Colorado mountains
From Long-Term Data to Understanding: Toward a Predictive Ecology
2015 LTER ASM Estes Park, CO - August 30 - September 2, 2015

Nitrogen Isotopes in the California Current Ecosystem-LTER Domain in August 2014

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Lihini Aluwihare
Brandon M. Stephens
Scott Wankel

We present here organic and inorganic nitrogen stable isotopes for the August 2014 California Current Ecosystem LTER process cruise. Three-day Cycles across 5 stations sampled within a lagrangian framework examined productive nearshore to oligotrophic ocean gyre ecosystem conditions. Nitrate data followed Rayleigh closed-system isotope fractionation allowing the nitrogen isotope singature of upwelling nitrate and fractionation factors to be estimated for this region. In addition, although oxygen and nitrogen isotopes of nitrate varied linearly the slope was greater than 1, which identifies nitrification as an important process in the euphotic zone during this time. Depleted isotope singatures in suspended POM also hinted at ammonium uptake and/or a relatively long lifetime (as accumulated product) for suspended POM in the upper ocean.  Sinking particulate organic matter collected from drifting sediment trap arrays (3-day deployment) showed little variation in their isotopic singature caross sites despite variable ntirate concentrations. This organic matter was consistently more enriched in its isotopic signature than nitrate. This can be either interpreted as the integration of sinking POM produced elsewhere or taken as further confirmation that the nitrate fuelling local production had its upwelling isotopic signature modified by local nitrification. Models to examine possible contributions of nitrified nitrate and mechanisms that confer enriched isotope singatures to sinking POM are being currently explored to aid further data interpretation.