Karl C. Bowles

The biotic ligand model: a historical overview

During recent years, the biotic ligand model (BLM) has been proposed as a tool to evaluate quantitatively the manner in which water chemistry affects the speciation and biological availability of metals in aquatic systems. This is an important consideration because it is the bioavailability and bioreactivity of metals that control their potential to cause adverse effects. The BLM approach has gained widespread interest amongst the scientific, regulated and regulatory communities because of its potential for use in developing water quality criteria (WQC) and in performing aquatic risk assessments for metals. Specifically, the BLM does this in a way that considers the important influences of site-specific water quality. This journal issue includes papers that describe recent advances with regard to the development of the BLM approach. Here, the current status of the BLM development effort is described in the context of the longer-term history of advances in the understanding of metal interactions in the environment upon which the BLM is based. Early developments in the aquatic chemistry of metals, the physiology of aquatic organisms and aquatic toxicology are reviewed first, and the degree to which each of these disciplines influenced the development of water quality regulations is discussed. The early scientific advances that took place in each of these fields were not well coordinated, making it difficult for regulatory authorities to take full advantage of the potential utility of what had been learned. However, this has now changed, with the BLM serving as a useful interface amongst these scientific disciplines, and within the regulatory arena as well. The more recent events that have led to the present situation are reviewed, and consideration is given to some of the future needs and developments related to the BLM that are envisioned. The research results that are described in the papers found in this journal issue represent a distinct milestone in the ongoing evolution of the BLM approach and, more generally, of approaches to performing ecological assessments for metals in aquatic systems. These papers also establish a benchmark to which future scientific and regulatory developments can be compared. Finally, they demonstrate the importance and usefulness of the concept of bioavailability and of evaluative tools such as the BLM.

Bioaccumulation and biomagnification of mercury in Lake Murray, Papua New Guinea

The bioaccumulation of mercury in the food webs incorporating the major piscivorous fish species of Lake Murray, Papua New Guinea, has been characterised. Methylmercury concentrations increased with trophic level and the proportion of total mercury present as methylmercury increased from <1% in plants to 94% in piscivorous fish. Methylmercury bioaccumulation factors (BAFs) were similar to those found in temperate environments, with a typical increase of 1 log unit between planktivore and piscivore trophic levels. The greatest bioaccumulation of methylmercury occurred between seston and the water column (log BAF of 5.36). The bioaccumulation of mercury to levels of regula tory concern by the lake’s piscivores was attributable to the biomagnification power of the plankton-based food chain comprising four trophic levels (phytoplankton, zooplankton, planktivore, piscivore) rather than any elevated concentra tions of mercury in waters or sediments. The methylmercury concentrations of individual piscivores were positively correlated with both trophic position, as indicated by d15N measurements, and fish size. Stable-isotope measurements were used to identify fish species where dietary changes occurring with age significantly augmented age-related bioaccumulation of mercury. Résumé: La bioaccumulation du mercure a été étudiée dans le réseau alimentaire reliant les principales espèces de poissons piscivores du lac Murray en Papouasie-Nouvelle-Guinée. Les concentrations de méthylmercure croissent avec le niveau trophique et le pourcentage du mercure total présent sous la forme méthylée augmente de <1% chez les plantes à 94% chez les poissons piscivores. Les facteurs de bioaccumalation du méthylmercure (BAFs) sont semblables à ceux que l’on retrouve dans les milieux tempérés et i l y a typiquement une augmentation d’une unité logarithmique entre les niveaux trophiques planctonophage et piscivore. La bioaccumulation la plus considérable a lieu entre le seston et la colonne d’eau (log BAF de 5,36). La bioaccumulation du mercure à des niveaux inquiétants (d’après les normes actuelles) par les piscivores du lac est causée par le pouvoir de bioamplification de la chaîne alimentaire à base de plancton qui couvre quatre niveaux (phytoplanctonique, zooplanctonique, planctonophage, piscivore) plutôt qu’à une augmentation des concentrations de mercure dans les eaux ou dans les sédiments. Les concentrations de méthylmercure chez les différents piscivores sont en corrélation positive à la fois avec leur position trophique, telle que déterminée par les mesures de d15N, et avec leur taille. Les analyses à l’aide d’isotopes stables ont permis de reconnaître les espèces de poissons chez lesquelles des changements de régime alimentaire au cours de la vie augmentent significativement la bioaccumulation du mercure reliée à l’âge. [Traduit par la Rédaction] Bowles et al. 897

