Testing the influence of sediment granulometry on heterotrophic respiration with a new laboratory flow-through system

Purpose Increased sedimentation due to land use intensification is increasingly affecting carbon processing in streams and rivers around the globe. This study describes the design of a laboratory-scale flow-through incubation system as a tool for the rapid estimation of sediment respiration. The measurements were compared with those obtained using an in situ closed chamber respiration method. The influence of sediment size on respiration rates was also investigated. Materials and methods Measurements were conducted on a pre-alpine gravel-bed river sediment separated into the following grain size fractions: >60 mm (14.3%), 60-5mm (60.2%), 5-2 mm (13.7%), 2-0.063 mm (11.1%) and <0.063 mm (0.6%). Concurrently, in situ and laboratory measurements were carried out on a naturally heterogeneous sediment. In situ respiration was determined in closed chambers as O-2 consumption over time, while in the laboratory, respiration was determined using flow-through respiration chambers. Oxygen concentrations were measured using a fibre-optic oxygen meter positioned at the inflow and outflow from the chamber. Results and discussion The mean respiration rates within naturally mixed riverbed sediments were 1.27 +/- 0.3 mg O-2 dm(-3) h(-1) (n = 4) and 0.77 +/- 0.1 mg O-2 dm(-3) h(-1) (n = 3) for the flow-through chamber system and closed chamber system, respectively. Respiration rates were statistically significantly higher in the flow-through chamber system (t test, p < 0.05), indicating that closed chamber measurements underestimated the oxygen consumption within riverbed sediments. Sediment grain size was found to significantly affect respiration rates in both systems (ANOVA, p < 0.001) with the fine sediment fraction (particle size < 0.063mm) having the highest respiration rate (r(flow-through) = 51 +/- 23 mg O-2 dm(-3) h(-1)). The smallest fractions (2-0.063 and < 0.063 mm), which represent approximately 12% of total sediment volume, contributed 60% of total respiration. Conclusions The study demonstrated that flow-through respiration chambers more accurately estimate the respiration rate within riverbed sediments than in situ closed chambers, since the former experiment imitates the natural conditions where continuous interstitial flow occurs in the sediments. We also demonstrated that fine sediments (< 5 mm) substantially contribute to heterotrophic respiration in the studied gravel-bed river.