Nepheloid layers impact on C:N ratio of settling organic particles in an ultraoligotrophic sea

The C:N ratio of settling particulate organic matter (POM) in the sea is directly related to both the availability of dissolved nitrogen for primary production and the efficiency of the carbon pump. Little is known about the way lateral transport via intermediate nepheloid layers of organic and lithogenic particles from the continental rivers/shelf/slope affects the C:N ratio of the settling POM at the deep-sea. In this study we analyzed C:N time series of sinking POM, collected by sediment traps at the DeepLev observatory, 50 km offshore Haifa, Israel, and of suspended particles along the water column at the ultra-oligotrophic southeastern Mediterranean Sea. POC:PN ratios in photic layer were 7.2 during HTP (high-transport period in winter) compared to 5.8 during LTP (low-transport period). Using reported POC:PN ratios in suspended sediments over the shelf, we calculated that the higher POC:PN ratios in the open sea sediment traps during HTP resulted from mean contribution of approximately 40% of shelf-derived POM arriving via lateral transport. Additionally, while the POC:PN ratio increase with depth occurred year-round, it was significantly steeper during LTP compared to HTP. This is attributed to the more refractory nature of the laterally transported POM and its association with ample lithogenic particles during HTP which may have hindered POM decay and increased settling velocity. Assuming similar mechanisms in other margins worldwide, this should have an impact on the C:N ratios of marginal seas and basins.