5 research outputs found
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): 1309â1332, 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 1â5 ”mol dpmâ1 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 ”m, 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 â 1356; Spain's Ministerio de EducaciĂłn 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 1â5 ”mol dpmâ1 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 diatomâdominated surface waters. Yet, the highest export efficiencies occurred in picoplanktonâdominated surface waters, where relative concentrations of diazotrophs were also elevated. Our results add to the increasing body of literature that picoplanktonâ and diazotrophâdominated 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/nanoplanktonâdominated 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 \u3e”m, 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
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Nitrogen fixation in the Gulf of California and the Eastern Tropical North Pacific
Di-nitrogen (Nâ) 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 Nâ 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 ÂČÂłâžU:ÂČÂłâŽTh disequilibria. Combined, these results suggest that Nâ 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. Nâ
fixation rates in the summer varied from 15-70 ÎŒmol N mâ»ÂČ dâ»Âč, with episodic blooms
contributing as much as 795 ÎŒmol N mâ»ÂČ dâ»Âč. While the estimated contribution of Nâ 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 Nâ 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 Nâ 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 Nâ 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 Nâ 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