South Slough’s oxygen-poor water may be due to climate change

Sediment from Coos Bay’s South Slough reveals increasing periods of low dissolved oxygen levels over the last 300 years and potentially opens a window on how organisms adapt and what’s to come with climate change, according to a University of Oregon study.

Enter from the Pacific Ocean at Charleston and turn right, rather than into the often-dredged ship channel, and you are in the South Slough National Estuarine Research Reserve, a 5,000-acre federally protected natural area of forests, wetlands, ponds, salt marshes, mud flats and meadows established in 1974.

Study co-authors Dan Gavin and Josh Roering check a core of sediment taken from the South SloughThe slough is a good, natural laboratory for beginning to understand seasonal fluctuations, said Geoffrey Johnson, a doctoral student in the Department of Geography and Environmental Studies Program.

In rainy seasons, freshwater flows into it. In summers, seawater takes over. But rainy seasons have become shorter, giving way to longer, hotter summers.

In recent summers, such as 2002 and 2006, areas along the Oregon coast have endured increasing upwelling in which cold, salty, nutrient rich, low oxygen water rises from the deep ocean. While upwelling serves up nutrients that boost fishing conditions, as that water moves into the estuaries the contents can result in a condition known as hypoxia.

“Those differences are likely to become more intense as freshwater feeding the slough decreases amid hotter, longer summers and is replaced by a noted increase in upwelling,” said Johnson, a native of Eugene. “This upwelling does create an amazing fishery off the coast, which is good. But in strong upwelling events, that low-dissolved-oxygen water suffocates aquatic organisms. Fish can escape. Bottom dwellers cannot.”

This pattern is seen in sediment cores – particularly in the concentration of metals worn down from rocks, taken in by organisms and deposited in the mud – taken from the South Slough and to the north in Haynes Inlet. The geochemical content is an archive of changing conditions dating to about 1680. In the South Slough, Johnson said, it matches up well with measured dissolved oxygen in the last 14 years.

“The important finding of this paper centers on what the sediment of the South Slough is made of, and how it responds to the chemistry of the water above it in this type of estuary,” Johnson said. “The amount of oxygen in the water, which allows organisms to breathe, is impacting the mud under the water. We could see almost annual changes in the composition of the sediment. We found chapters of low-oxygen periods in the history of the estuary that were there long before Europeans arrived.”

Until recently, however, periods of impaired oxygen levels were likely rarer, he added.

“Over the span of our reconstruction, our geochemical proxy had a pattern that is robust with changes in climate,” he said.

To the north in Haynes Inlet, a different story emerged. The cores reflected a faster addition of sediment in recent years. Evidence of low-dissolved oxygen was found in the deeper past.

An earlier study, led by Dave Sutherland, a co-author on the new study, had focused only on observational data on the main channel, finding no evidence of hypoxic water infiltrating into it. The Johnson-led study, Sutherland said, provides a different metric for identifying dissolved oxygen stress, one involving the addition of biological processes, in the nearby South Slough and potentially applicable to other estuaries.

“While the new study doesn’t really get at the mechanisms at play, the real novelty is the technique, using cores to get at dissolved oxygen levels in the past,” he said.

The difference between the two locations likely relates to dredging to keep the channel open to ships coming and going from Coos Bay, Johnson said.

“The dredging has increased the volume of water in the estuary so much that the periods of low-dissolved oxygen that potentially could be hazardous to organisms in the estuary may have been buffered by having a large volume of water available,” he said. “Dredging has opened up the channel that feeds water farther in. Historically, the channels filled in and were much shallower in the past.”

The study, Johnson said, potentially sets a starting point for exploring the historical record of estuaries to the north in the Pacific Northwest, many of which have been more prone to disruptions generated by hypoxia than the more southern estuaries.

The mostly untouched South Slough, Johnson said, appears to reveal how an unmodified estuary behaves.

“Seasonality in these estuaries is important,” he said. “We need to better understand what has happened and what may happen under changing climate conditions. Have changes, including human impacts, been recorded over longer periods of time? Having more information could have ramifications for estuaries’ management.”

Co-authors on the study, which was published in the journal Estuaries and Coasts, were Sutherland and Josh Roering, professors in the Department of Earth Sciences; Nathan Mathabane, a former graduate student in Roering’s lab; and Daniel Gavin, a professor in the Department of Geography.

Oregon Sea Grant, with funding from the National Oceanic and Atmospheric Administration and the U.S. Department of Commerce, supported the project. Additional funding came from the state of Oregon and the National Estuarine Research Reserve System Science Collaborative.

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