An Evaluation of the Efficacy of Shell Hash for the Mitigation of Intertidal Sediment Acidification

Our objectives were twofold: (1) to determine whether the addition of shellhash to intertidal sediments would mitigate porewater acidification and(2) whether its effectiveness was dependent on the type of sediment asdescribed by organic matter (OM) and particle grain size (PGS). Field experi-ments were conducted at two sites within Burrard Inlet, British Columbia;Maplewood Mudflats (MM), high in OM and silt and Whey-ah-Wichen/CatesPark (WAW), low in OM and an equal PGS among very coarse, coarse, finesand, and silt. Shell hash was added to triplicate treatment plots matched withtriplicate controls at each site and porewater pH measured at flood and ebbtide over eight tidal cycles. Sampling occurred during June and July when tidalcycles were at their maximum inundation and exposure. Porewater pH wassignificantly greater for ebb versus flood tide and also between sites with MMsignificantly lower (7.59) as compared to WAW (8.03). Although pH was notmitigated by the shell hash, for WAW, variation in pH was reduced as com-pared to MM, as indicated by coefficients of variation over the 6-week sam-pling period. We suggest that the application of shell hash to reduce theimpact of ocean acidification (OA) on intertidal sediments will be site depen-dent. The combined processes of eutrophication in sediments with high OMand respiration of infauna, especially at high densities, could act in concertwith OA to create an intertidal region unsuitable for bivalve larvae settlementand development.