Factors influencing the use of dissolved aluminum as a source tracer in the East China Sea and adjacent waters

To better understand the biogeochemical cycles of dissolved aluminum (Al) in the East China Sea (ECS) and adjacent waters in summer, a large spatial coverage investigation based on four expeditions was conducted in July 2012 and 2013. The concentration of dissolved Al decreased across the ECS shelf as the distance from the coast increased. The maximum concentration (97.7 nmol/L) was found in the near-bottom waters off the Min-Zhe coast. This value occurred in water depths with lower salinity and higher turbidity, indicating sources from fluvial input and sediment resuspension. Significant biological removal of dissolved Al occurred in the upper waters (>30 m) in the central northern ECS. This finding was supported by the negative correlation between dissolved Al and chlorophyll fluorescence. Higher levels of dissolved Al occurred in the subsurface layers of stations HR2 and HR9 along the Kuroshio Waters (KW) above the Okinawa Trough, indicating contributions of terrestrial Al from the ECS shelf. The behavior of dissolved Al is mostly conservative in the southern ECS, particularly in subsurface layers. The presence of an Al plume indicated that cross-shelf transport over the ECS shelf was evident at the isopycnal layers at s_q = 23.5 ± 0.5 kg/m³. The possible transport passage originated from the near-bottom layer of the Min-Zhe coast, extended north-eastward across the PN section and then shifted south-eastward through the ECS shelf to the KW at 30° N. Due to the seasonal variation of the physical fields in the ECS, the cross-shelf transport in summer 2013 differed from that in spring 2011, where the transport passage and the export position to KW moved approximately a latitude northward, and the flux decreased during our field observations.