Long‐Range Lateral Transport of Dissolved Manganese and Iron in the Subarctic Pacific.

Dissolved manganese (dMn) and iron (dFe) are important micronutrients for marine microorganisms. In this study, we presented comprehensive distributions of dMn in the subarctic Pacific and compared our Mn data with those of dFe in the same region. dMn and dFe exhibited a scavenged‐type and nutrient‐type vertical profile, respectively. We also determined the size fractionation of Mn at two stations and found that most of dMn was in the soluble fraction, in contrast to dFe, which included colloidal organic particles. In the coastal surface waters of the Gulf of Alaska, Mn was supplied from a riverine source. Drawdowns of dFe, dMn, and macronutrient were observed along the salinity gradient. Both dMn and dFe were transported eastward, with log‐scale transport distances of 3,600 and 3,300 km, respectively in the upper intermediate water, and 4,700 and 3,400 km, respectively in the lower intermediate water. In the upper intermediate water, dMn showed a positive relationship with apparent oxygen utilization, suggesting that regeneration processes could supply dMn to these waters. However, mixing with water masses from the subtropical North Pacific reduced dMn concentration. High dissolved oxygen concentrations in these waters also increased the rate of dMn loss relative to the lower intermediate water. The transport distances of dMn found in this study were the highest in the Pacific region. This long‐range transport of dMn and dFe could eventually fuel phytoplankton growth at the surface via upwelling processes. Plain Language Summary: Phytoplankton requires manganese and iron to grow. In our study, we revealed the distributions of dissolved manganese in the subarctic Pacific and compared our results with those of dissolved iron in that region. Concentrations of dissolved manganese were high at the surface and decreased with depth, while concentrations of dissolved iron were low at the surface and increased with depth. Most dissolved manganese was smaller in size than dissolved iron, which existed in large organic complexes. Dissolved manganese concentrations decreased with increasing salinity in the surface waters of the Gulf of Alaska, suggesting that manganese may be supplied from freshwater sources such as rivers. In the intermediate water, both dissolved manganese and iron were transported for long distances, which we found to be the longest in the Pacific region. We found that remineralization of particulate organic matter may supply manganese to the upper intermediate water. However, dissolved manganese concentration decreased when mixed with waters coming from the subtropical North Pacific. High concentration of dissolved oxygen also removed dissolved manganese by oxidizing manganese into nonsoluble manganese oxides. Dissolved manganese and iron in the intermediate water may eventually reach the ocean surface, providing phytoplankton with additional micronutrients for growth. Key Points: Temporal variability of dissolved manganese in intermediate waterTransport distances of dissolved manganese and iron in intermediate water were the longest in the PacificLow pH and dissolved oxygen conditions facilitated long‐range transport in intermediate water [ABSTRACT FROM AUTHOR]