Impact of recently upwelled water on productivity investigated using in situ and incubation‐based methods in Monterey Bay

Abstract

Photosynthetic conversion of CO2 to organic carbon and the transport of this carbon from the surface to the deep ocean is an important regulator of atmospheric CO2. To understand the controls on carbon fluxes in a productive region impacted by upwelling, we measured biological productivity via multiple methods during a cruise in Monterey Bay, California. We quantified net community production and gross primary production from measurements of O2/Ar and O2 triple isotopes ( 17Δ), respectively. We simultaneously conducted incubations measuring the uptake of 14C, 15NO3−, and 15NH4+, and nitrification, and deployed sediment traps. At the start of the cruise (Phase 1) the carbon cycle was at steady state and the estimated net community production was 35(10) and 35(8) mmol C m−2 d−1 from O2/Ar and 15N incubations, respectively, a remarkably good agreement. During Phase 1, net primary production was 96(27) mmol C m−2 d−1 from C uptake, and gross primary production was 209(17) mmol C m−2 d−1 from 17Δ. Later in the cruise (Phase 2), recently upwelled water with higher nutrient concentrations entered the study area, causing 14C and 15NO3− uptake to increase substantially. Continuous O2/Ar measurements revealed submesoscale variability in water mass structure and likely productivity in Phase 2 that was not evident from the incubations. These data demonstrate that O2/Ar and 15N incubation‐based NCP estimates can give equivalent results in an N‐limited, coastal system, when the nonsteady state O2 fluxes are negligible or can be quantified.

Refined Estimates of Net Community Production in the Subarctic Northeast Pacific Derived From ΔO2/Ar Measurements With N2O‐Based Corrections for Vertical Mixing

“…The supply ratio was therefore characterized independently for each cruise as the slope through the respective pooled data set of all profile measurements in the 150 m below the mixed layer. A similar approach was previously used by Castro-Morales et al (2013) and Manning et al (2017) in estimating vertical fluxes from O 2 gradients. As discussed below (section 4.1.1), the magnitude of the NCP correction is sensitive to the approach used to estimate the supply ratio term but is largely insensitive to the depth of integration or the threshold criterion of the mixed layer depth (mld) estimate.…”

Changes in gross oxygen production, net oxygen production, and air-water gas exchange during seasonal ice melt in Whycocomagh Bay, a Canadian estuary in the Bras d'Or Lake system

“…Geen (1965) measured primary production by 14 C uptake in the Bras d'Or Lake during summer 1962-1964 and found an average uptake rate at 5 m depth, the depth of maximum photosynthesis, of ∼ 4 mmol C m −3 d −1 (50 mg C m −3 d −1 ) (Geen and Hargrave, 1966). These rates are based on 6 h daytime incubations and ignore nighttime respiratory loss of 14 C, and therefore approximate net primary production (NPP, gross primary production minus autotrophic respiration) or something between NPP and gross primary production (GPP) (Bender et al, 1999;Marra, 2002Marra, , 2009Quay et al, 2010). Assuming a C : O 2 ratio of 1.1-1.4 (Laws, 1991), the equivalent O 2 production based on the 14 C incubations is 4.4-5.6 mmol O 2 m −3 d −1 .…”

“…The NOP/GOP ratio provides an estimate of the export efficiency, i.e., the fraction of autotrophic production available for export to waters below the mixed layer. The time-series approach allowed us to quantify non-steady-state O 2 fluxes, which can be a significant fraction of the total O 2 flux in many regions (Hamme et al, 2012;Tortell et al, 2014;Manning et al, 2017b). We quantify GOP with discrete measurements of the triple oxygen isotopic composition of O 2 (Luz et al, 1999;Luz and Barkan, 2000;Juranek and Quay, 2013) and NOP with nearcontinuous measurements of the O 2 /Ar saturation anomaly (Cassar et al, 2009).…”

Changes in gross oxygen production, net oxygen production, and air-water gas exchange during seasonal ice melt in the Bras d'Or Lake, a Canadian estuary

“…CC BY 4.0 License. below the mixed layer are small and the physical fluxes are large and highly uncertain, as they are driven by lateral and vertical mixing rather than air-water gas exchange (Giesbrecht et al, 2012;Munro et al, 2013;Manning et al, 2017b).…”

“…The time-series approach allowed us to quantify non-steady state O 2 fluxes, which can be a significant fraction of the total O 2 flux in many regions (Hamme et al, 2012;Tortell et al, 2014;Manning et al, 2017b). We quantify GOP with discrete measurements of the triple oxygen isotopic composition of O 2 and NOP with near-continuous measurements of the O 2 /Ar saturation anomaly 5 (Cassar et al, 2009).…”