum concentrations, although total soluble phosphorus concentrations remained low in the water provided alum was present. Bioturbation did not appear to play a major factor with respect to P release in these sediments. Given the current concentration of phosphorus in the Spring Lake sediments, internal P loading can continue for another 40 years, even if all external P sources were immediately eliminated.
NB13B-05 14:30h
Quantifying the Importance of Marine-derived Nutrients in Atlantic Coast Streams Using Stable Isotopes: Prospects and Challenges.
Ecologists have long recognized the importance of essential elements, particularly nitrogen, delivered by anadromous fishes to nutrient poor streams of the Pacific coast. On the Atlantic coast, this process is less widespread but still potentially important. Due to distinct gradients in stable isotope signatures between freshwater and the sea, stable isotope analysis (SIA) offers a mechanism of identifying areas that receive large influxes of marine nutrients in the form of excreta, eggs, carcasses, and young-of-the-year fishes, and the possibility of quantifying such fluxes. However, there are a variety of limitations in using this approach that must be considered. The purpose of this talk will be to describe some of these limitations using original data from three river systems on both sides of the Atlantic Ocean. We present interactions between anadromous fishes (Atlantic salmon, Salmo salar and blueback herring, Alosa aestivalis) and freshwater consumer groups (the sculpins Cottus cognatus and C. gobio, and several benthic invertebrate species). Interpretation of isotope patterns can be confounded by variable baseline signatures, reproductive status of consumers, and spawner densities. The successful use of SIA in determining the importance of marine-nutrients to Atlantic coast streams requires awareness of natural and man-made patterns of isotope ratios, as well as life history characteristics of stream-dwelling organisms.
NB13B-06 14:45h
Biogeochemistry of a California Floodplain Revealed by High Resolution Temporal Sampling
Previous weekly monitoring of a floodplain in the California Central Valley confirmed the dynamic spatiotemporal nature of biogeochemical floodplain processes, but did not resolve short-term (<1 day) temporal dynamics. This study examined water quality on a fine temporal scale during and after flooding to elucidate short-term variations in biogeochemical processes. In February-March 2004, we collected water samples using autosamplers at the entrance and exit of the floodplain. We detected diel patterns in chlorophyll-a, with concentrations fluctuating 50% - 200% over 12 hours. Nitrate (NO3--N), ammonium (NH4+-N) and orthophosphate (PO43--P) concentrations were highest during flooding (0.68 ppm, 0.35 ppm and 101 ppb, respectively). NO3--N and PO43--P decreased 0.12 ppm and 4 ppb per day during draining and remained constant during ponding; while NH4+-N remained constant during draining and increased 0.02 ppm per day during ponding. Total suspended solids (TSS) were 25% - 65% greater at the entrance than exit; fluctuated daily with the river hydrograph and decreased while volatile suspended solids increased 9% of TSS per day as hydrologic residence time increased. Our data demonstrate dramatic floodplain water quality changes occurring over a few hours that depend on connectivity phase and hydrologic residence time.