SCIENCE:

CO2 absorption and nutrient runoffs increase acidity of Puget Sound

A combination of carbon dioxide emitted by human activities and nutrient runoff is transforming the chemistry of Washington state's Puget Sound, according to a new study.

Without intervention, the one-two punch could threaten the area's shellfish industry, said lead author Richard Feely, a senior scientist at the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory in Seattle.

Feely and his team sampled waters in Puget Sound in February and August 2008, discovering that the estuary's waters were surprisingly acidic -- and in some areas, likely corrosive to many shelled creatures.

The ocean averages a pH of 8.1, on a 14-point scale where battery acid tests at 0, pure water at 7 and drain cleaner at 14.

But in Puget Sound's Hood Canal, Feely and his colleagues sampled water with a pH of just 7.4 in summer 2008. Water in the sound's main basin carried a pH of 7.7.

The scientists say a combination of processes is to blame. One is the ongoing acidification caused by oceans' ongoing absorption of CO2 from the atmosphere. As emissions of the greenhouse gas have risen, the average ocean pH has dropped from 8.2 to 8.1. -- a seemingly tiny jump that represents a 30 percent increase in acidity.

In Puget Sound, nutrient runoff also plays a role in shifting water chemistry. It encourages growth of phytoplankton and zooplankton that add to the sound's acid burden when they die and sink into the water column, where they decompose -- releasing CO2.

"These processes combined together to decrease pH further than what we would expect from one or another by themselves," Feely said.

One of his previous studies demonstrated that corrosive water is already upwelling each summer off North America's Pacific coast, reaching levels of acidity that scientists had predicted wouldn't occur until 2050.

Bad news for a struggling shellfish industry

Some of that water is making its way into Puget Sound through the Strait of Juan de Fuca, Feely said, and the CO2 produced by decaying plankton just adds to the acidification caused by carbon pollution.

He and his colleagues estimate that CO2 emissions have caused 24 to 29 percent of the pH drop in Hood Canal, compared to the preindustrial level.

Over time, they say, the effect of CO2 from human activities could begin to dwarf the contribution from decaying plankton. If the level of carbon dioxide in the atmosphere hits 560 parts per million (ppm), it could account for 49 to 82 percent of the pH drop at that point in time.

The current level of CO2 in the atmosphere stands at 390 ppm, up from a preindustrial level of 280 ppm.

Feely said his findings should be of concern to the Pacific Northwest's shellfish industry, which has struggled in recent years.

Beginning in 2008, hatcheries have experienced a sharp increase in oyster larvae mortality. Feely says some recent studies conducted at Oregon's Whiskey Creek hatchery "showed that when waters were upwelled, that's when larvae were dying."

Meanwhile, Taylor Shellfish Farms has struggled with oyster larvae die-offs at its hatchery on Puget Sound's Dabob Bay (ClimateWire, April 23).

Said Feely, "We're beginning to wonder if the corrosive water in the deeper water of Hood Canal and maybe Puget Sound gets pushed up on surface waters during storm events, and then that corrosive water gets into hatcheries."

The study will be published next month in the journal Estuarine, Coastal and Shelf Science.