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Arctic coastal margins receive organic material input from rivers, melted sea ice, and coastal erosion, phenomena that are all undergoing changes related to global climate. The optical properties of coastal Arctic waters contain information on this organic material, and we examined three optical properties of seawater (absorption, backscatter, and fluorescence) for their relationships to variability in dissolved and particulate organic carbon (DOC and POC) in Stefansson Sound, Alaska, a coastal Arctic embayment. During open water periods in 2018 and 2019, DOC was inversely correlated with salinity (r2 = 0.97) and positively correlated with dissolved organic matter fluorescence (fDOM; r2 = 0.67). DOC showed strong correlation with the nonparticulate absorption coefficient at 440 nm (ag(440)) only in 2018 (r2 = 0.95). The vertical structure of fDOM in Stefansson Sound aligned with density profiles more strongly in 2018 than in 2019, and higher levels of fDOM, ag(440), and backscatter seen near the bottom in 2019 suggest wind‐driven mixing and/or bottom resuspension events. In both years, DOC correlated strongly with the spectral slope of the absorption coefficient between 412 and 550 nm (r2 = 0.70), and POC was well correlated with spectral backscattering at 470, 532, and 660 nm (r2 = 0.90, 0.71, and 0.59). These interannual differences in the spatial and vertical distributions of DOC and POC, and their respective correlations with optical proxies, likely reflect regional climatological factors such as precipitation over the adjacent watersheds, wind patterns, and residual sea ice in late summer. Plain Language Summary: Arctic coastal waters are experiencing drastic changes in response to changing global climate, many of which can affect the biology and biogeochemistry of these waters in the ice‐free summer months. Optical methods are potentially valuable for measuring changes in the amount of particulate and dissolved organic carbon in seawater. To study the biogeochemistry of organic carbon in the coastal Arctic using optical properties, we must first understand their relationships in these optically complex waters. This study examines these relationships within a 2‐year study in Stefansson Sound, a coastal embayment along the Alaskan Arctic Ocean. We found that across 2 years with differing residual sea ice in summer and rainfall trends in the adjacent watersheds, some optical properties can be useful proxies for dissolved and particulate carbon in seawater. For these two specific years, we observed that the amount of dissolved organic carbon found in this region is likely related to the precipitation over the adjacent watersheds, which increases the river flow and delivery of terrestrial organic carbon into these coastal waters. Key Points: Absorption, fluorescence, and spectral slope are conditionally accurate optical proxies for dissolved organic carbon in nearshore coastal Arctic watersRiver discharge appears to be the primary driver of spatial distributions of organic matter in Stefansson Sound during the open water seasonThe backscattering coefficient can serve as a conditional proxy for particulate organic carbon concentration in nearshore coastal Arctic waters [ABSTRACT FROM AUTHOR]

Although bacterioplankton and phytoplankton are generally perceived as closely linked in marine systems, specific interactions between discrete bacterioplankton and phytoplankton populations are largely unknown. However, measurements of bacterioplankton distributions during phytoplankton blooms may indicate specific microbial lineages that are responding to phytoplankton populations, and potentially controlling them by producing allelopathic compounds. Here we use a comprehensive molecular approach to identify, characterize and quantify bacterioplankton community responses to an Oregon coast diatom bloom. Total DAPI counts increased by nearly sevenfold in bloom samples, reaching 5.7 × 109 cells l−1, and lineage-specific cell counts using fluorescence in situ hybridization (FISH) indicated that Bacteria accounted for approximately 89% of observed increases. Several dominant members of the bacterial community present outside the bloom (SAR11 and SAR86) did not contribute significantly to observed increases in bloom samples. Clone library and FISH data indicated that uncultured planctomycetes most closely related to Pirellula, and members of the OM43 clade of beta proteobacteria, reached 0.5 × 108 and 1.2 × 108 cells l−1, respectively, and were among the dominant lineages in bloom samples. [ABSTRACT FROM AUTHOR]