Hydraulic Control of Pore Water Geochemistry within the Oxic-Suboxic Zone of a Permeable Sediment

The geochemical behavior of the top 70 cm of permeable reef flat sediments on Checker Reef, Oahu, Hawai'i was examined using spatial and temporal changes in pore water O2, NO- 3, NO- 2NH+ 4, and N2O concentrations. The shallow depth of the sediment-water interface relative to the height of waves propagating across the reef creates an environment in which pore water mixing and transport are dominated by wave-induced mechanisms. Dissolved oxygen penetrated 15-50 cm in the four sites studied. The depth-integrated concentration of dissolved oxygen was greater under larger wave conditions and in more permeable sediments, which suggests that dissolved oxygen may be a good indicator of the relationship between sediment metabolism and the physical nature of the sediment environment. The absence of any subsurface N2O maxima, limited temporal variability in pore water N2O concentrations, and vertically coherent profiles of O2, NO- 3, NO- 2, and N2O suggest that suboxic processes are typically restricted to spatial scales that are smaller than for oxic processes in these sediments. The geochemical response of the sediments to a natural perturbation was monitored by examining the composition of pore waters immediately following a large wave event and for several weeks thereafter. Significant changes in the depth-integrated concentrations of O2, NO- 3, NO- 2, and NH+ 4were observed following the perturbation; however, only changes in NO- 3lasted for several weeks after the event, whereas most other species returned to previous conditions within two weeks or less.