Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Supplemental Files
• Related Datasets
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Sampling was conducted on R/V Pelican cruise PE22-03 in the Northern Gulf of Mexico in August 2021. All CTD casts were conducted according to standard ship operator procedure.

The data were collected using a SeaBird 911 plus system with twelve 12-liter Niskin bottles. The instruments included in the 911 plus suite are dual Temperature (SBE 3), dual Conductivity (SBE 4), and dual Oxygen (SBE 43) sensors. Additionally, the suite included an SBE 27 pH/Oxidation, Chelsea Aquatracka 3 Chl-a, and a Wetlabs CDOM sensor. Serial numbers and calibration information is stored in the Header Information included in the .cnv files (see Supplemental Files).

Data Processing:
The data were processed using Seabird's SBEDataProccessing_Win32 software. The data were converted from hexadecimal to engineering units first.

Using the software's Bin Averaging tool, it was also averaged in bins of 0.25 meters. For the binned data, see the related dataset https://www.bco-dmo.org/dataset/879704.

The attached Supplemental File, "PE22-03_Seabird_Unbinned_CTD_File_Header.txt", contains the complete Seabird header from one cast (C6C_2).

BCO-DMO Processing:
- captured starting latitude, starting longitude, and starting date/time from the Seabird headers as columns;
- replaced "-9.990e-29" with "nd" (no data);
- concatenated separate files (one per cast) into one dataset;
- converted starting latitude and longitude to decimal degrees (rounded to 5 decimal places);
- converted starting date/time to ISO8601 format;
- created Cast column based on the original file name;
- renamed fields to comply with BCO-DMO parameter naming conventions.

NSF Award Abstract:
Decreasing marine dissolved oxygen (DO) is a widespread and growing global problem. Among the chief causes for coastal losses of DO are anthropogenic nutrient inputs that lead to seasonal hypoxia- DO concentrations below 2 mg/L. Microorganisms are the primary agents of oxygen removal, and although we have a basic mechanistic understanding of how nutrient enrichment combines with stratification to stimulate microbial metabolism and oxygen drawdown, we still do not know which microorganisms are ultimately responsible for oxygen consumption leading to, and during, coastal hypoxia in any particular region. Nor do we know the extent to which each hypoxic system arises from universal microbial mechanisms or whether there may be unique microorganisms and metabolic pathways involved in each locale. Hypoxic regions are increasing in number and size around the globe, amplifying the need to better understand the microbial processes responsible for oxygen consumption. In response, the project pursues an integrated research and education effort to study the microbial mechanisms of oxygen respiration in two coastal zones where DO depletion occurs: the northern Gulf of Mexico ?dead zone?, and the Southern California Bight. This work identifies the microorganism actively consuming oxygen in these systems, what fuels them, and whether/how those taxa respond to environmental changes. The investigators are generating microbial DO consumption rate, genetic, and taxonomic data critical to better constraining respiration models focused on water column DO depletion. In doing so, hundreds of undergraduate and students and dozens of high school STEM teachers contribute valuable data by participating in modern marine microbiological research. The project also improves integration of authentic research experiences into college and high-school classrooms in the context of a problem of global relevance.

Regions of low DO take multiple forms, from vast open ocean oxygen minimum zones (OMZs) which hover in the upper water column, to shallower coastal zones of bottom water hypoxia fueled by close proximity to fluvial inputs of human supplied nutrients. Microbial respiration is primarily responsible for the ultimate consumption of DO, and therefore understanding the microorganisms that inhabit these systems and their metabolic capabilities is critical for improving our ability to predict the timing, extent, and severity of DO depletion, and how these factors relate to environmental change. While there has been substantial research into the microbiology of OMZs, we know comparatively little about the microbial dynamics, and in particular, the microorganisms responsible for oxygen consumption, in coastal hypoxia. This integrated research and education effort is framed by the following objectives:

1. Determine the microorganisms and metabolic processes responsible for actively respiring water column DO prior to, and during, hypoxia.
2. Quantify microbial respiration rates for communities and representative water column species in both the planktonic and particle-associated fractions.
3. Integrate authentic microbiology research on a globally relevant topic into undergraduate and high school classrooms.

The work uses a combination of advanced cultivation-independent and pure culture measurements to discriminate between oxygen consumption by planktonic and particle-associated microbial fractions, as well as by communities at the surface and those in bottom waters, in two different systems of coastal DO depletion (the northern Gulf of Mexico hypoxic zone and the Southern California coastal shelf). Direct assessment of actively respiring taxa are connected with their metabolic potential, gene expression, and respiration rates. This project is determining, for the first time, the taxa actively consuming oxygen in these systems. The results also provide size fractionated respiration rates and bacterial growth efficiency (BGE) with depth across multiple sites and seasons, and importantly, also yield cell-specific respiration rates and BGE for active taxa within these systems. This data will constrain variable respiration across differing environmental conditions. The investigators are experimentally testing how alterations in environmental variables affect these respiration rates and BGEs, leading to greater predictive insight for the range of effects climate forcing will have on DO consumption. This information also facilitates comparisons between multiple marine systems to identify whether common or distinct organisms and metabolic processes are operating to remove DO.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.