William Z. Haskell

Annual Cyclicity in Export Efficiency in the Inner Southern California Bight

The balance of marine autotrophy and heterotrophy regulates the oceans ability to serve as a CO2 sink, as organic material produced by autotrophs sinks into the ocean interior to drive the biological pump. Marine ecosystems over the continental margins, especially coastal upwelling regions, account for a disproportionate amount of carbon export, thus even small fluctuations in export in these regions can have a large impact on the global carbon cycle. In this study, we estimated the rate of gross oxygen production (GOP), stoichiometrically related to gross primary production, by combining measurements of the triple isotope composition of dissolved oxygen with estimates of vertical advection, eddy diffusion and air-sea gas exchange in a one-dimensional two-box non-steady state model of the euphotic zone. Net oxygen production (NOP) estimates based on O2/Ar were then combined with GOP to estimate the NOP/GOP ratio, or potential export efficiency, out of the euphotic zone at the San Pedro Ocean Time series (SPOT) during an 18-month period between January 2013 and June 2014. GOP estimates ranged from 161 ± 44 to 477 ± 155 mmol m−2 d−1 during this period, peaking in May each year, and NOP/GOP ratios ranged from 0.05 ± 0.10 to 0.65 ± 0.28. The highest export efficiency occurred in late February/early March, following the onset of spring upwelling, declining as the upwelling season continued. This study demonstrates that export efficiency changes through time in this temperate coastal upwelling region on a repeated annual cycle and the magnitude of export efficiency suggests efficient photosynthetic energy conversion by phytoplankton in spring.

Estimates of vertical turbulent mixing used to determine a vertical gradient in net and gross oxygen production in the oligotrophic South Pacific Gyre

Mixed layer (ML) gross (GOP) and net (NOP) oxygen production rates based on in situ mass balances of triple oxygen isotopes (TOI) and O2/Ar are influenced by vertical transport from below, a term traditionally difficult to constrain. Here, we present a new approach to estimate vertical eddy diffusivity (Kz) based on density gradients in the upper thermocline and wind-speed based rates of turbulent shear at the ML depth. As an example, we use this Kz, verified by an independent 7Be-based estimate, in an O2/TOI budget at a site in the oligotrophic South Pacific Gyre (SPG). NOP equaled 0.31 ± 0.16 mmol m-2 d-1 in the ML (~55-65 m depth) and 1.2 ± 0.4 mmol m-2 d-1 (80%) beneath the ML, while GOP equaled 74 ± 27 mmol m-2 d-1 (86%) in the ML and 12 ± 4 mmol m-2 d-1 (14%) below, revealing a vertical gradient in production rates unquantifiable without the Kz estimate.

Contextualizing time-series data: quantification of short-term regional variability in the San Pedro Channel using high-resolution in situ glider data

Oceanic time-series have been instrumental in providing an understanding of biological, physical, and chemical dynamics in the oceans and how these processes change over time. However, the extrapolation of these results to larger oceanographic regions requires an understanding and characterization of local versus regional drivers of variability. Here we use highfrequency spatial and temporal glider data to quantify variability at the coastal San Pedro Ocean Time-series (SPOT) site in 20 the San Pedro Channel (SPC) and provide insight into the underlying oceanographic dynamics for the site. The dataset was dominated by four water column profile types that typified active upwelling, a surface bloom, warm-stratified-low-nutrient conditions, and a subsurface chlorophyll maximum. On weekly timescales, the SPOT station was on average representative of 64% of profiles taken within the SPC. In general, shifts in water column profile characteristics at SPOT were also observed across the entire channel. On average, waters across the SPC were most similar to offshore profiles suggesting that SPOT 25 time-series data would be more impacted by regional changes in circulation than local, coastal events. These results indicate that high-resolution in situ glider deployments can be used to quantify major modes of variability and provide context for interpreting time-series data, allowing for broader application of these datasets and greater integration into modeling efforts.