Publication Date

Spring 2026

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Moss Landing Marine Laboratories

Advisor

Holly Bowers; Maxime Grand; Pam Krone; Tom Connolly

Abstract

Agricultural plastics are an emerging source of microplastics (MPs) to coastal marine environments, yet their effects on marine phytoplankton remain poorly understood. In coastal regions surrounding Monterey Bay (California, USA), plastic mulch used in agriculture can fragment into MPs during weathering and removal, entering marine waters via runoff and wind transport. This study investigated the effects of environmentally relevant concentrations of agricultural MPs on the growth and physiology of four marine phytoplankton species: two harmful algal bloom (HAB) taxa (Alexandrium sp. and Pseudo-nitzschia multiseries) and two non-HAB taxa (Thalassiosira sp. and Prorocentrum sp.). Each species was cultured independently and exposed to virgin and field-weathered polyethylene (PE) and biodegradable (BD) MPs. Growth rates, pulse amplitude modulated (PAM) fluorometry, and chlorophyll-a per cell were used to assess responses. Growth of P. multiseries, Alexandrium sp., and Prorocentrum sp. was not significantly affected by MP exposure. In contrast, Thalassiosira sp. exhibited reduced growth under high concentrations of weathered BD MPs, accompanied by increased chlorophyll-a per cell, suggesting a shift in cellular resource allocation toward maintaining photosynthetic capacity rather than cell division. These findings demonstrate that agricultural MPs, particularly weathered BD plastics, can differentially affect marine phytoplankton with potential consequences for species composition and primary productivity in coastal ecosystems.

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