Transverse spatiotemporal variability of lowland river properties and effects on metabolic rate estimates

Abstract

Variability of river properties such as temperature, velocity, dissolved oxygen (DO), and light at small scales (centimeters to meters) can play an important role in the local exchanges of energy and mass. We hypothesize that significant transverse cross-sectional DO variation is observable within a river. Such variation may influence conventional single-station metabolic rate (primary production and respiration) estimates with respect to DO probe location, and reveal important connections between physical and biogeochemical processes and their drivers in rivers. Using a mobile sensor system, we measured riverproperties across a bend in the lower Merced River in Central California under stationary flow conditions in April and September. Cross-sectional temperature, DO, and chlorophyll-a concentrations exhibited modest but significant gradients, which varied in magnitude anddirection on a diel basis. The spatiotemporal variation was consistent with reachgeomorphology and incident light patterns. Gross primary production (GPP), community respiration (CR24), and net ecosystem production (NEP) rates estimates derived from local DO and temperature time series varied by 3&ndash10% over the river cross section, with greatervariation in late summer. The presence of transverse metabolic rate gradients in this relatively simple reach implies the existence of substantial gradients in more complex riverregimes, such as those spanning distinctively different microhabitats, transient storagezones, and related distributed biogeochemical zones.