An evaluation of the efficacy of shell hash for the mitigation of intertidal sediment acidification

Abstract

Our objectives were twofold: (1) to determine whether the addition of shell hash to intertidal sediments would mitigate porewater acidification and (2) whether its effectiveness was dependent on the type of sediment as described by organic matter (OM) and particle grain size (PGS). Field experiments were conducted at two sites within Burrard Inlet, British Columbia; Maplewood Mudflats (MM), high in OM and silt and Whey-ah-Wichen/Cates Park (WAW), low in OM and an equal PGS among very coarse, coarse, fine sand, and silt. Shell hash was added to triplicate treatment plots matched with triplicate controls at each site and porewater pH measured at flood and ebb tide over eight tidal cycles. Sampling occurred during June and July when tidal cycles were at their maximum inundation and exposure. Porewater pH was significantly greater for ebb versus flood tide and also between sites with MM significantly lower (7.59) as compared to WAW (8.03). Although pH was not mitigated by the shell hash, for WAW, variation in pH was reduced as compared to MM, as indicated by coefficients of variation over the 6-week sampling period. We suggest that the application of shell hash to reduce the impact of ocean acidification (OA) on intertidal sediments will be site dependent. The combined processes of eutrophication in sediments with high OM and respiration of infauna, especially at high densities, could act in concert with OA to create an intertidal region unsuitable for bivalve larvae settlement and development.

Effects of ancient anthropogenic clam gardens on the growth, survival, and transcriptome of Pacific littleneck clams (Leukoma staminea)

“…For example, the application of very finely crushed shell hash could have a negative effect by decreasing interstitial spaces in the sediment and thus, as seen with silt, decreasing sediment permeability. Recent studies investigating the efficacy of shell hash to mitigate acidification of intertidal sediments found it unsuccessful at raising pore-water pH of acidic sediments (Beal et al 2020, Doyle and Bendell 2022). Doyle et al (2022) found the addition of shell hash to be site dependent as it reduced variation in pH at an intertidal site with pH 8.03 but did not mitigate the pH of mud flats with pH 7.59.…”

“…Recent studies investigating the efficacy of shell hash to mitigate acidification of intertidal sediments found it unsuccessful at raising pore-water pH of acidic sediments (Beal et al 2020, Doyle andBendell 2022). Doyle et al (2022) found the addition of shell hash to be site dependent as it reduced variation in pH at an intertidal site with pH 8.03 but did not mitigate the pH of mud flats with pH 7.59. At this point it appears that more research is required to comprehensively understand the effects of shell hash on bivalve growth and survival.…”

Variability and drivers of carbonate chemistry at shellfish aquaculture sites in the Salish Sea, British Columbia

“…One of the beach locations in Okeover is a shell midden. Shell middens are prominent features along the coast of British Columbia and are being evaluated for their potential for mitigating the effects of OA in the region, as it is thought that dissolution of the shell hash will add TA back into the water and elevate pH (e.g., Doyle and Bendell, 2022;Kelly et al, 2011).…”