Diatom-Diazotroph Associations in hydrographically defined habitats of the South China Sea

The South China Sea (SCS) is a hydrographically complex and physically dynamic marginal sea of great economic importance. Primary production in the SCS experiences strong seasonal forcing through the monsoon cycle, which affects both riverine runoff and circulation within the basin. The summer monsoon in particular produces a mix of waters affected by the Mekong outflow and coastal upwelling embedded within a dynamic wind-driven surface circulation. Here, we discuss the distribution, abundance, and symbiotic state of a suite of host diatoms and Diatom-Diazotroph Associations (DDAs) in different habitats defined in terms of the physical and biological characteristics of the SCS during the early stages of the SW Monsoon of 2016. DDA host diatoms were broadly distributed throughout our study region, and we found intact symbioses in all of the habitats sampled, though infection rates (abundance of hosts bearing symbionts) and infection intensities (number of symbionts per host) were lowest in waters affected by coastal upwelling. Host infection rates tended to be highest in offshore waters, and DDA host diatoms generally varied widely in size and infection intensity both within and among defined habitats. These differences may reflect different optimal strategies for allocating biomass and energy between host and symbiont.