LSNM-NP CDGT Zn-Tox: Demonstration of commercial DGT passive sampler's ability to better predict marine toxicological endpoints of Zinc
In recognition of the need to revisit Marine Toxics Substances Criteria for Zn in surface waters – in a manner that better quantifies bioavailability to sensitive life forms and stages – the Puget Sound Naval Shipyard, the Pacific Northwest National Laboratory and the Naval Information Warfare Center’s Bioassay Laboratory, under the Navy’s ENVironmental inVESTment Project (ENVVEST), investigated the causal links between toxicological endpoint moderation toward Strongylocentrotus purpuratus and Mytilus galloprovincialis larvae and dissolved organic carbon (DOC) quantity and quality, in coastal seawaters. Median effective concentrations (EC50) were determined by diffusive gradients in thin-films (DGT), which can mimic uptake of labile analytes from the environment by diffusion-limited organisms and integrate exposure quantification over windows most relevant to developmental stress. Testing was carried out in seawaters from a range of coastal environments to elucidate the dominant controlling factor of DOC induced ligand competition. EC50s, as quantified by the operationally defined dissolved fraction of Zn (ZnDISS) and by the operationally defined labile fraction of Zn (CDGT Zn), ranged from 96.2 to 161 μg/L as ZnDISS and 47.7 to 74.6 μg/L as CDGT Zn for S. purpuratus. M. galloprovincialis EC50s were unaffected by DOC concentration under positioning within the dilution series. The absolute concentrations of DOC explained 87% of S. purpuratus EC50 variability as CDGT Zn, between marine sites, such that higher concentrations resulted in elevated EC50s, and characterization of DOC by fluorescence spectroscopy confirmed a range of autochthonous and allochthonous ligand mixtures, within the expected range of coastal marine pools. Improved correlations between EC50s as CDGT Zn, relative to EC50s expressed as ZnDISS, and DOC qualitative assessments indicate that DGT labile Zn may be the superior expression of bioavailability toward larval-stage organisms in the marine environment. Irrespective of the observed correlations between CDGT Zn EC50s and DOC quality, the relative homogeneity of ligands in coastal marine waters allowed for protective effects to be estimated solely from CDGT and DOC concentrations while providing an improved ecological assessment relative to that of ZnDISS (R2 = 0.68) – an improvement of 19%. Based on the current findings, and application of the approach taken in U.S. EPA (1995b) and Strivens et al. (2020a), toward protection of S. purpuratus a proposed criterion continuous concentration (CCC) for Sinclair Inlet of 27.4 μg/L as CDGT Zn can be used for conservative ecological assessments of field data; however, application of this interpretive threshold must consider averaging of small scale spatial variability and is subject to adjustments as physicochemical and antagonistic effects on lability are further characterized in industrial mixing zones.