Non-target tandem mass spectrometry enables the prioritization of anthropogenic pollutants in seawater along the northern San Diego coast

Anthropogenic pollutants inundate marine ecosystems as human population growth and urbanization rapidly increase along the coast. The analytical methods applied to detect anthropogenic imprints in coastal seawater are historically aimed at measuring and monitoring a restricted number of compounds. More recently applied non-targeted approaches are typically still limited by spectral and structure library coverage. To prioritize anthropogenic impacts in marine systems beyond compounds present in libraries and suspect lists, we applied non-targeted liquid chromatography tandem mass spectrometry in combination with molecular networking. We integrated an advanced data analysis pipeline that allows scalable comparison of chemotypes between samples in addition to expanded compound annotation using molecular networking. Using this workflow, we explored the chemical impacts of a major rain event in January 2018 in coastal San Diego, USA in a total of 120 samples. We detected 4384 ion features and could annotate 92 MS/MS spectra by spectrum library matching of which 40 annotations were putative xenobiotics. The observed seawater chemotype shifted significantly after the rain event. Molecular drivers of this shift could be attributed to multiple anthropogenic compounds, such as pesticides (Imazapyr and Isoxaben), cleaning products (Benzyl-tetradecyl-dimethylammonium) and chemical additives (Hexa(methoxymethyl)melamine). Expanding the search of identified xenobiotics to other public tandem mass spectrometry datasets, we could further contextualize their possible origin and show their potential importance in other ecosystems. Ultimately, the mass spectrometry and data analysis pipelines applied here offer a scalable framework for future molecular mapping and monitoring of marine ecosystems, which we hope will contribute to a more deliberate assessment of how chemical pollution impacts marine environments.