OFF TO WORK: Two underwater robots, called gliders, are deployed last year to observe water quality along the state’s coastal shelf. Gliders move up and down through the water column and take measurements every two seconds. (Photo credit: Chip Haldeman, Rutgers University)

Two Rutgers University scientists recently discussed the possible implications of their findings last summer of low dissolved oxygen and pH off the New Jersey coast, which concurred with numerous reported mortalities of fish, lobsters and crabs.

Grace Saba and Josh Kohut work within Rutgers’s Center for Ocean Observing Leadership, in the Department of Marine and Coastal Sciences. The center, as the university notes, maintains the world’s most advanced coastal ocean observatory, with platforms consisting of satellite imagery, a radar network for surface current mapping and waves, and a fleet of long-duration autonomous underwater vehicles, called gliders, equipped with physical, chemical and biological sensors.

Saba, an associate professor, and Kohut, a professor, employed gliders to map ocean water quality measures along the coast, surface to bottom, from late April to late September 2023. As they explained, “From August through September, much of the bottom water sampled from Sandy Hook south to Tuckerton, and from nearshore to deeper depths, exhibited dissolved oxygen concentrations less than 5 mg/liter and pH values less than 7.75.

“Coast-wide, hypoxic levels of dissolved oxygen (concentrations of less than 3 mg/liter) were observed at shallower, more inshore locations. In addition to low pH measured in bottom waters, which is indicative of ocean acidification, aragonite saturation state – a relevant metric for biological impacts of ocean acidification – was calculated to be less than 1 in several locations. Normal, more optimal levels in seawater typically include dissolved oxygen concentrations of more than 7 mg/liter, pH of 8.1, and aragonite saturation states of more than 3.”

These values are concerning, said Saba and Kohut, as oxygen is essential to ocean animals. “Dissolved oxygen concentrations at or below 5 mg/liter is considered problematic for marine life,” they noted. “Although concentrations between 3-5 mg/liter may not be low enough to directly cause death in many marine animals, research focused on New Jersey species has identified other negative impacts such as reduced metabolism, feeding, growth and reproduction at these levels.

“Lower hypoxic concentrations of dissolved oxygen (less than 3 mg/liter) have been directly associated with mortalities in some organisms in New Jersey and in other coastal regions around the world,” they added.

The many mortalities of fish, lobsters and crabs during the time period of Saba and Kohut’s project were observed in bottom waters primarily off the coast of Monmouth and Ocean counties. Mortalities for American lobsters, Jonah crab, Atlantic rock crab, spider crabs, black sea bass and tautog were reported not only in pots, where the trapped organisms would not have been able to escape poor conditions, but also on the open bottom. “This observation,” said the scientists, “suggests that if low dissolved oxygen and/or pH were indeed the culprit for these reported mortalities, the area may have been extensive enough that they could not escape in time.”

As Saba and Kohut pointed out, mortalities associated with low dissolved oxygen in New Jersey coastal waters or other locations is not new. “The most extreme hypoxic event documented in the state occurred during the summer of 1976, whereby mass mortalities of marine organisms occurred,” they noted. “More recently, numerous fish, lobster and crab mortalities were associated with low dissolved oxygen in Cape Cod Bay in September 2019, and hypoxic conditions have been identified as the culprit of mortalities and changes in bottom water communities in the Gulf of Mexico’s notorious summertime ‘Dead Zone’ since the beginning of annual observations that started in 1985.”

The cause of low dissolved oxygen and pH in bottom water, the professors explained, is a seasonal phenomenon: “strong summer stratification, whereby cold water near the bottom is capped off by warm, highly oxygenated water near the surface. This creates a two-layer stacking of water off the New Jersey coast with little exchange of oxygen between them.”

In addition, nutrients from rivers and estuaries foster the growth of phytoplankton in the surface layer, which produces surface layer oxygen through photosynthesis. “However, these surface plants eventually die and sink to the bottom layer where bacteria break them down,” said Saba and Kohut. “As bacteria metabolize, they remove oxygen from the bottom layer and produce carbon dioxide.

“Without the periodic input of surface water oxygen, the bottom layer can reach much lower dissolved oxygen levels that can lead to hypoxia (low dissolved oxygen) and acidification (low pH) in bottom waters. These seasonal patterns of biologically-driven seawater pH change are occurring simultaneously with ongoing global ocean acidification caused by the ocean’s absorption of the increasing concentrations of atmospheric carbon dioxide since the beginning of the Industrial Revolution.”

The poor conditions this past summer could have been more pronounced than in previous years, the researchers believe. Currently, Rutgers staff are continuing to analyze the data collected to identify the specific causes of the low dissolved oxygen and pH event.

As Saba and Kohut remarked, “Events such as these that may prevent the ability to sustain normal populations of marine organisms are concerning, not only for the ocean ecosystem but also for the local economy and commercial and recreational fishing industries. Understanding the factors that cause these events will aid in projecting the severity and duration of these events under ongoing climate change and provide important support for guiding the state’s policy options and identifying priorities for science and monitoring.”

To report any organism mortalities observed over this past summer, email Saba at saba@marine.rutgers.edu.

Juliet Kaszas-Hoch, The SandPaper, 12 January 2024. Article

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