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Nitrogen Cycling in the Euphotic Zone of the California Current


Deep ocean nitrate supply to the surface euphotic zone of the California Current Ecosystem (CCE) increases rates of productivity and leads to an overall increase in the fixed repository of dissolved and particulate organic matter (DOM and POM, respectively). Chapter II demonstrates that DOM production, in carbon units, can represent 12 ± 8% of nitrate-based production during low production periods and up to 28 ± 15% during highly productive periods. DOM can also accumulate in the surface CCE during post-bloom, surface stratified or iron-limited conditions, and therefore can represent a potentially exportable reservoir when subsequently subducted from the surface ocean. DOM production rates are comparable to regional sinking particle export rates, the combination of which matches net oxygen production rates as estimated in previous studies. While Chapters I and II demonstrate that deep ocean nitrate concentrations track patterns of surface productivity in the CCE, Chapter III tests whether all nitrate used in the surface ocean is derived from the dark ocean via upwelling. Stable isotopes of nitrate estimate that 6 - 36% of nitrate uptake is generated by nitrification occurring within the euphotic zone of the CCE. Rates of euphotic zone nitrification spanned 4 - 103 nmol L-1 d-1, rates of which extend to the surface ocean. A strong correlation between euphotic zone nitrification and nitrite concentrations led to the inference that either increased POM substrate or reduced phytoplankton abundance to be determinants of nitrification. Use of nitrate stable isotopes in Chapter IV demonstrates that surface nitrate utilization was enhanced at inshore CCE stations during the 2014 warm anomaly. The effect of utilization was detected as an isotopic enrichment in sinking and suspended POM and Calanus pacificus copepods. Nitrate sourced from the remnant mixed layer depth supports the food web, and was a source that was at times decoupled from deeper (200 - 400 m) nitrate. A relatively low 3.0 ± 0.5‰ nitrate uptake isotope effect corresponded with phytoplankton communities dominated by chlorophytes and flagellates. Data from Chapter IV support hypotheses that isotopes of upper ocean N reservoirs in the CCE will primarily reflect surface ocean nitrate dynamics.

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