Lateral Advection of Particulate Organic Matter in the Eastern Indian Ocean

The Argo Abyssal Plain (AAP), south of Java and northwest of Australia in the tropical eastern Indian Ocean, is an oligotrophic region (low in chlorophyll and nutrients), downstream of the Indonesian throughflow. The processes that supply nitrogen to the AAP and support ecosystem production are unknown. Here we quantified lateral advection of particulate organic matter (POM) and the role of this advected POM in supplying new nitrogen (i.e., allochthonous nitrogen supply) to the mixed layer of the AAP using 14 yr of remotely sensed data, combined with sampling undertaken on a research cruise in February 2022. Our results indicate that the average net primary productivity of phytoplankton in this offshore oligotrophic region is 98.5 Gg C d(-1) and that lateral advection transports 1.21 Gg C d(-1) of particulate organic carbon (POC) into the region. We find that lateral advection of POM within the mixed layer supplies an annual average of 12% (95% C.I. = 5.2%-49%) of allochthonous "new" nitrogen supporting phytoplankton productivity, if regenerated to nutrients through microbial processes. Accounting for lateral transport in the deep euphotic zone, lateral transport supplies an average of 32% (10%->100%) of new nitrogen, although this calculation is less certain due to inability to constrain subsurface POM fields. Our data suggest that lateral advection is a quantitatively important but not dominant source of nitrogen supporting new production. Overall, approximately half of the lateral transport is driven by transient eddies (mostly during winter) highlighting the importance of mesoscale circulation.