Measuring heterotrophic respiration rates of suspended particulate organic carbon from stream ecosystems

David C. Richardson, J. Denis Newbold, Anthony K. Aufdenkampe, Philip G. Taylor, Louis A. Kaplan

Limnol. Oceanogr. Methods 11:247-261 (2013) | DOI: 10.4319/lom.2013.11.247

ABSTRACT: Integrated carbon budgets of terrestrial and aquatic ecosystems indicate that particulate organic carbon (POC) plays an important role in the transport, storage, and turnover of carbon during its transit from land to sea. However, little is known about the rates at which POC in suspension is metabolized during downstream transport. We address this deficiency by improving existing respiration methods and models to assess the biological lability of POC suspended in a headwater stream. Our method involves concentration of stream particles by tangential flow filtration, extended incubations (35-40 d) using conditions to ensure particle suspension and prevent particle aggregation, correction for the simultaneous respiration of dissolved organic carbon, and conversion from oxygen measurements into carbon with a respiratory oxidation ratio (OR) of 1.30 O2:C. We include analysis of the choice of OR. The POC turnover times estimated with the improved methods in this study are ~10 days. These respiration rates are among the highest reported for either suspended or benthic POC in streams and suggest that suspended POC is mineralized closer to its point of origin than was previously assumed. During incubation, keeping POC in suspension can increase respiration rates as much as 2-fold compared with allowing particles to settle and physical inhibition of POC aggregation can increase rates by 1.2-fold compared with allowing POC to aggregate. Methods that explicitly incorporate suspension and discourage aggregation of POC and longer incubation times during respiration measurements will generate data that improve our understanding of the dynamic role of POC in aquatic ecosystems.