Determination of the mercury complexation capacity of natural waters by anodic stripping voltammetry

Abstract The complexation of mercury by dissolved organic matter (DOM) in natural waters was investigated using Osteryoung square wave anodic stripping voltammetry (OSWASV). The method employed a gold disc electrode and utilised medium exchange (0.1 M perchloric acid/2.5 mM hydrochloric acid) to avoid interferences during the stripping step. The limit of detection (3σ) was 0.11 nM (22 ng l−1) and the relative standard deviation at a mercury concentration of 5.0 nM was 2.8%. Mercury complexation capacities were determined on a range of fresh and saline water samples. In all samples, ligand concentrations were greatly in excess of dissolved mercury concentrations and ranged from 1.35 to 4.47 nM. Log conditional stability constants of the mercury-DOM complexes were in the range 9.7–10.8. The results suggest that the speciation of mercury in natural waters is dominated by complexation with natural organic matter.

A Comparison of Copper Speciation Measurements with the Toxic Responses of Three Sensitive Freshwater Organisms

Environmental Context. A rapid Chelex resin method is shown to be a valuable speciation screening tool for use in a tiered risk assessment of copper toxicity in fresh waters. It is a more conservative measure than toxicity testing with sensitive biota, but a better indicator of toxicity than a dissolved copper measurement. Abstract. Twelve natural fresh waters with similar pH and hardness, but varying dissolved organic carbon (DOC) and copper concentrations, were assessed for (a) toxicity to an alga (Chlorella sp. 12), a cladoceran (Ceriodaph- nia cf. dubia) and a bacterium (Erwinnia sp.), and (b) copper speciation using a rapid Chelex extraction method, diffusive gradients in thin films (DGT) and anodic stripping voltammetry (ASV). In synthetic fresh water with no added DOC, at pH 7.0 and low hardness, the toxic responses (EC/IC50) of all three organisms to copper were similar. However, in the toxicity of copper added to natural water samples exhibited a negative linear relationship to DOC (r 2 = 0.82-0.83), with respective slopes for algae, cladocerans and bacteria decreasing in the ratio 7.4 : 3.5 : 1. The marked difference in responses in the presence of natural dissolved organic matter indicated that not all of the organisms conformed to the free ion activity model (FIAM). This was confirmed by copper ion selective electrode measurement of copper ion activity. Copper toxicity to algae in the presence of DOC was overestimated by free ion activity possibly due to surface binding of DOC. Copper toxicity to the bacteria was greater than predicted and was shown to be a result of bioavailability of some copper complexes formed with organic matter. Cladocerans appear to more closely follow FIAM predictions. These findings have important implications for attempts to extend predictive models of metal toxicity beyond fish to more sensitive freshwater species. The measured labile copper concentra- tions of copper-spiked natural waters varied from 0 to 70% of total copper concentrations. There was no clear relationship between the three measurement techniques. Good correlations were obtained between both algal and bacterial growth inhibitions measured on copper-spiked natural waters and the corresponding Chelex-labile copper concentrations. A single natural water sample was manipulated to different pH and hardness values, spiked with copper, and tested using the above three organisms with the Chelex method.Toxicity test results generally agreed with studies performed in synthetic fresh waters, showing that the relationships between toxicity, pH and hardness were organism-specific. Chelex-labile copper was always over-predictive of toxicity but significantly better (P ≤ 0.05) than dissolved copper concentrations, as it only detects the fraction of total copper that is reactive over biologically-relevant timescales. Colloidally-bound copper and copper associated with strong ligands are not detected. The Chelex method is therefore useful as a measure where speciation is accepted in water quality regulations.

