Hydrographic and dissolved oxygen variability in a seasonal Pacific Northwest estuary

Abstract

Hypoxia is an issue of growing concern for coastal communities. In the California Current System, a prototypical eastern boundary current, attention has been focused on explaining the trend of increasing shelf hypoxia. Despite the regional focus on hypoxia in eastern boundary regions, relatively few studies have examined smaller estuarine systems. Here, we present results from an observational study in Coos Bay, a small estuary on the southern Oregon coast, subject to seasonal upwelling/downwelling winds, strong tides, and wide fluctuations in freshwater input. Coos Bay exhibits characteristics of a salt-wedge type estuary under high river discharge conditions (>150 m 3 s-1), a wellmixed estuary under low discharge conditions (0-30 m 3 s-1), and partially-mixed estuary during times of moderate discharge (30-150 m 3 s-1). The observed vertical stratification and along-estuary salinity gradients correlate significantly with river discharge and tidal forcing. Despite a strong coupling with coastal waters where hypoxia has been present, we do not find evidence for pervasive hypoxia in Coos Bay. We find that upwelling on the shelf advects low dissolved oxygen water into the estuary on synoptic timescales. Early in the upwelling season (April and May), dissolved oxygen minima are found at the estuary mouth, while later in the summer (September), dissolved oxygen minima are found at the riverine end, presumably due to decreased discharge and increased productivity. However, in a given year, the overall strength of the upwelling season is not a good predictor of low dissolved oxygen levels in the estuary.

Estuarine Exchange Flow Variability in a Seasonal, Segmented Estuary

“…Estuaries in Mediterranean climates have particularly strong seasonal river discharge variability, with long periods of low discharge during the dry season and event-driven periods (typically 1-5 days) of intense discharge during the wet season. These seasonal discharge variations result in large seasonal changes in the salinity distribution (Banas et al 2004;Nidzieko and Monismith 2013;Sutherland and O'Neill 2016), and in extreme cases the salinity gradient can reverse in the dry season due to evaporation (Largier et al 1997). The seasonal change in salinity distribution has been shown to alter the strength of gravitational circulation and subtidal currents (Nidzieko and Monismith 2013), however, the importance of the subtidal contribution to the exchange flow depends on the strength of tidal dispersive processes.…”

“…Many estuaries on the U.S. West Coast can be characterized as Mediterranean due to their small watersheds with minimal snowmelt. In summer, precipitation is rare and freshwater input decreases substantially, leading to large seasonal shifts in the salinity distribution in small to midsize estuaries (Banas et al 2004;Sutherland and O'Neill 2016). Conditions in these estuaries can also be affected by shelf processes that modify coastal water properties and sea level (Hickey and Banas 2003;Giddings and MacCready 2017).…”

Field measurements, sediment tracer study, and numerical modeling at Coos Bay Inlet, Oregon

“…For an environmental study in the Coos Bay estuary, an ADCP was deployed in the bay (Sutherland and O'Neill 2016). Figure 2-3 shows the location of the upward-looking ADCP moored at a water depth of 10.3 m, which is off the deepest part of the channel near mid-estuary (RM 8.0).…”

“…The monthly simulation corresponds to the latest ADCP survey period by Sutherland and O'Neill (2016). The CMS calibration was performed using the measured WSE and current at the bay location (RM 8.0).…”

Temporal Changes in Water Quality Parameters of Two Sections of the Ancient Canal: A Case of a Reach for Yangnong Chemical Plant

“…DO is one of the most important indexes for evaluating water quality of rivers, reservoirs and other water bodies [13][14][15]. Equation (2) was adopted to calculate monthly SFI of DO (P i ) from October 2015 to September 2016, and the results are shown in Figure 5.…”