Risk characterization of microplastics in San Francisco Bay, California

Abstract

Assessing microplastics risk to aquatic ecosystems has been limited by lack of holistic exposure data and poor understanding of biological response thresholds. Here we take advantage of two recent advances, a toxicological meta-analysis that produced biotic response thresholds and a method to quantitatively correct exposure data for sampling methodology biases, to assess microplastic exposure risk in San Francisco Bay, California, USA. Using compartment-specific particle size abundance data, we rescaled empirical surface water monitoring data obtained from manta trawls (> 333 μm) to a broader size (1 to 5000 μm) range, corrected for biases in fiber undercounting and spectroscopic subsampling, and assessed the introduced uncertainty using probabilistic methods. We then compared these rescaled concentrations to four risk thresholds developed to inform risk management for California for each of two effect categories/mechanisms - tissue translocation-mediated effects and food dilution - each aligned to ecologically relevant dose metrics of surface area and volume, respectively. More than three-quarters of samples exceeded the most conservative food dilution threshold, which rose to 85% when considering just the Central Bay. Within the Central Bay, 38% of the samples exceeded a higher threshold associated with management planning, which was statistically significant at the 95% confidence interval. For tissue translocation-mediated effects, no samples exceeded any threshold with statistical significance. The risk associated with food dilution is higher than that found in other systems, which likely reflects this study having been conducted for an enclosed water body. A sensitivity analysis indicated that the largest contributor to assessment variability was associated with estimation of ambient concentration exposure due to correcting for fiber undercounting. Even after compensating for biases associated with fibers and other small particles, concentrations from the trawl samples were still significantly lower than the 1-L grab samples taken at the same time, suggesting our SFB risk estimates are an underestimate. We chose to rely on the trawl data because the 1-L grab sample volume was too small to provide accurate spatial representation, but future risk characterization studies would be improved by using in-line filtration pumps that sample larger volumes while capturing a fuller range of particle size than a towed net.

Maximizing Realism: Mapping Plastic Particles at the Ocean Surface Using Mixtures of Normal Distributions

“…Since the errors in these parameters are known (Table ), uncertainty in the parameters can also be taken into account probabilistically. As recently demonstrated, having mathematical equations for ecologically realistic mixtures of microplastics (i.e., eq ) offers great opportunities to quantify and align the ecologically relevant metrics (ERMs) used in microplastic risk characterization. ,,,− …”

“…Kooi and Koelmans (2019) have taken a first step in building new best practises . They propose to move away from the sole use of discrete classifications and to map the characteristics of plastics through continuous distributions. , Besides capturing the heterogeneous multidimensional nature of plastics, parametrizations of these distributions allow for comparison between studies, irrespective of laboratory or sampling setups, and will be helpful in probabilistic risk modeling. ,,− …”

The emergence of microplastics: charting the path from research to regulations

“…38,39 Currently, hazard thresholds for marine ecosystems are only available for surface water, so occurrence data in the other matrices listed were not evaluated for risks. 9 Surface water data were assessed for quality according to the criteria dened in Koelmans et al, 40 and while data obtained using manta trawl nets was considered appropriate for risk assessment, data obtained by 1 L grab samples could not be used due to the low sampling volume (at least 500 L required). 9 To enable comparisons between the ambient concentrations obtained via manta trawl and the ecological risk thresholds developed by Mehinto et al, 10 three types of corrections were performed: 9 monitoring data (>333 mm) was rescaled to the hazard thresholds size distribution (1 to 5000 mm) using probability distributions; systematic exclusion of microber particle counts by analysts was accounted for based on a subset of manta trawl samples in which all bers were counted (i.e., 41 ); and overcounting of plastic particles due to spectroscopic interference was corrected for using FTIR data reported in Zhu et al,.…”

“…38 Uncertainties due to corrections were propagated probabilistically using best available techniques, including derivation of probability distributions and Monte-Carlo modelling. 9 The majority of corrected surface water samples from San Francisco Bay (82%) exceeded the most conservative risk threshold (i.e., 0.3 particles per L based on food dilution), indicating that additional investigative monitoring would be warranted according to the risk assessment framework developed by the expert workshop (i.e., 10 ). 9 Far fewer samples exceeded thresholds recommending more costly regulatory actions such as discharge monitoring (27%), management planning such as setting a total maximum daily load or related measures (21%) or controlling MPs emissions at sources (3%).…”

“…7 However, recent developments to address the multidimensionality of this emerging contaminant suite now allow for more reliable assessments of risks than previously possible (albeit still with signicant uncertainty). [8][9][10] These advances provide the scien-tic foundation for governmental bodies to make regulatory decisions regarding MPshowever some may choose to act in a precautious manner regardless. 7 While these and other recent scientic advances provide fundamental and long-awaited tools to monitor and assess risks of MPs in the environment, managing MP pollution will likely require far-reaching and disruptive actions across many facets of society.…”