Wintertime particulate organic matter distributions in surface waters of the northern California current system

Semi-automated sampling via the surface underway systems of research vessels was used to explore the distribution and composition of particulate organic matter in surface waters of the northern California Current ecosystem during winter, a poorly studied period that is characterized by downwelling favorable winds and elevated discharge by coastal rivers. New wintertime observations were compared to those measured during a summer cruise characterized by strong upwelling and highly reduced coastal river flows. Particulate organic carbon (POC) concentrations in surface waters along the Oregon shelf during winter periods were significantly lower (7.0 ± 5.0 μM) than those measured in the same region during summer (21 ± 25 μM) with similar seasonal contrasts in chlorophyll (Chl) concentrations (1.7 ± 1.7 mg Chl m−3 in winter and 8.7 ± 3.8 mg Chl m−3 in summer). The combination of POC, Chl, and particle beam attenuation (cp) measurements revealed spatial and temporal distributions that confirm the importance of physical drivers such as wind, waves and river discharge in influencing the biogeochemistry of eastern boundary current systems during non-upwelling conditions. Elevated contributions of allocthonous particulate materials with distinct compositional characteristics, including low Chl:POC and POC:cp ratios, were measured during winter in low-density, low-salinity surface waters influenced by coastal river discharge. In contrast, mid-salinity, intermediate-density surface waters exhibited higher concentrations of POM with elevated Chl:POC and POC:cp ratios, which approached those measured during highly productive summer upwelling periods. These results are solid evidence for wintertime phytoplankton productivity along this margin under elevated buoyancy and nutrient contributions from the discharge of coastal rivers.

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