Combining observations and numerical model results to improve estimates of hypoxic volume within the Chesapeake Bay, USA

Abstract

[1] The overall size of the ''dead zone'' within the main stem of the Chesapeake Bay and its tidal tributaries is quantified by the hypoxic volume (HV), the volume of water with dissolved oxygen (DO) less than 2 mg/L. To improve estimates of HV, DO was subsampled from the output of 3-D model hindcasts at times/locations matching the set of [2004][2005] stations monitored by the Chesapeake Bay Program. The resulting station profiles were interpolated to produce bay-wide estimates of HV in a manner consistent with nonsynoptic, cruise-based estimates. Interpolations of the same stations sampled synoptically, as well as multiple other combinations of station profiles, were examined in order to quantify uncertainties associated with interpolating HV from observed profiles. The potential uncertainty in summer HV estimates resulting from profiles being collected over 2 weeks rather than synoptically averaged $5 km 3 . This is larger than that due to sampling at discrete stations and interpolating/extrapolating to the entire Chesapeake Bay (2.4 km 3 ). As a result, sampling fewer, selected stations over a shorter time period is likely to reduce uncertainties associated with interpolating HV from observed profiles. A function was derived that when applied to a subset of 13 stations, significantly improved estimates of HV. Finally, multiple metrics for quantifying bay-wide hypoxia were examined, and cumulative hypoxic volume was determined to be particularly useful, as a result of its insensitivity to temporal errors and climate change. A final product of this analysis is a nearly three-decade time series of improved estimates of HV for Chesapeake Bay.

The competing impacts of climate change and nutrient reductions on dissolved oxygen in Chesapeake Bay

“…The Estuarine-CarbonBiogeochemistry (ECB) component of the model (Feng et al, 2015) was developed originally from a continental shelf application (Hofmann et al, 2011), using dissolved organic matter cycling similar to that described in Druon et al (2010). With only single phytoplankton and zooplankton classes and only one limiting nutrient (nitrogen), the ECB model is simpler than that employed by the Chesapeake Bay Program (Cerco et al, 2010) but is more complex than simple dissolved-oxygen models that utilize a constant oxygen consumption rate (e.g., Scully, 2010;Bever et al, 2013). ChesROMS-ECB has been previously shown to adequately resolve the spatial and temporal variability of key physical and biological variables such as temperature, salinity, nitrogen, and DO (Feng et al, 2015;Irby et al, 2016).…”

“…Hypoxic volume is a commonly used metric to quantify the amount of water that experiences a given level of DO concentration over a specific time (e.g., Murphy et al, 2011;Bever et al, 2013). In this study, two metrics related to hypoxic volume are computed: cumulative hypoxic volume (CHV) and hypoxic duration (HD).…”

“…In this study, two metrics related to hypoxic volume are computed: cumulative hypoxic volume (CHV) and hypoxic duration (HD). CHV is calculated as the sum of each day's hypoxic volume over a year (Bever et al, 2013), and HD is computed as the number of days that have a hypoxic volume greater than 1 km 3 . While traditional DO concentration levels of hypoxia (< 2 mg L −1 ) and anoxia (< 0.2 mg L −1 ) are examined, this research also considers impacts of low-DO, defined here as DO < 5 mg L −1 .…”

Challenges associated with modeling low-oxygen waters in Chesapeake Bay: a multiple model comparison

“…Li et al, 2015). Bever et al (2013) specifically demonstrated that multiple models of varying complexity are able to generate skillful estimates of hypoxic volume in the bay. Some of these models are being used in the bay to simulate short-term and/or seasonal forecasts of DO conditions.…”

“…Since the middle of the last century, anthropogenic impacts have dramatically decreased water quality throughout the Chesapeake Bay (Boesch et al, 2001), one of the largest estuaries in North America. Land-use change along with the industrialization and urbanization of the Chesapeake Bay watershed have caused dramatic increases in nutrient inputs to the bay (Kemp et al, 2005), spurring additional primary production and phytoplankton abundance (Harding Jr. and Perry, 1997).…”