Science Advisory Report 2022/049

Evaluation of factors affecting the ion-selective electrode (ISE) electrochemical measurement of total free sulfide in marine sediments

Summary

• Sediment sulfide concentrations are used as an indicator of oxic state and biodiversity in soft sediments. Under the DFO Aquaculture Activities Regulations (AAR) and provincial regulations, aquaculture industry operators are required to conduct monitoring of marine sediment sulfide near finfish aquaculture sites. Sediment sulfide concentrations are used to assess the potential impact of organic matter on the benthic environment. Should regulatory thresholds be exceeded, management actions are required.
• The ion-selective electrode (ISE) method is the prescribed method for the measurement of sulfide concentrations from sediment samples in the monitoring standard associated with the AAR, as well as provincial aquaculture regulatory documents specific to New Brunswick and Nova Scotia.
• Sulfide concentrations in marine sediment are difficult to measure because they can change when exposed to oxygen, which is likely to occur during sampling and/or analysis. Multiple factors increase the error and variability of results generated using the ISE method. These include differences in procedures (e.g., sampling, sample storage, and analysis protocols), as well as potential lack of consistency in the implementation by analysts and laboratories across the country, resulting in varying levels of differences in measured sulfide concentrations.
• As compared to the total free sulfide measured immediately after sample collection, the effect of storing sediment samples on the measurement of total free sulfide by ISE is unpredictable and inconsistent. Within four hours, the magnitude of the differences can exceed either a decrease or increase of 15% relative to the initial measurement and, in some cases, can be much greater than the initial measurement. Sulfide in different sediment compositions responds differently to the same storage conditions and duration.
• To minimize uncertainties associated with sediment storage, ISE measurements on sediment samples ideally should be conducted as soon as the sample is collected. Where this is not feasible, the interpretation of the results should take into account the uncertainty associated with storage. At this time, it has not been determined which storage conditions and/or durations result in expected total free sulfide measurements within ± 5%, 10% or 15% of the value obtained from measuring total free sulfide immediately following sample collection.
• The refinement and nation-wide adoption of an updated standard operating procedure (SOP) for ISE measurement of total free sulfide from marine sediment will reduce uncertainty.
• When equipment is properly and newly calibrated and samples are not stored, the precision associated with the ISE analytical methodology is typically about an order of magnitude less than the spatial variability among sediment samples collected at various locations around a marine finfish aquaculture site or reference site.
• Despite the uncertainties, the ISE method has the ability to resolve differences between low (e.g., hundreds µM, i.e., oxic) and high (e.g., thousands µM, i.e., anoxic) sulfide concentrations characteristic of enriched sediments. However, in the middle ranges there is increased variability due to the patchiness of organic deposition and other sources of variability. In this case, sampling design may be modified to account for this variability.
• It is recommended that the ISE method be formally validated for its use in regulatory monitoring and decision-making to ensure increased robustness and confidence in results with respect to regulatory thresholds.
• The strengths and weaknesses of ISE-based measurements of sulfide in the context of potential alternative measurement methods and regulatory thresholds could also be examined to determine if more suitable or additional alternatives exist.

This Science Advisory Report is from the May 10-12, 2022 National Peer Review on the Evaluation of factors affecting the ion-selective electrode (ISE) electrochemical measurement of total free sulfide in marine sediment. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

Science Advisory Report 2022/049 (PDF, 2,274 KB)

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