Alternative detection methods for performance indicators of the oxic state of bottom sediments: indicator inter-calibration and thresholds



Potential impacts associated with biochemical oxygen demanding (BOD) matter effluents at aquaculture sites are assessed by monitoring the oxic state of the bottom sediment. Performance indicators, such as sulfide concentrations and Redox Potential (Eh), are used to determine the effects of BOD matter deposits. Previous research had shown that the standard method for quantifying total free sulfides in sediments can provide inaccurate results as a result of contamination with mineral sulfides, calibration variability, and inappropriate sample storage. Eh measurements were also known to have high variability. There is ongoing research to develop and validate alternative methods for measuring total free sulfides and dissolved oxygen in sediments that solve many of these problems. This project aimed to inter-calibrate available oxic state indicators to ensure they provide comparable information on sediment oxic state, and develop thresholds for the oxic state indicators that equate to the management objective of no greater than 50% biodiversity loss. Additional thresholds were identified which can be used to classify the oxic status of sediments over the full range of possible biological effects from sediment organic enrichment (i.e. oxic to anoxic). A new pore water sampling technology was developed that will greatly reduce aquaculture monitoring costs while also eliminating concerns over the capacity of present sampling approaches (grabs and cores) to provide undisturbed surficial sediment and pore water samples.


The standard ion selective electrode (ISE) method for total free sulfide analysis generally provides biased results related to sample contamination with non-toxic mineral sulfides and/or the oxidation and volatilization of free sulfides prior to analysis. Alternative field and laboratory protocols, based on pore-water analysis, were developed to alleviate these problems. These protocols also included a practical approach for simultaneously measuring pore-water dissolved oxygen concentrations using commercially available micro-sensors. A simple ultra-violet (i.e., UV) spectrophotometry method was extensively field tested and shown to provide rapid and accurate total free sulfide measures in the field. A sample preservation and laboratory-based microplate methylene blue method was simultaneously tested and shown to provide sulfide results comparable to the UV method. In contrast, the ISE method was confirmed to give biased results that were poorly correlated with those obtained with both alternative methods. The simultaneous use of sulfide and oxygen indicators in sediment evaluations more accurately describes the geochemical alterations of sediment that affect benthic communities.


Cranford, P.J., L. Brager, and D. Wong. 2017. A dual indicator approach for monitoring benthic impacts from organic enrichment with test application near Atlantic salmon farms. Marine Pollution Bulletin. 124:258-265.


Innovative aquaculture tools

Program Name

Program for Aquaculture Regulatory Research (PARR)


2016 - 2017

Principal Investigator

Peter Cranford
Marine Ecologist, Bedford Institute of Oceanography