Storage stability : effect of storage time, temperature and preservation method on total free sulfide measurements in marine benthic sediment

Abstract

Within Canada, the main regulatory indicator of impact to soft bottom benthos, used as a marker to gauge the oxic state of the seabed stemming from finfish aquaculture activities, is total free sulfide in sediment. Each region follows their own standard operating procedure (SOP) for monitoring this parameter, whether federally as part of the Aquaculture Activities Regulations (DFO 2018a) which British Columbia follows (DFO 2018b) or provincially, New Brunswick (NB DELG 2018) or Nova Scotia (NS DFA 2021). Determination of sulfide requires collection of sediment followed by quantification using ion selective methodology (Wong and Page, 2025). At present, depending on the jurisdiction, samples must be analysed within 5 minutes of collection in British Columbia (DFO 2018b), within 36 h as stipulated in the federal AAR (DFO 2018a), or within 72 h in New Brunswick (NB DELG 2018) and Nova Scotia (NS DFA 2021). Differences in these time frames have raised questions regarding the effect of storage on the accuracy of the generated sulfide data. The only known research conducted to try to answer this knowledge gap has been performed by Wildish et al. (1999) – sediment details not explicit in their report, and Wong (not published) whose experiments focused on muddy sediment types obtained from intertidal and subtidal sediments collected using hand scoops and surface deployed grabs respectively. Their work investigated whether sulfide could be stabilised in sediment by adding sulfide antioxidant buffer (SAOB), a solution comprising alkaline EDTA and L-ascorbic acid, to sediment samples at the start of storage. The function of SAOB is detailed in Wong and Page (2025). Wong (not published) also explored the possibility of maintaining sulfide levels by vacuum sealing sediment to remove oxygen and thus potentially reducing the oxidation of sulfide. Both of these procedures failed to maintain sulfide levels at, or near the initial determined concentration (hereafter referred to as ‘baseline’ in this report) during the investigated storage periods. Storage of sediment without any preservation methods also did not maintain sulfide levels. Data derived from experiments conducted by Wildish et al. (1999) and Wong (not published) indicated that sulfide’s degradation response is highly variable and unpredictable, and is most likely dependent on substrate type, and environmental/aquaculture setting (e.g., degree of organic loading, oxic/anoxic). Therefore to derive the most accurate sulfide concentration for regulatory purposes, it is recommended that sediment samples should be analysed for total free sulfide immediately after collection and not stored.