Coastal N₂ Fixation Rates Coincide Spatially With Nitrogen Loss in the Humboldt Upwelling System off Peru

Abstract

Marine nitrogen (N2) fixation supports significant primary productivity in the global ocean. However, in one of the most productive regions of the world ocean, the northern Humboldt Upwelling System (HUS), the magnitude and spatial distribution of this process remain poorly characterized. This study presents a spatially resolved data set of N2 fixation rates across six coastal transects of the northern HUS off Peru (8°S–16°S) during austral summer. N2 fixation rates were detected throughout the waters column including within the Oxygen Minimum Zone (OMZ) between 12°S and 16°S. N2 fixation rates were highest where the subsurface OMZ (O2 < 20 μmol L−1) was most intense and estimated nitrogen (N) loss was highest. There, rates were measured throughout the water column. Hence, the vertical and spatial distribution of rates indicates a colocation of N2 fixation with N loss in the coastal productive waters of the northern HUS. Despite high phosphate and total dissolvable iron (TdFe) concentrations throughout the study area, N2 fixation was still generally low (1.19 ± 3.81 nmol L−1 d−1) and its distribution could not be directly explained by these two factors. Our results suggest that the distribution was likely influenced by a complex interplay of environmental factors including phytoplankton biomass and organic matter availability, and potentially iron, or other trace metal (co)‐limitation of both N2 fixation and primary production. In general, our results support previous conclusions that N2 fixation in the northern HUS plays a minor role as a source of new N and to replenish the regional N loss.

Hydrodynamic regimes modulate nitrogen fixation and the mode of diazotrophy in Lake Tanganyika

“…Selden et al 22 , and references therein). While rates vary widely from below the limit of detection to as high as 100 nM d −1 13 , 15 , 22 – 30 , they do not re-supply the N lost before/during upwelling as predicted by previous biogeochemical models 21 . More recent estimates indicate that N 2 fixation contributes little as a new N source in upwelling regions, contributing only ~ 1% to the export of organic N and carbon 15 .…”

“…The observed physical-biological coupling in Lake Tanganyika, a model system with respect to its hydrodynamics, may help to explain why N 2 fixation rates are generally low (<5 nM d −1 ) in some marine upwelling regions 15 , 22 , 26 – 28 , 30 , 80 , 81 . Many of these upwelling systems are also associated with gammaproteobacterial heterotrophic and often chemotactic diazotrophs 79 , like Pseudomonas 24 , 31 , 35 , 80 , 82 .…”

Heterogeneous Diazotroph Communities in the Subtropical‐Subantarctic Transition and Aphotic Zones Off the Coast of Patagonia, Eastern South Pacific Ocean

“…(2015) also documented N 2 fixation in the OMZ. N 2 fixation occurs in the OMZ water off the coast of Peru (Fernandez et al., 2011; Kittu et al., 2023; Loescher et al., 2014), and this water mass is transported south by the Peru‐Chile Undercurrent (Schneider et al., 2003). The OMZ in this study region represented by a minimum oxygen concentration of 138.64 μmol L −1 , whereas oxygen concentrations in OMZ waters where N 2 fixation has been observed are subzero (Fernandez et al., 2011, 2015; Kittu et al., 2023; Loescher et al., 2014).…”

Aspectos ecológicos de la macroalga roja Chondracanthus chamissoi (C. Agardh) Kützing en la región La Libertad, Perú A1.3

“…Los resultados de la salinidad se debieron posiblemente a que las praderas tuvieron mucha influencia de los drenes de agua dulce desde el continente. En general, estos valores son normales y son característicos de las aguas costeras frías (34.8 a 35.0 ups), las cuales están asociadas al sistema de afloramiento costero en el Perú (Morón 2011;Kittu et al, 2023). Los valores mínimos presentados en los meses de primavera/verano pueden estar asociados a un efecto de los poblados cercanos a las praderas o por la influencia de aguas ecuatoriales superficiales (34.00 a 34.80).…”