US EPA

The Ecological Risk Assessment Support Center (ERASC) announced the release of the final report, Determination of the Biologically Relevant Sampling Depth for Terrestrial and Aquatic Ecological Risk Assessments. This technical paper provides defensible approximations for what the depth of the biologically active zone, or “biotic zone” is within certain environments. The methods used in this study differ somewhat between Part 1 (Terrestrial Biotic Zone) and Part 2 (Aquatic Biotic Zone). In Part 1, biological activity was quantified in forests and grasslands as a function of depth across selected metrics. In Part 2, the biotic zone(s) in various habitats was based on the 80th percentile of abundance or biomass depth distributions.

Part 1 (Terrestrial Biotic Zone) of this study uses a meta-analysis approach to quantify the zone of highest biological activity for soil-dwelling ecological receptors commonly utilized in ecological risk assessments (ERAs). Endpoints evaluated include: invertebrate density, microbial biomass carbon (C), microbial density, mycelium production, root biomass, root production and total phospholipid fatty acids (PLFA). Results suggest sampling strategies should be adaptive allowing for variable depths. If constant depths are utilized, our results suggest that samples should be collected to a depth of approximately 25−30 cm.

Part 2 (Aquatic Biotic Zone) explores data from a wide realm of habitat types in an attempt to develop habitat specific practical default values for the depth of the biotic zone, where most organism-substrate interactions occur. We recommend that the depth of the biotic zone be based upon the 80th percentile of abundance or biomass depth distributions. The biotic zone, based on benthic abundance, in most estuarine and tidal freshwater environments is 10 or 15 cm. Exceptions are oligohaline and polyhaline mud (5 cm) and oligohaline sand (5 cm). In marine muds (both coastal and offshore), the biotic zone is 15 cm. In other marine substrates it is 10 cm (marine coastal mixed and marine offshore sand) or 5 cm (marine coastal sand). In lentic environments, the biotic zone is 15 cm. The biotic zone tends to be deeper when biomass is taken into account. The biotic zone in lotic systems varies from 15 to 35 cm depending upon water/habitat type. In areas populated by a high density of deep dwelling organisms such as the examples provided, the biotic zone may be somewhat deeper than our recommended values.

Ecological risk assessors are frequently faced with the challenge of defining the biologically active zone, or “biotic zone,” in soils and sediments during the design and interpretation of soil and sediment sampling programs. Knowledge of the biotic zone is necessary when evaluating sediment/soil concentrations, calculating risks to ecological receptors, and attempting to delineate the relevant depth for remediation at sites where an action is needed.

As current practice with regards to determining the biotic zone is quite varied, EPA’s Ecological Risk Assessment Forum (ERAF) submitted a request to Office of Research and Development (ORD)’s Ecological Risk Assessment Support Center (ERASC) to develop a scientifically defensible definition for the depth of the biotic zone in soils and sediments. In response to the ERAF request, the present document attempts to provide defensible approximations for what the depth of the biotic zone is within certain environments. Actual sampling depths may be modified by the assessor based on the purpose of the assessment.

The primary audience for this document is Superfund staff and contractors, and ecological risk assessors, though general ecologists should find the information useful as well. With respect to terrestrial assessments, this study uses a meta-analysis approach to quantify the zone of highest biological activity for soil-dwelling ecological receptors commonly utilized in ERAs.

For decisions related to ecological assessment or remediation in aquatic scenarios, we develop practical default values for the depth of the biotic zone (i.e., biologically relevant sampling depth) in various habitats based on the 80th percentile of abundance or biomass depth distributions.

This technical paper provides defensible approximations for what the depth of the biologically active zone, or “biotic zone” is within certain environments. With respect to terrestrial assessments, this study uses a meta-analysis approach to quantify the zone of highest biological activity for soil-dwelling ecological receptors commonly utilized in ecological risk assessments. For decisions related to ecological assessment or remediation in aquatic scenarios, we develop practical default values for the depth of the biotic zone in various habitats based on the 80th percentile of abundance or biomass depth distributions.

This is the final report.

U.S. EPA. Determination of the Biologically Relevant Sampling Depth For Terrestrial and Aquatic Ecological Risk Assessments (Final Report). U.S. Environmental Protection Agency, Ecological Risk Assessment Support Center, Cincinnati, OH, EPA/600/R-15/176, 2015.