Seasonality of Dissolved Organic Carbon in the Upper Northeast Pacific Ocean

Abstract

From seasonal cruises in the NE Pacific Ocean during 2017, we (1) determined dissolved organic carbon concentrations; (2) calculated net community production (NCP) from nitrate drawdown; and (3) established relationships between NCP and seasonal dissolved organic carbon (DOC) accumulation in the upper 75 m. The fraction of NCP that accumulated as DOC, hereafter referred to as the net dissolved production ratio, was calculated for several stations during spring and summer. The net dissolved production ratio was about 0.26 at the oceanic station Ocean Station Papa during different seasons and years. Using nitrate concentration profiles obtained from Bio‐Argo floats during 2009–2018 operating near Ocean Station Papa, we calculated NCP at high temporal resolution and then applied the 0.26 constant in order to (4) estimate DOC variability for the 9‐year period. We found strong seasonality near Ocean Station Papa, with NCP maxima during summers ranging from 0.3 to 2.9 mol C/m2 and surface DOC concentrations estimated from 56 μmol/kg in winters to 73 μmol/kg in summers. There was a 10‐fold interannual variability in the seasonally accumulated inventory of DOC, ranging from 0.078 to 0.75 mol C/m2. This study reinforces the value of deploying floats equipped with chemical sensors in order to better understand marine biogeochemical cycles, especially when high resolution data cannot be obtained otherwise. Given that ~26% of NCP accumulates as DOC in the central Gulf of Alaska, the remaining balance of ~74% is available for export as sinking biogenic particles.

Warm Events Induce Loss of Resilience in Organic Carbon Production in the Northeast Pacific Ocean

“…We determined NCP from NO 3 − variability through time for the period between 2009 and 2018, following the detailed methodology of Bif and Hansell (2019). Briefly, we considered February as the winter and initial condition for all years, when NO 3 − concentrations reach maximum concentrations in the northeast Pacific's upper layer.…”

“…Briefly, we considered February as the winter and initial condition for all years, when NO 3 − concentrations reach maximum concentrations in the northeast Pacific's upper layer. We present the NCP profiles (mol C/m 3 ) and NCP stocks (mol C/m 2 ), with the latter estimated by integrating the upper 75 m. In order to compare NCP across the considered years, we chose August as the reference month to estimate anomalies since the peak of NCP is typically observed then (Bif & Hansell, 2019;Plant et al, 2016). The mean NO 3 − concentration for February was used to calculate NO 3 − (mol N/m 3 ) over the following 12 months.…”

“…The autonomous floats generated highresolution vertical profiles from the upper 2,000 m of depth at~5-day intervals, following a quasi-Lagrangian path. Data quality control for each float, particularly for NO 3 − concentrations, was performed following Plant et al (2016) and Bif and Hansell (2019). The quality control analysis considered internal temporal drifts and offsets in NO 3 − concentrations as well as offsets between different floats when available.…”

“…Briefly, we considered February as the winter and initial condition for all years, when NO 3 − concentrations reach maximum concentrations in the northeast Pacific's upper layer. Only the upper 75 m of the water column were considered in the calculations since most NCP happens there in this region (Bif & Hansell, 2019). The mean NO 3 − concentration for February was used to calculate NO 3 − (mol N/m 3 ) over the following 12 months.…”

“…Since the magnitude of NCP is controlled by nutrients that allow new primary production, divergence zones that experience high nutrient input produce most of the global ocean NCP (Chavez & Toggweiler, 1995;Hansell & Carlson, 1998;Peña & Varela, 2007;Stabeno et al, 2017). The northeast Pacific Ocean is an example of a highly seasonal environment whereby NCP mainly takes place during spring and summer, fueled by vertical nutrient inputs from the preceding winter mixing events (Bif & Hansell, 2019;Plant et al, 2016;Wong et al, 2002). Frequently, extreme climate events driven by ocean atmosphere oscillations, such as El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO), warm the northeast Pacific upper ocean.…”