Abiotic controls on periphyton accrual and metabolism in streams: Scaling by dimensionless numbers

Increasingly available high-resolution topographic data from remote sensing motivates the search for topographic features that predict abiotic controls on the distribution and performance of biota. We investigated the extent to which periphyton distribution and stream ecosystem metabolism in a steep upland river drainage network could be predicted from physical conditions that varied with local topography. During the summers of 2003 and 2004, we measured periphyton standing crops and gross primary production and ecosystem respiration rates along a 5 km reach of the South Fork Eel River and six of its tributaries in northern California (39°44'N, 123°39'W). We also measured wetted stream width (B), cross-sectionally averaged stream velocity (U), and streambed photosynthetically active solar radiation (PAR) at each site to investigate the degree to which periphyton abundance and metabolism were related to these indicators, which in turn are partially predictable from models relating environmental parameters to the topographic settings. Dimensional analysis, a technique widely used in the field of fluid mechanics, was used to investigate how biotic and abiotic variables may be interconnected in stream environments. Nondimensional groups of variables were formulated on the basis of our field estimates of chosen biotic and abiotic variables. Periphyton biomass was controlled by B^9/5, exposure to light PAR^1/5, nutrient concentration N^5/6, and inverse stream depth H^-5/6 and U^-1/2. The autotrophic-heterotrophic balance, quantified by the gross primary production to ecosystem respiration rate, scaled with the stream aspect ratio (B/H)^3/5 and Peclet number Pe = (UB/u*H)^3/10, where u* is the shear stress velocity. The scaling relationships were validated against reported field measurements from other geographical areas. The results show nonlinear dependencies among periphyton biomass, stream metabolism, and abiotic variables. These nonlinear relationships point to a need for detailed quantification of biotic and abiotic variables over a range of scales.