Controls on surface distributions of dissolved organic carbon and nitrogen in the southeast Pacific Ocean

Abstract

Here we use data from three meridional survey cruises that took place in 1994, 2007/2008 and 2016/2017 in the southeast Pacific Ocean to investigate controls on surface distributions of dissolved organic carbon and nitrogen (DOC and DON, respectively). While DOC and DON production occur simultaneously in the euphotic layer of the gyre margins, budgets that account for horizontal transport, production and consumption are not balanced across the region. Our results show that while DOC concentrations increase inside the gyre, DON remains at similar levels in comparison to margins. There is little interannual variability in DOC and DON trends, except near the equator, explained by ENSO phases and seasonality. While the effect of evaporation (precipitation) did not significantly contribute to variable DOC and DON concentrations, differences in the naturally occurring 15N/14N isotope ratios from the most recent cruise suggest net consumption of allochthonous DON inside the gyre. In the northernmost and southernmost stations, where surface [NO3−] was detected, 15N-DON showed low signatures in the upper 300 m of 3.8 ± 1.5‰ and 2.5 ± 1.6‰, respectively. Inside the gyre where [NO3−] was undetectable, higher values averaging 5.0 ± 0.9‰ were observed. This suggests that a fraction of DON turns over more rapidly than the time scale of horizontal transport, with the isotopic change reflecting the gradient in 15N-DON of newly produced organic matter. Thus, allochthonous DON is likely a significant source of nitrogen to microbial communities inside this nitrogen-limited environment and should be taken into consideration when closing nutrient budgets to estimate productivity in the South Pacific Subtropical Gyre.