Small phytoplankton drive high summertime carbon and nutrient export in the Gulf of California and Eastern Tropical North Pacific

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 29 (2015): 1309–1332, doi:10.1002/2015GB005134.Summertime carbon, nitrogen, and biogenic silica export was examined using 234Th:238U disequilibria combined with free floating sediment traps and fine scale water column sampling with in situ pumps (ISP) within the Eastern Tropical North Pacific and the Gulf of California. Fine scale ISP sampling provides evidence that in this system, particulate carbon (PC) and particulate nitrogen (PN) concentrations were more rapidly attenuated relative to 234Th activities in small particles compared to large particles, converging to 1–5 µmol dpm−1 by 100 m. Comparison of elemental particle composition, coupled with particle size distribution analysis, suggests that small particles are major contributors to particle flux. While absolute PC and PN export rates were dependent on the method used to obtain the element/234Th ratio, regional trends were consistent across measurement techniques. The highest C fixation rates were associated with diatom-dominated surface waters. Yet, the highest export efficiencies occurred in picoplankton-dominated surface waters, where relative concentrations of diazotrophs were also elevated. Our results add to the increasing body of literature that picoplankton- and diazotroph-dominated food webs in subtropical regions can be characterized by enhanced export efficiencies relative to food webs dominated by larger phytoplankton, e.g., diatoms, in low productivity pico/nanoplankton-dominated regions, where small particles are major contributors to particle export. Findings from this region are compared globally and provide insights into the efficiency of downward particle transport of carbon and associated nutrients in a warmer ocean where picoplankton and diazotrophs may dominate. Therefore, we argue the necessity of collecting multiple particle sizes used to convert 234Th fluxes into carbon or other elemental fluxes, including <50 µm, since they can play an important role in vertical fluxes, especially in oligotrophic environments. Our results further underscore the necessity of using multiple techniques to quantify particle flux given the uncertainties associated with each collection method.NSF Grant Numbers: OCE-0726290, OCF-0962362, OCE-0726543, OCE-0726422; EU Grant Number: FP7-MC-IIF-220485; MEC Grant Number: CTM2007-31241-E/MAR; ICREA Academia; MERS Grant Number: 2014 SGR – 1356; Spain's Ministerio de Educación y Ciencia Grant Numbers: AP-2009-4733, BES-2004-3348; NASA New Investigator Award Grant Number: NNX10AQ81G; Sloan Research Fellowship2016-02-2

Small Phytoplankton Drive High Summertime Carbonand Nutrient Export in the Gulf of California and Eastern Tropical North Pacific

Summertime carbon, nitrogen, and biogenic silica export was examined using 234Th:238U disequilibria combined with free floating sediment traps and fine scale water column sampling with in situ pumps (ISP) within the Eastern Tropical North Pacific and the Gulf of California. Fine scale ISP sampling provides evidence that in this system, particulate carbon (PC) and particulate nitrogen (PN) concentrations were more rapidly attenuated relative to 234Th activities in small particles compared to large particles, converging to 1–5 µmol dpm−1 by 100 m. Comparison of elemental particle composition, coupled with particle size distribution analysis, suggests that small particles are major contributors to particle flux. While absolute PC and PN export rates were dependent on the method used to obtain the element/234Th ratio, regional trends were consistent across measurement techniques. The highest C fixation rates were associated with diatom‐dominated surface waters. Yet, the highest export efficiencies occurred in picoplankton‐dominated surface waters, where relative concentrations of diazotrophs were also elevated. Our results add to the increasing body of literature that picoplankton‐ and diazotroph‐dominated food webs in subtropical regions can be characterized by enhanced export efficiencies relative to food webs dominated by larger phytoplankton, e.g., diatoms, in low productivity pico/nanoplankton‐dominated regions, where small particles are major contributors to particle export. Findings from this region are compared globally and provide insights into the efficiency of downward particle transport of carbon and associated nutrients in a warmer ocean where picoplankton and diazotrophs may dominate. Therefore, we argue the necessity of collecting multiple particle sizes used to convert 234Th fluxes into carbon or other elemental fluxes, including \u3c50 \u3eµm, since they can play an important role in vertical fluxes, especially in oligotrophic environments. Our results further underscore the necessity of using multiple techniques to quantify particle flux given the uncertainties associated with each collection method

Nitrogen fixation in the Gulf of California and the Eastern Tropical North Pacific

Di-nitrogen (N₂) fixation plays a well-recognized role in the enhancement of primary production and arguably particle export in oligotrophic regions of the subtropical and tropical oceans. However, recent evidence suggests that N₂ fixation may also be significant in regions of the surface ocean proximate to or overlying zones of intense subsurface denitrification. In this study, we present results from a series of research cruises in the Gulf of California (GoCal) and adjacent waters of the eastern tropical North Pacific (ETNP). Measurements included microscopy, genomic analyses, incubations, stable isotopic measurements, and sediment traps coupled with ²³⁸U:²³⁴Th disequilibria. Combined, these results suggest that N₂ fixing microorganisms are present and active throughout the region, with larger sized Richelia and Trichodesmium spp. recorded within the warmer waters at the entrance to and within the GoCal, and smaller, unicellular diazotrophs observed in the cooler waters of the northern ETNP. N₂ fixation rates in the summer varied from 15-70 μmol N m⁻² d⁻¹, with episodic blooms contributing as much as 795 μmol N m⁻² d⁻¹. While the estimated contribution of N₂ fixation to particle export was highly variable, blooms of diatom-Richelia symbioses accounted for as much as ~44% of the measured summer carbon flux at 100 m. Alternately, evaluation of the N isotopic composition of sinking material and the magnitude of measured N₂ fixation rates indicate negligible to small enhancements of new production when blooms of either colonial Trichodesmium spp. or unicellular diazotrophs were encountered. Consistent with previous research, we also found that while fluxes of C to sediment traps are similar in winter and summer months, the efficiency of C export (export/surface productivity) in the GoCal region is elevated during summer relative to the more productive diatom-dominated winter phase of the seasonal cycle. The episodic and variable nature of N₂ fixation recorded in this region make it unlikely that new production via diazotrophic activity is solely responsible for the observed patterns of C transport efficiency; rather, we hypothesize that eolian inputs and/or efficient transport of picocyanobacterial biomass via grazing or aggregation may further explain the enhanced export efficiency observed in the GoCal summer. In sum, diazotrophy typically supports <10%, but as much as 44% of export production. The high variability of direct measurements of N₂ fixation implies that other mechanisms contribute to the seasonal invariance of C flux in this region. If this region is indicative of other oxygen minima zones with active diazotrophs, our results indicate that export-mediated feedback mechanisms between N₂ fixation and denitrification are not as strong as previously hypothesized.Keywords: Export production, Denitrification, Nitrogen fixation, Oxygen minima zonesKeywords: Export production, Denitrification, Nitrogen fixation, Oxygen minima zone