Comparing multiple metrics of ecosystem function reveals underlying controls on variation among and within headwater streams

Measurements of ecosystem function integrate the activity of organisms across multiple trophic levels by quantifying rates of elemental transformations, and are frequently promoted as potential bioassessment tools. Comparisons among multiple metrics may reveal fundamental drivers of ecosystem function, strengthening their use in assessment; however, logistical challenges frequently preclude simultaneous collection of multiple functional metrics. We compared rates of nutrient uptake (short-term enrichment), metabolism (data-logging sondes), leaf litter decomposition (litter bags), macroinvertebrate secondary production (monthly benthic samples), and hydrological and geomorphological variables from May 2003-2006 in two 100m reaches of 3 forested headwater streams.  Across streams, we found positive correlations among nutrient uptake, secondary production, and decomposition rates, which were positively related to presence of large inorganic substrata and increasing discharge.  Within streams, relationships among functional measurements were variable and related to temporal variation in organic and inorganic substrata and water chemistry. Functional metrics describing seemingly disparate processes revealed similar underlying geomorphologic and hydrological controls among streams, but at the reach scale controls on functional metrics were variable.  Though managers and researchers will benefit from the use of functional metrics for comparison among streams, we note that geographic proximity does not portend consistency in factors controlling functional measurements within a stream.