Nutrient Dynamics V
Presiding: J Beaulie, University of Notre Dame; N Lottig, Virginia Tech University
NB42F-01 10:30h
The Effects of Zebra Mussels on Nitrogen Cycling and Sediment Physiochemical Characteristics: A Laboratory Experiment.
Many ecosystems experiencing anthropogenic nitrogen loading have also been invaded by zebra mussels (ZM), therefore it is important to understand how these human-induced changes interact. In particular, ZM may impact biogeochemical transformations. We hypothesized that high NH4+ waste from ZM will increase sediment nitrification rates, and increase NO3--N available for denitrification. To test this prediction, aquaria were stocked with either no ZM or 10,000 individuals m-2. Sediment nitrification (via nitrapyrin-inhibition technique), denitrification (via chloramphenicol-amended acetylene block technique), and sediment O2 profiles (using microelectrodes) were measured weekly for 1 month. The presence of ZM increased nitrification and denitrification rates over initial conditions throughout all weeks (RM ANOVA, p<0.05). Sediment nitrification in the ZM aquaria increased to 47 ± 2.1 Îœ g NH4+ -NgAFDM-1h-1 by the 3rd week in comparison to 24 ± 4.4 Îœ g NH4+ -NgAFDM-1h-1 in control aquaria. Sediment denitrification peaked at week 2 in the ZM aquaria at 330 ± 24 Îœ g N2O gAFDM-1h-1 but remained low in control aquaria throughout. There was a stronger correlation between nitrification and denitrification rates in the presence of ZM (R=0.32, 0.12 respectively). These results, in addition to more frequent hypoxia in sediment oxygen profiles, suggest closer coupling between nitrification and denitrification when ZM are present.
NB42F-02 10:45h
Source and Fate of Phosphorus in a Constructed Wetland in the Northern Everglades: Evidence From Oxygen Isotopes
High phosphorus influx into wetland ecosystems in the Florida Everglades continues to be a problem. Although the effects of phosphorus (P) loading on plant communities have been well documented, relatively little is know about how those changes affect the biogeochemical processes regulating the nutrient availability and cycling in aquatic cosystems. It has been suggested that oxygen isotopes in phosphate may provide a useful tool for tracing the source of phosphorus in aquatic systems. In this study, we measured the oxygen isotopic composition of total phosphate in sediments collected from an artificial wetland constructed to remove phosphorus from runoff water in the northern Everglades. The d18O values of total phosphate in sediment samples range from 8.3 to 13.3%. These values are very different from those of the phosphate in fertilizers commonly used in this area. These initial data suggest that biologically recycled phosphate is significantly depleted in oxygen-18 compared to the fertilizers and that these sediments are dominated by organic P. Analyses of oxygen isotopic composition of total phosphate in soil samples collected from the Everglades Agricultural Area about 10-12 months after fertilization also indicate the dominance of organic P in the soil, suggesting rapid removal of fertilizer P from the soil through plant uptake and leaching. These data show: 1)d18O decreases from the inflow area (i.e., highly polluted area) to a less polluted area in the constructed wetland; and 2) d18O decreases with depth in the same sediment core. This trend may be explained by less influence of fertilizer at depth and also farther away from the source of P.
NB42F-03 11:00h
Nitrogen Retention in the Riparian Zone of Watersheds Underlain by Discontinuous Permafrost in Interior Alaska
Riparian zones function as important ecotones for reducing nitrate concentration in ground water and inputs into streams. In the boreal forest of interior Alaska, permafrost confines subsurface flow through the riparian zone to shallow organic horizons, where plant uptake of nitrate and denitrification are typically high. Two research questions were addressed in this study: 1) how does riparian zone nitrogen retention vary in watersheds underlain by discontinuous permafrost, 2) what is the contribution of denitrification to riparian zone nitrogen retention? To estimate the contribution of the riparian zone to watershed nitrogen retention, we analyzed groundwater chemistry using an end-member mixing model. To assess the importance of denitrification as a mechanism of nitrogen retention, we conducted field denitrification assays using the acetylene block technique. Nitrogen retention averaged 8.9 and 5.7 mgN m-2 d-1 in low and high permafrost watersheds, respectively, over the summer. Compared with the fluvial export of nitrogen, the retention rate of nitrogen in the riparian zone is 10 - 15% of the loss rate in stream flow. Denitrification accounted for a small proportion (3%) of nitrogen retention in the riparian zone. The interaction between permafrost, hydrology, and biological processes appears to enhance riparian nitrate retention in this system.
NB42F-04 11:15h
Storage and Transformation of Artificial and Natural Salmon-Derived Nutrients in the Hyporheic Zone of a Southeast Alaska Stream
Adding nutrients in organic pellets (analogs) or salmon carcasses (SDN) is one strategy resource managers use to enhance productivity in streams where natural salmon nutrient subsidies have been reduced. We compared hyporheic storage and transformation of nutrients from carcasses to those from analogs added to surface water of two tributaries of a salmon stream in Southeast Alaska. Hyporheic sediments responded differently to the two sources with some responses detectable the following summer. Average hyporheic and phreatic SRP concentrations in the analog treatment were 2.5 and 3.5 times greater, respectively, than controls one month after the August additions. Subsurface SRP was again higher the following spring and summer in the analog treatment. Surface water SRP was higher in the analog treatment in late summer a year after the addition. Respiration in phreatic zones in analog and control reaches remained under 1.1 mg DO L sediment-1 h-1. However, respiration rates were elevated in phreatic zones in the carcass treatment reach (3.0 mg DO L sediment-1 h-1) the summer after the addition, suggesting delayed use of stored carcass carbon. These results support the hypothesis that hyporheic zones provide long-term storage and remobilization of SDN thereby enhancing stream productivity in subsequent years.
NB42F-05 11:30h
Seasonal nutrient dynamics in three stream types in SE Alaska
The Tongass National Forest encompasses over 5,000 salmon streams draining a wide variety of lithologies, microclimates and landforms. To predict management outcomes and responses to climate change, managers need an understanding of how ecosystem processes in streams vary over large spatial scales and with major controlling landscape variables. We measured forms of N, P and DOM in three common stream types: glacial, brownwater and clearwater near Juneau, AK. Glacial and clearwater streams showed strong seasonal trends in nitrate and total nitrogen related to snow melt and summer uptake, whereas brownwater streams were variable but not highly seasonal. Total nitrogen concentrations were dominated by inorganic forms in clearwater streams, organic forms in brownwater streams and varied seasonally between organic and inorganic forms in glacial streams. DOC concentrations were low in clearwater and glacial streams and varied little seasonally. Brownwater stream DOC values were high, variable and varied with discharge, with an increasing trend during summer. The Tongass NF recently classified SE Alaska forest lands into ecological subsections based on large-scale geophysical factors such as lithology and surficial geology. Partitioning Tongass streams using this approach may lead to better management predictions by more accurately incorporating natural variation in baseline stream dynamics.