Diurnal and Seasonal Variability of Near‐Surface Oxygen in the Strait of Georgia

Abstract

Dissolved oxygen (O2) concentrations in waters of the ocean surface mixed layer are generally close to thermodynamic equilibrium with atmospheric concentrations. However, near‐surface O2 levels are also affected by other processes, including primary productivity, and thus measurements of near‐surface O2 can in theory be used to estimate productivity. Here we discuss variations in near‐surface O2 concentrations in the Strait of Georgia by examining a variety of data sets, focusing primarily on ferry‐based measurements over a 3‐year period (2015–2017). Both diurnal, seasonal, and interannual variations are quantified, and various fluxes into and out of the surface waters are estimated so that the degree to the measured oxygen variations representing biological activity can be assessed. On average, advective and vertical transport makes only a small negative contribution, while other budget terms show a strong seasonal cycle, lowest in winter, peaking in spring and slowly dropping to winter level during summer and autumn. For most of the year, the air‐sea flux is the largest term, but the storage term is important during the spring blooms. Diurnal variations of 5–10% saturation in measured oxygen levels must therefore largely reflect diurnal cycles in productivity and respiration. We estimate respiration effects from the rate of decay of O2 at night, which yields an average respiration rate of 459 mmol·m−2·day−1. This value is almost an order of magnitude larger than the other budget terms we have discussed, suggesting that a future focus on diurnal variations might provide most insight into biological processes in the Strait.

Validation of Non-photochemical Quenching Corrections for Chlorophyll-a Measurements Aboard Ships of Opportunity

“…The dataset is comprised of measurements acquired with autonomous sensors on board the ferry including salinity, above-water solar radiation, in vivo Chl-a fluorescence, and water samples from which Chl-a was extracted using HPLC methods. The approximate depth of intake for the ferry instruments was 2 m (Halverson and Pawlowicz, 2013;Wang et al, 2019). Salinity was used to separate plume dominant plume and oceanic dominate waters, and, as a proxy to determine the photosynthetic active radiation (PAR) attenuation coefficient at 2 m depth (k z ).…”

“…To address these limitations, in vivo fluorometers are used to continuously measure the fluorescence of the chlorophyll-a pigment, following the principles introduced by Lorenzen (1966). In vivo fluorometers are routinely installed on floats, gliders, profiling systems (Davis et al, 2008;Thomalla et al, 2018), moored platforms (Falkowski and Kolber, 1995), animals (Biermann et al, 2015;Xing et al, 2012), and ships of opportunity (Holley et al, 2007;Halverson and Pawlowicz, 2013;Anderson et al, 2017;Wang et al, 2019).…”

“…Fluorometers installed on ships of opportunity measure in vivo fluorescence (fChl-a) at the depth of intake, typically within the surface mixed layer (or within a few meters of the surface). These systems have the advantages of operating year round at high spatial and temporal resolution, and in some cases surveying the same region multiple times per day (Halverson and Pawlowicz, 2013;Wang et al, 2019). The temporal resolution of these datasets is particularly advantageous for capturing short term events, such as phytoplankton blooms (Holley et al, 2007), and spatial variability in dynamic environments (Halverson and Pawlowicz, 2013).…”

“…In addition, these data can also be used for validating satellite derived Chl-a products (Petersen et al, 2008;Lavigne et al, 2012;Jackson et al, 2015). As such, Chla fluorometers have been operational aboard ferries in Europe (Petersen, 2014;Anderson et al, 2017), Japan (Harashima and Kunugi, 2000), Canada (Halverson and Pawlowicz, 2013;Wang et al, 2019) and the United States (Codiga et al, 2012).…”

Suppression of CO2 Outgassing by Gas Bubbles Under a Hurricane

“…A recent observational study (Leighton et al, 2018), however, concluded that bubble‐induced supersaturation is more important than previously thought, implying that existing CO 2 calculations neglecting bubble‐induced supersaturation may overestimate (underestimate) efflux (influx). On the other hand, the importance of bubble‐induced supersaturation for less soluble gases such as O 2 (e.g., Bushinsky et al, 2016; Vagle et al, 2010; Wang et al, 2019), inert gases (Stanley et al, 2009), and their ratios (e.g., Hamme & Emerson, 2006) has already been recognized. It was also established in those studies that accurate quantification of the total air‐sea gas flux requires the separation of the total gas flux into three components, viz., the gas flux through the ocean surface, that through bubbles that completely dissolved, and that through bubbles that eventually burst at the ocean surface (partially dissolved).…”