A novel and robust device for repeated small-scale oxygen measurement in riverine sediments - implications for advanced environmental surveys

H.Wolfgang Riss, Elisabeth I. Meyer,Olaf Niepagenkemper

Limnol. Oceanogr. Methods 6:200-207 (2008) | DOI: 10.4319/lom.2008.6.200

ABSTRACT: A novel sampling methodology was developed to measure in situ oxygen concentrations at well-defined spatial resolutions in riverine sediments. Stainless steel tubes of different length, perforated and shielded at the lower end, and bundled in units, were inserted into the streambed to serve as long-term sampling points. The samplers were given a physically robust design to withstand a certain degree of bed load discharge. Oxygen measurements were obtained by introducing into the sampling tubes a fine tolerance metal tube containing a fiber optic oxygen sensor (optode). Small volumes of interstitial water were withdrawn and the partial pressure of dissolved oxygen in the sample water was measured continuously as it passed the oxygen sensor. The optode has distinct advantages over previous oxygen sensors in that it neither consumes oxygen nor requires a significant flow of water past the sensor to obtain reliable readings. A sample volume of less than 1 cm3 is sufficient to obtain interstitial water with a purity of more than 97%. Residual water from inside the steel tubes caused only a low contamination level. Measurement bias as a consequence of vacuum withdrawal and consequent changes to atmospheric pressure did not exceed one percent under normal conditions. The measurement technique presented in this paper offers advantages over previous methodologies in that it permits more accurate in situ oxygen measurement at smaller spatial scales and especially at low oxygen levels. After installation of the sampling tubes in the streambed, and following an initial stabilization period (approx. 40 d), field measurements revealed vertical and horizontal variability in interstitial oxygen concentrations. Installed sample arrays allowed repeat measurement of interstitial conditions over a 12-month period.