Determination of methylmercury in sediments by steam distillation/aqueous-phase ethylation and atomic fluorescence spectrometry

Abstract A method is described for the determination of methylmercury in natural sediment samples using steam distillation followed by aqueous phase derivatisation and gas chromatography-atomic fluorescence spectrometry. Recoveries of methylmercury spikes to wet sediments were >95% except for an industrially-contaminated, sulfidic sediment (76% recovery). The method precision was typically

MERCURY CYCLING IN LAKE GORDON AND LAKE PEDDER, TASMANIA (AUSTRALIA). II: CATCHMENT PROCESSES

The sources and concentrations of total mercury (total Hg) and methylmercury (MeHg) in the upper catchment of the Lake Gordon/Lake Pedder system in Tasmania, Australia were investigated. The catchment area, which contains over 50% wetlands, is located in a temperate region with no obvious point sources of mercury. Surface waters in the region had concentrations of total Hg ranging from 1.2 to 14.4 ng L-1 and MeHg from < 0.04 to 1.4 ng L-1. MeHg concentrations were seasonally dependent, with the highest concentrations occurring in summer. Sediments/soils in the catchment had concentrations of total Hg ranging from 4.0 to 194 ng g-1 and MeHg from <0.02 to 20.1 ng g-1. The low concentrations of total Hg confirmed that this region is pristine as regards mercury and has no geological enrichment of total Hg. The highest total Hg and MeHg concentrations in both sediment/soils and waters were found in bogs whereas the lowest concentrations typically occurred on the wetlandplains. MeHg concentrations, in bog and swamp sediments were correlatedwith the organic matter content (r = 0.942, P < 0.001). Acid volatile sulfide (AVS) measurements indicate that in most sediments AVS was greater than total Hg. Given the high reactivity of inorganic mercury and sulfide, this suggests that most of the particulate mercury in sediments is present as mercuric sulfide. The yield of MeHg from the catchment was estimated to be 3.2 mg ha-1 yr-1 and is higher than published rates measured in non-contaminated temperate catchments in the northern hemisphere. The higher yield was attributed to the generally warmer climatic conditions that favour net methylation and the relatively high rainfall (2–3 m yr-1) of the region, which supplies reactive inorganic mercury to the active zones ofmercury methylation and also flushes MeHg from the catchment.

Determination of butyltin species in natural waters using aqueous phase ethylation and off-line room temperature trapping

Monobutyltin (MBT), dibutyltin (DBT) and tributyltin (TBT) were determined in natural water samples by aqueous phase ethylation with sodium tetraethylborate (STEB), room temperature trapping of the resulting volatile derivatives on Tenax TA®, followed by gas chromatography–quartz furnace atomic absorption spectrometry (GC–QFAAS). Recoveries of butyltin spikes from natural water samples were 90–109% at concentrations of ∼100 ng Sn/l. The method precision at ∼100 ng Sn/l was ≤6% RSD for butyltins spiked into natural waters. The detection limits for 1 l water samples were <1 ng Sn/l for all butyltin species. Sample throughput of the method is high (greater than three samples per hour) due to the two-stage nature of the procedure, which allows derivatisation/trapping and GC–QFAAS quantitation to be performed separately. Off-line trapping is also advantageous as it extends the life of the GC column and quartz furnace to at least 12 months due to minimisation of carry-over of co-purged material.

