Fig 1. Retrieval of the CTD rosette on the M/V Pelican. The rosette contains 12 12-liter Niskin bottles and with accompanying electronic instrumentation (CTD strapped to the bottom of the rosette frame) is used to vertically profile the chemical and physical properties of the seawater. The instrument was deployed to depths of 1,300 meters during the cruise. Pictured on the left is Michael Scaboo (University of Delaware) and on the right is John Lehrter (University of South Alabama/Dauphin Island Sea Lab).

(April 2017) -- Dr. John Lehrter and Rebecca Pickering-Turner of the University of South Alabama and Dauphin Island Sea Lab participated in a research cruise aboard the M/V Pelican from April 5-17, 2017. 

The cruise involved 10 scientists working collaboratively to investigate the role of the Mississippi River in coastal acidification and hypoxia (O2 < 2 mg/L).  

Dr. Lehrter served as co-chief scientist for the cruise and Mrs. Pickering-Turner conducted her Ph.D. thesis research and assisted with Dr. Lehrter’s experiments. 

The Experiments

Fig 2. Extracting a sediment core from a sediment sample retrieved on the M/V Pelican. Box cores (0.25 m2) were obtained from three sites across the shelf at depths of 20, 33, and 56 m. On the left, Michael Scaboo holds box coring instrument in place while John Lehrter extracts a sediment core. The extracted core is shown on the right. Triplicate sediment cores were extracted to measure sediment-water exchange rates of oxygen, pH, inorganic carbon, alkalinity, nutrients, and dinitrogen gas. These processes contribute to the development of hypoxia (O2 < 2 mg/L) and acidification (declining pH) in the bottom waters of the Louisiana shelf.

In total for the cruise, seawater samples were collected at 86 sites across the broad Louisiana continental shelf and slope covering a depth range from 5 to 1,300 meters. At each site, the water-column was sampled vertically (Fig. 1) to provide a three-dimensional view of shelf, seawater chemistry. At three of the sites, sediment cores were collected (Fig. 2) to assess the contribution of sediment-water exchanges of carbon, oxygen, and nutrients to variations in pH in the bottom waters, which is where the most extreme acidification and hypoxia occurs on the Louisiana shelf.  

At 41 of the sites, water-column primary production and respiration rates were measured (Fig. 3) to determine the contributions of these processes to observed carbon dioxide and oxygen changes. 

Other experiments conducted on board, such as those conducted by Mrs. Pickering Turner on silica weathering (Fig. 4), assist in linking carbon dynamics with the nutrient cycles that drive production and respiration. Continuous underway measurements of carbon dioxide, pH, oxygen, argon and other seawater properties were made over a total cruise track of more than 3,000 km. 

Collaborative Project

Fig 3. Deck incubator with incubation bottles. The incubator was used to measure water-column primary production and respiration in up to 6 light treatments. The incubator was pumped with continuous surface seawater to maintain ambient temperatures during 24-hour incubations.

This research cruise is a part of a collaborative project to quantify the physical and biogeochemical mechanisms that control acidification and hypoxia in coastal waters. 

The work is being led by researchers at the University of Delaware, Louisiana Universities Marine Consortium, University of Massachusetts Dartmouth, Dalhousie University, and University of South Alabama/Dauphin Island Sea Lab through a grant from the National Science Foundation.

Why this research?

Globally, the ocean takes up approximately 25 percent of the anthropogenic carbon dioxide emitted to the atmosphere.  On the one hand, this is good because the oceans reduce the greenhouse gas concentration in the atmosphere.  

On the other hand, the uptake of carbon dioxide is changing the ocean’s chemistry in a process called ocean acidification whereby some of the carbon dioxide in seawater forms carbonic acid that results in declining pH in the ocean. 

Fig 4. Samples derived from Rebecca Pickering-Turner’s experiment on reverse weathering of silica (upper photo) turn into long nights in the radiation lab processing samples (lower photo). Rebecca is working on her Ph.D. in Dr. Jeffrey Krause’s laboratory at the Dauphin Island Sea Lab.

In the northern Gulf of Mexico, marine life may be at particular risk owing to acidification and hypoxia (O2 < 2 mg/L) that also form as a result of river inflows.  The land-based river inflows of nutrients and organic matter cause acidification in addition to that occurring in open ocean waters. As a result, the combined acidification and hypoxia in coastal regions is a growing threat to marine biota. 

Research and monitoring programs are needed to quantify this threat, to understand the underlying mechanisms and effects on biota, and to assess what management options may be effective in reversing or stabilizing acidification and hypoxia trends.