Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • BCO-DMO Processing Description
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Benthic fluxes were measured along the northern Gulf of Mexico shelf during cruises aboard the R/V Pelican in August 2018 and July 2019.  Sediment cores, incubation experiments, and in-situ measurements from a custom made benthic lander all provided data on benthic fluxes of nutrients, dissolved inorganic carbon (DIC), and total alkalinity (TALK) as well as estimates of total oxygen and diffusive oxygen utilization. 

Intact sediment cores were collected in triplicate from ____ (** how many stations? **  five stations each year?) each station using an Ocean Instruments MC800 multi-corer and utilized for benthic flux measurements through whole core incubation technique (** reference? **). Temperature-controlled recirculating water baths adjusted to the bottom water temperature were used for all incubation experiments. Filtered bottom water was used to gradually fill up the core tube with minimum disturbance to the sediment-water interface. The overlying water column height was adjusted to 25 cm from the sediment surface to  ensure similar water column volume among all cores using custom PVC core caps. Care was taken to avoid any visible air bubbles or headspace. The custom designed lids are fitted with two O-rings for gas-tight incubations, and include one tube attachment for the corresponding water reservoir, one tube attachment for sample extraction, and two tube attachments that connect to each other in a peristaltic pump, allowing continuous flow and circulation in the core for the entire duration of the incubation. Sediment cores were fully submerged into the temperature-controlled water bath, and reservoir tanks along with the incubation water bath were covered with an opaque shroud to ensure no primary production. 

A custom benthic lander system was deployed with minimum sediment disturbance at each of  the stations to carry out in situ flux measurements for total oxygen utilization (TOU). The lander system was  equipped with chambers that each seal a sediment surface area of 840 square centimeters and a volume of  approximately 16 liters. Each chamber included a magnetically coupled stirrer that mixed the  water within the chamber uniformly in about 20 minutes at 12-16 rpm and produced a 300-400 μm diffusive boundary layer. Leakage was calculated from the loss of a CsBr tracer from samples taken from each chamber every 4 hours. Three of the benthic chambers were fitted with Seaguard dissolved oxygen (DO)  optode sensors that directly measured DO every 15 minutes for the duration of the 22  hour deployment.

Sediment oxygen penetration depth (OPD) was measured with a Unisense in situ 130 microprofiler system, which was equipped with two oxygen sensors (250 μm) and a resistivity sensor. The position of the sediment-water interface (SWI) was determined relative to the in situ oxygen profiles using a modified version of the technique of Sweerts 133 et al.  (** ? what is this reference?**) (Revsbech 1989; Sweerts et al. 1989; Rabouille et al. 2003). Diffusive oxygen utilization (DOU) was calculated with Cai and Sayles’s adaptation of Fick’s law of diffusion using the oxygen gradient in coastal sediments.

Measurement details:

• DIC was measured with an Apollo SciTech DIC multisample analyzer (AS-C6L) with a laser-based CO2 detector. Certified reference material from batch 180 provided by Dr. Andrew Dickinson was used for calibration for DIC and TA measurements.
• TA was measured with an Apollo SciTech Total Alkalinity titrator (AS-ALK2). Certified reference material from batch 180 provided by Dr. Andrew Dickinson was used for calibration for DIC and TA measurements.
• NO2+NO3 was measured using EPA method 353.4
• NH4 was measured using EPA method 350.1
• PO4 was measured using EPA method 365.1
• SiO4 was measured using using the method described in Strickland and Parsons (1968).
• Dissolved nutrinet analysis was caried out using a Seal Analytical Auto Analyzer.
• Oxygen penetration depth (OPD) was measured in situ with a Unisense microprofiler and utilized to caluclate DOU.
• TOU was measured using custom benthic lander system.

Diffusive oxygen utilization (DOU) was calculated with Cai and Sayles’s adaptation of Fick’s law of diffusion using the oxygen gradient in coastal sediments.

(** need more information here on how this was done and what the gradient was **)

- Converted date format from m/d/yy to yyyy-mm-dd (ISO Date 8601 format)
- Modified parameter (column) names to conform with BCO-DMO naming conventions. The only allowed characters are A-Z,a-z,0-9, and underscores. No spaces, hyphens, commas,

NSF Award Abstract:

In many coastal regions fertilizer washed down rivers leads to high production of algae. Consumption of organic material produced by the algae in deeper waters or in sediments consumes oxygen, leading to low-oxygen "dead zones" in many places like the northern Gulf of Mexico. This process of nutrient input, algal growth, and loss of oxygen is called eutrophication. Consumption of the organic matter also releases dissolved carbon dioxide into the water, and contributes directly and indirectly to ocean acidification. In order to understand and predict this impact on acidification, it is important to understand how much of the organic matter degradation occurs in the water column versus in the sediments, how variable it is between seasons and years, and how it is affected by processes such as storms that disturb the bottom sediments. In this project, investigators from three institutions along with their graduate and undergraduate students will conduct a combination of field observations and computer modeling to address these questions in the northern Gulf of Mexico. They will share their results with the public through local outreach activities, and with secondary school students in Louisiana through partnerships and curriculum development with local teachers.

The proposed research will use a combined observational and numerical modeling approach to better understand the role of shelf sediment in driving bottom water dissolved inorganic carbon (DIC) and pH dynamics and acidification at seasonal scales. Past and current studies have not addressed this mechanism. This current lack of knowledge makes it difficult to construct a comprehensive carbon budget for this region. The proposed research will (i) quantify the role of benthic fluxes in DIC production leading to acidification in the bottom water; (ii) determine the importance of the seasonally changing benthic DIC flux in acidifying the bottom water; and (iii) explore the importance of episodic resuspension events in modulating benthic fluxes of DIC. Seasonal sampling will be carried out in Louisiana shelf using state of the art benthic lander platform to record in situ sediment and bottom water oxygen consumption rates, organic matter remineralization rates, sediment oxygen penetration depths, benthic fluxes of DIC and accompanying pH drop. Bottom current velocities and turbidity will also be recorded in conjunction with sediment porewater and water column sampling. Complementing these detailed near-bed and seabed observations, we will utilize a recently developed coupled hydrodynamics, sediment transport and biogeochemistry model (HydroBioSed) to scale up observed estimates of benthic fluxes to an annual scale.

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.

Related Project note:

There are overlapping cruises with the project "nGOMx acidification" https://www.bco-dmo.org/project/751332. Thus, while all benthic data can be found under this project "Sed Control on OA", some water column data can be found under the "nGOMx acidification" project.