MERCURY CYCLING IN LAKE GORDON AND LAKE PEDDER, TASMANIA (AUSTRALIA). I: IN-LAKE PROCESSES

The processes affecting the concentrations of total mercury (total Hg) and methylmercury (MeHg) in a freshwater system comprising two connected reservoirs in southwest Tasmania were investigated. Surface concentrations of total mercury (total Hg)were temporally and spatially uniform in both Lake Gordon (2.3±0.4 ng L-1, n = 27) and Lake Pedder (2.3±0.3 ng L-1, n = 11). The surface concentrations of MeHg in Lake Gordon (0.35±0.39 ng L-1, n = 25) were more variable than total Hg and MeHg typically comprised 10–20% of total Hg. The relatively high amount of total mercury present as MeHg in Lake Gordon was attributed to the high proportion of wetlandsin the upper catchment (50% of total area) and in-lake contributions (ca. 40% of total MeHg). Despite the close proximity of the two lakes, MeHg concentrations in Lake Pedder were consistently lower than in Lake Gordon. This phenomenon canbe explained in part by the greater contribution of direct rainfall to Lake Pedder leading to the dilution of MeHg. Water column MeHg concentrations were higher in warmer months in bothlakes, reflecting increased net methylation of inorganic mercury.Unlike previous studies of seasonally anoxic lakes, depth profiles of total mercury and MeHg in Lake Gordon were uniform and were not affected by water column stratification occurringin the summer months, and oxygen depletion with depth. This suggests that redox cycling and accumulation of MeHg in the hypolimnion following seasonally-induced anoxia is not a significant part of the mercury cycle in Lake Gordon. The primary location of MeHg production within the lakes water column is not conspicuous. Mercury speciation measurements made above and below the lake system over a period of 19 months indicates that after 20 yr of impoundment, the reservoirs are not significantly affecting MeHg concentrations in the downstreamriverine environment.

Depuration of perfluoroalkyl substances from the edible tissues of wild-caught invertebrate species.

Detection and quantification of poly- and perfluoroalkyl substances (PFASs) in aquatic organisms is increasing, particularly for saltwater species. Depuration can remove PFASs from the tissues of some species once they are removed from the contaminant source, but it is not known if this process occurs for saltwater crustaceans. Such information is important for managing human health risks for exploited migratory species following exposure. We present the results of a depuration trial for School Prawn (Metapenaeus macleayi) and Mud Crab (Scylla serrata), two commercially important crustaceans in Australia. Perfluorooctane sulfonate (PFOS), perfluorohexane sulfonate (PFHxS), and perfluorooctanoic acid (PFOA) were present in samples of both species collected following exposure under natural conditions in contaminated estuaries. Depuration was tested in uncontaminated water for 33days. PFOA was present at levels close to LOR in both species, and was not detected after 4.5h and 72h in School Prawn and Mud Crab respectively. PFHxS was rapidly depurated by School Prawn, and had a depuration half-life of 5.7h. PFOS was also depurated by School Prawn, with a depuration half-life of 158.5h. PFHxS and PFOS concentrations were highly variable in Mud Crab both at the start, and during the depuration experiment, and a depuration model could not be fitted to the data. For School Prawn, depuration of total PFASs to the relevant screening value for protection of human health (9.1μgkg-1) occurred within 7.1h. Rapid depuration of PFASs in School Prawn indicates that human health risks associated with consumption may decrease as this species migrates away from the contamination source. Further research is required to better understand the relationships between contaminant load and life-history characteristics (such as growth, reproduction, and moult cycle) in Mud Crab, and future work should target broader time frames for depuration in this species.

Mercury speciation in waters and sediments of Lake Murray, Papua New Guinea

The speciation of mercury in Lake Murray, Papua New Guinea and nine surrounding rivers was investigated. Total mercury concentrations in waters (0.52-9.5 ng L-1) and sediments (56-177 ng g-1 dry mass) were similar to those found in other remote lake environments. Total methylmercury (MeHg) concentrations in waters and sediments were also low. Of the 50 water samples analysed during three surveys, only six samples had total MeHg concentrations between 0.1 and 0.64 ng L-1 and 25 of the samples had total MeHg concentrations < 0.05 ng L-1. Methylmercury concentrations were very variable and could not be linked with parameters known to affect mercury methylation such as pH, dissolved organic carbon and sulfate concentrations. A statistically significant and unexplained correlation was observed between filterable MeHg and filterable silicon. Intermittent inputs of turbid water from the Strickland River inject particulates and filterable MeHg into the southern end of Lake Murray. This has resulted in the formation of a depositional footprint that contains higher concentrations of particulate mercury and other elements compared with the rest of the lake.