Lateral advection supports nitrogen export in the oligotrophic open-ocean Gulf of Mexico

Abstract

In contrast to its productive coastal margins, the open-ocean Gulf of Mexico (GoM) is notable for highly stratified surface waters with extremely low nutrient and chlorophyll concentrations. Field campaigns in 2017 and 2018 identified low rates of turbulent mixing, which combined with oligotrophic nutrient conditions, give very low estimates for diffusive flux of nitrate into the euphotic zone (< 1 µmol N m−2 d−1). Estimates of local N2-fixation are similarly low. In comparison, measured export rates of sinking particulate organic nitrogen (PON) from the euphotic zone are 2 – 3 orders of magnitude higher (i.e. 462 – 1144 µmol N m−2 d−1). We reconcile these disparate findings with regional scale dynamics inferred independently from remote-sensing products and a regional biogeochemical model and find that laterally-sourced organic matter is sufficient to support >90% of open-ocean nitrogen export in the GoM. Results show that lateral transport needs to be closely considered in studies of biogeochemical balances, particularly for basins enclosed by productive coasts.

A gulf-wide synoptic isoscape of zooplankton isotope ratios reveals the importance of nitrogen fixation in supporting secondary production in the central Gulf of Mexico

“…Nitrate vs. fixed N supporting upper water column production in the deep waters of the GM While we do not discount subsurface nitrate inputs to the euphotic layer of the deep water region of the GM, zooplankton d 15 N values in our study suggest that nitrate cannot be the sole N source supporting primary and secondary production, contrary to what has been suggested by Knapp et al (2021) and Kelly et al (2021) for the northeastern gulf in the region of influence of the Loop Current. Knapp et al (2021) used sediment traps drifting below the euphotic layer in the north-central GM for 2-4 days during late spring to sample sinking particles, and measured their isotopic composition as well as that of subsurface nitrate. They used a box model approach and an d 15 N budget to evaluate the relative importance of N 2 fixation and subsurface NO 3 to export production.…”

“…Specifically, low d 15 N values of zooplankton (1.8‰ to 4‰) have been associated with high N 2 fixation rates measured in the GM during the summer (Holl et al, 2007), and Dorado et al (2012) reported low d 15 N values of zooplankton in the northern oceanic GM (2.8 ± 1.4‰) suggesting that N 2 fixation by Trichodesmium is an important N source in the LC and in LCE. In contrast, Knapp et al (2021) and Kelly et al (2021), based on indirect estimates, reported low N 2 fixation rates in the GM as a whole, suggesting that N 2 fixation is too small to be considered as a relevant N source; these authors also suggested that the low d 15 N values of POM were due to remineralization of primary producers supported by subsurface nitrate (Knapp et al, 2021;Kelly et al, 2021). On the other hand, cyclonic eddies with divergent conditions at their core lead to a shallower nitracline and pumping of subsurface NO 3 to the euphotic zone (Lee-Sańchez et al, 2022), while transport of NO 3 toward the surface can also occur due to a deepening of the mixed layer in winter (Müller-Karger et al, 2015;Damien et al, 2018).…”

“…They found a pronounced latitudinal gradient in the d 15 N values of zooplankton, with the highest d 15 N values in the northern gulf and lower values in the central oligotrophic gulf, but did not examine source contributions. More generally, studies in the GM have focused on the region of influence of the Loop Current (Howe et al, 2020;Knapp et al, 2021), the northwestern gulf (Holl et al, 2007), the west Florida Shelf (Mullholand et al, 2006;Mullholand et al, 2014), and the central, oligotrophic region (Hernańdez-Sańchez et al, 2022).…”

In situ estimates of net primary production in the open‐ocean Gulf of Mexico

“…4c) could not reveal the increased mid-depth nitrate supply. Alternatively, such high NPP signal at mid-depth could also be a result of the lateral transport of organic matter (Kelly et al 2021).…”

The Relationship Between Nitrate and Potential Density in the Ocean South of 30°S

“…Without the Southern Ocean nutrient source, the sum of the vertical processes that transport nutrients upwards across the pycnocline in the mid-and low-latitude ocean would be insufficient to maintain the biological productivity (Sarmiento et al, 2004). The horizontal transport of nitrate has been proved extremely important for productivity in subtropical circulation oligotrophic systems (Kelly et al, 2021;Letscher et al, 2016;Williams & Follows, 1998). It is foreseeable that the abundant nitrate in the upper seawater of the Antarctic Circumpolar Current (ACC) can be transported to the subtropical oligotrophic basin, according to the northward Ekman transport driven by westerly wind (Manabe et al, 1990).…”