Porewater temporal variability in a wave-impacted permeable nearshore sediment

Porewater samples were collected from a nearshore permeable sediment on Oahu, Hawaii over a variety of surface gravity wave conditions to evaluate the effect of ocean swells and their corresponding bottom currents on porewater dissolved oxygen and inorganic nutrient dynamics. Our results indicate that swells with significant wave heights of ~ 1.3 m, with resulting nearbed velocities reaching at least 0.30 m s− 1, flush the porewater to a depth of ~ 7.5 cm by enhancing exchange between the upper sediment and the overlying water column. Upper-sediment dissolved oxygen inventories were positively correlated with nearbed velocities, and decline linearly versus time during background conditions following upper-sediment flushing, corresponding to an apparent upper-sediment deoxygenation rate of 0.31 mmol m− 2 d− 1. Similarly, upper-sediment nutrient inventories were observed to recover linearly after swell events, resulting in sediment recovery rates for silicate and soluble reactive phosphate of 35 μmol m− 2 d− 1 and 3.5 μmol m− 2 d− 1, respectively. The observed elemental ratios of regenerated nutrients suggest that the majority of organic matter undergoing remineralization is planktonic in origin. Wave-enhanced exchange is associated with a fluorescence response in the bottom of the overlying water column, suggesting that sediment flushing and subsequent nutrient input to the water column may be important to nutrient budgets and photosynthetic communities in nearshore oligotrophic waters.