Saltwater Changes Everything: Why Coastal and Ocean Microplastic Detection Requires New Field Methods

Oceans and saltwater bodies comprise approximately 97 % of Earth’s water by volume and form the dominant component of the planet’s hydrosphere. Despite this overwhelming scale, virtually all existing microplastic and nanoplastic detection methods have been developed and validated in freshwater systems, simplified laboratory matrices, or centralized lab workflows. Practical, scalable, field-ready methods optimized for the high-ionic-strength, dynamic conditions of coastal, estuarine, and open-ocean environments remain almost nonexistent. This paper presents a field-oriented framework for scalable microplastic and nanoplastic screening in saltwater and coastal environments using a smartphone-based zero-shear optical interaction platform. Supporting evidence includes controlled saltwater laboratory studies, mixed microplastic–nanoplastic experiments, time-resolved optical behavior, and real-world field validation in San Francisco Bay under fully uncontrolled outdoor conditions. These findings demonstrate that effective ocean monitoring requires methods engineered specifically for real marine environments rather than adapted from freshwater or idealized lab conditions. Saltwater is not a niche use case — it is the dominant matrix on Earth and one of the greatest opportunities for meaningful environmental impact.