LTER ASM Workshop: Exploring nitrogen dynamics in streams: using models to scale up from headwater reaches to stream networks

Exploring nitrogen dynamics in streams: using models to scale up from headwater reaches to stream networks

Organizers: Sherri Johnson, AND; Bruce Peterson, PIE; Jack Webster, COW; Steve Hamilton, KBL

Most research on nutrient processes occurs in small streams. With anthropogenic inputs of nitrogen increasing in many areas, understanding the driving or controlling factors for downstream nutrient concentrations is becoming critical, especially in basins which provide drinking water to communities. Do differing landuses impact instream rates of uptake and transformation? Do concentrations of nutrients increase downstream? How do processes of uptake and transformation change in larger streams?

Bruce Peterson provided some context for the importance of understanding processes driving nutrient concentrations across a range of sizes of streams. Then three short presentations of potential ways to scale up from headwater studies started the workshop discussion. Wil Wollheim, Steve Thomas, and Geoff Poole gave presentations and shared their ideas of model applications to scaling, especially across a range of land use types, such as occur in larger basins. For example, in LINX, a cross-site project, involving 8 of the LTER sites with streams, mechanisms of nitrogen uptake and transformation are being studied in headwater streams and background synoptic nutrient concentration data are being collected throughout larger stream networks. This synoptic data will be used as validation for scaling up models of nutrient dynamics.

Discussion then focused on how to proceed to address these questions.

Synthesis needs are:

- use null models of network dynamics and examine why models don’t match observed distributions

- research to determine functional trait that drives/controls uptake across biomes

o suggestions included hydrologic retention time, uptake/biomass, water velocity, particle size, substrate surface area

- comparison of existing V_f (a metric of potential control of nutrient uptake processes) in many stream systems of varying size across biomes

- experiments to modify V_f and see responses to uptake and transformation rates

- examine other metrics – such as uptake/unit area of substrate in small vs large streams

- when or where does hydrology trump biology for nutrient uptake versus transport?

- begin to examine effects of seasonality and flow regimes on nutrient processes and retention

- what are management implications of increased understanding of nutrient uptake and transformation – especially for instream restoration and riparian guidelines?

Follow up to this workshop:

- Parameterize model(s) presented with similar data bases conduct sensitivity analyses for controlling functional traits – starting with uptake, denitrification, mineralization

- Continue to plan workshops in conjunction with existing meetings (NABS, AGU). LINX group has been meeting the day before the annual NABS meeting for over 6 yrs to coordinate research.

- Apply for NCEAS workshop for synthesis of existing data and to explore several models (potential date of 2005)

- Contribute to LTER Initiative to raise profile of existing successful cross-site comparisons in stream research and for funding of this broader synthesis needs.

Those participating and interested in potential follow up workshop on scaling up of nutrient processes to larger basins:

Bruce Peterson Peterson@mbl.edu

Jack Webster jwester@vt.edu

Bob Hall bhall@uwyo.edu

John Roach john.roach@asu.edu

Julie Simpson simpson@lifesci.ucsb.edu

Chuck Hopkinson chopkins@mtsl.edu

Kevin McGuire Kevin.mcguire@oregonstate.edu

Nadia Maddem nadia.madden@unh.edu

Brynne Bryan Brynne@sunites.upr.clu.edu

Tim Robinson trobinson@bren.ucsb.edu

Stan Gregory Stanley.Gregory@oregonstate.edu