The production of plastic around the world has rapidly increased since the 1950s leading to large amounts of plastic entering the environment. Small plastic particles, referred to as microplastics (MP; 1–1000 µm), are found in high levels in aquatic environments around the globe. Previous studies have mainly sampled larger sized MP with nets that have a mesh that captures MP > 333 µm in size, thus underestimating the levels of MP. This study proposed a protocol for sampling smaller sized MP between 3–500 μm in estuarine and marine surface water and utilized this protocol to measure the MP concentration in three highly urbanized aquatic environments surrounding Long Beach, California. The largest concentrations of MP was found in the Los Angeles River at an estimated average of 641,292 MP m⁻³, followed by 113,627 MP m⁻³ in the Long Beach Harbor, and 63,359 MP m⁻³ found in the San Gabriel River. To test the toxicity of these environmentally relevant levels of MP, Daphnia magna were exposed to different concentrations of microplastic beads (MPP) and microplastic fibers (MPF) in order to assess the effects of MPP and MPF on algal ingestion and reproduction. Only a concentration two orders of magnitude above environmentally relevant levels significantly decreased algal ingestion, and no concentration of MPP or MPF tested significantly impacted Daphnia reproduction. To fully address risk, more studies need to quantify the abundance of smaller sized MP, of which environmentally relevant levels are expected to exponentially increase due to the projected increase in plastic production.