Aquatic sediments play an important role in the cycling of biogeochemical compounds (e.g. nutrients). However, our understanding of these processes is limited by the need to collect intact sediment cores and extract the porewater for chemical analysis in order to measure the concentrations and gradients of the compounds involved, precluding observations in dynamic areas. Here we demonstrate a new approach for performing in situ time series measurements of nitrate concentrations in sediment pore water. An autonomous lab-on-chip sensor which performs spectrophotometric nitrate measurements was deployed in an estuarine river on the UK’s south coast. Rhizon filters were attached to the sensor’s sample intakes and inserted horizontally into the sediment to extract pore water for analysis, with one measuring the nitrate in the overlying water. Repeat measurements at 2.5-3 cm depth (n=124 over 5 days in winter; n=124 over 3 days in summer) revealed tidal and seasonal influences on pore water nitrate concentration. In winter, the pore water nitrate ranged from 0-30 µM with a tidal cycle out of phase with that of the water above. In summer the pore water nitrate ranged 0-8 µM with a tidal cycle in phase with the overlying water. This powerful technique can reveal temporal variability in porewater biogeochemistry, which is hard to capture by traditional methods. It could be expanded to examine any compound which can be measured with lab-on-chip sensors (e.g. phosphate, silicate, pH, total alkalinity) to enable new understandings of human impacts and natural biogeochemical processes.

Primary Presenter: Allison Schaap, National Oceanography Centre UK (allison.schaap@noc.ac.uk)

Authors:

Anna Lichtschlag,   (alic@noc.ac.uk)