Ecosystem export efficiency in an upwelling region: a two-year time series study of vertical transport, particle export and in-situ net and gross oxygen production

• Main Author: Haskell, William Ziegler
• Format: Dataset
• Language: English
• Published: University of Southern California Digital Library (USC.DL) 2015
• Subjects: Ocean Sciences; upwelling; biological production; carbon export; oxygen; stable isotopes; radioactive isotopes; Curl; North Atlantic
• Online Access: https://dx.doi.org/10.25549/usctheses-c3-606385; https://digitallibrary.usc.edu/asset-management/2A3BF1LN3O2C

A common observation in studies of biological production and carbon export in the oceans has been that ecosystems with higher nutrient input (i.e. upwelling zones, high latitudes, etc.) have higher export efficiency (export/gross production) than ecosystems that are nutrient limited. However, there have been very few time‐series studies in high‐efficiency regimes which are able to capture the nutrient input, production and export over time necessary to define the role nutrient input plays in determining the efficiency of the biological pump. In the last decade, O₂/Ar ratios and the triple oxygen isotope composition (TOI; ¹⁷Δ) of dissolved O₂ in the surface mixed layer have been used in many regions to simultaneously estimate net (NOP) and gross oxygen production (GOP) in the surface ocean. Both NOP and GOP can be stoichiometrically related to carbon production and therefore, the NOP/GOP ratio reflects the efficiency of an ecosystem to export, rather than recycle, organic carbon. This approach has been a significant advancement in marine geochemistry, however it has been limited in regions with strong vertical transport because transport is often difficult to determine, but has a profound effect on these production estimates. This is especially an issue when the majority of biological production occurs beneath the surface mixed layer. ❧ In this study, profiles of the concentration and isotopic composition of oxygen were combined with concurrent estimates of upwelling velocity and eddy diffusivity, based on water column budgets of ⁷Be (t₁/₂=53d) and ²³⁴Th (t₁/₂=24d). This approach, unlike many other metrics used to estimate vertical transport, is able to resolve the dynamics of upwelling and eddy diffusivity over weekly timescales, the same timescale as the dissolved oxygen approach and biological bloom development. Mass balances for oxygen and its isotopes were based in a 1‐D, non‐steady state, two‐box model of the upper ocean, which includes production in the entire euphotic zone. Observations of both at ∼two week intervals at the San Pedro Ocean Time‐series (SPOT) in the Southern California Bight over two annual upwelling cycles were used to constrain non‐steady state model parameters. Sediment traps and a ²³⁴Th budget were also used to estimate particulate export at 100m and 200m for comparison to our net production estimates. An oxygen optode in the ship's underway system was used to constrain temporal and spatial variability in the region. ❧ During 18 months between January 2013 and June 2014, upwelling velocities ranged from 0.0 ± 0.5 to 2.8 ± 1.4 m d⁻¹ at SPOT, which agreed within uncertainty with the monthly Bakun Index, a pressure‐field based approach, every month except June each year. Hydrographic parameters and a heat budget suggest upwelling at this site is dominated by wind stress curl‐driven interior water column upwelling. Upwelled nutrients supported GOP rates of 134 ± 62 to 665 ± 164 mmol m⁻² d⁻¹ and NOP rates of 13 ± 23 to 210 ± 63 mmol m⁻² d⁻¹, which translates to Net Community Production (NCP) in the euphotic zone of up to 150 mmol C m⁻² d⁻¹. NOP/GOP ratios ranged from 0.06 ± 0.10 to 0.63 ± 0.27 over the same period and peaked prior to the maximum in upwelling velocity and GOP, suggesting that the timing of upwelling initiation each year may influence export efficiency. Organic carbon export during spring months, measured in sediment traps and using a water column ²³⁴Th budget, ranged from 2.6 ± 1.2 to 25.3 ± 12.4 mmol C m⁻² d⁻¹ at 100m and 1.9 ± 1.3 to 16.3 ± 10.1 mmol C m⁻² d⁻¹ at 200m. Export fluxes followed a temporal trend similar to NCP in the euphotic zone at both depths, but were ∼7x and ∼12x smaller due to remineralization above the trap depths. ❧ Results of this study show that it is possible to apply the dissolved O₂/Ar and TOI tracer pair to estimate production rates along the ocean margins, and that the export efficiency of this ecosystem may not depend on upwelling velocity alone, but is most likely influenced by the timing of the initial upwelling ‘pulse’ and the composition and structure of the biological community during this critical transition period from winter into the spring upwelling season. Furthermore, export efficiency slowly rose from its annual low in October through January, suggesting that there may be a fundamentally similar mechanism leading to the annual spring bloom at SPOT that has been documented previously in a high latitude region, and that bloom theory previously applied to the North Atlantic spring bloom may be a plausible explanation for ecosystems in temperate coastal upwelling regions, as well.