Spatial and Temporal Variations of Microplastics within Humboldt Bay, California

This study aimed to quantify microplastic (MP) concentration and analyze the spatial and temporal variabilities of the concentrations during the tidal cycle in Humboldt Bay, California. To get an approximation of MP concentration, both water and sediment samples were taken at five different stations, twice during one tidal cycle. Sampling was conducted during two different cruises, on the 19th and 21st of September 2020. The samples were processed in the lab using a density separation procedure and filtration. MP concentrations in the different samples were determined using an average optical microscopy count. Comparison of the water column MP concentrations during ebb and flood tides shows higher concentrations during flood tide, 49.0 particles/L ± 32.37 (flood) vs 34.4 particles/L ± 16.32 (ebb), indicating that MPs are brought into Humboldt Bay from the ocean. The comparison of the MP concentrations during lower energy and higher energy conditions indicates that concentrations in the water column were elevated when there was greater tidal kinetic energy, approximated by the covariance of the measured velocity in North Bay Channel. This result was assumed to be caused by the strong tidal currents stirring up both sediments and the settled MPs into the water column. Due to lower tidal kinetic energy on the sediment sampling cruise day, we could not confirm that assumption. Water samples indicated that MPs are heterogeneously distributed in the bay, with higher concentrations found near the Entrance Channel and lower concentrations found further north in the bay. Sediment samples also indicate a heterogeneous distribution of MPs in the bay, with the lowest concentrations near the Entrance Channel, 15 particles/kg, where high tidal currents inhibit settling of particles.