Many aquatic species, from shrimp to sharks, are vulnerable to low levels of dissolved oxygen, but what level is too low? An oxygen level that endangers one species might be just fine for another, according to scientists studying the oxygen needs for various aquatic animals.

Building upon this concept, researchers with the Puget Sound Institute are layering data about low-oxygen tolerance, species by species, onto the Salish Sea Model, which is designed to predict oxygen levels in Puget Sound. The goal is to quantify risk for any given species at any place and time within the Sound.

One outcome of the project is a series of maps that display danger zones for various species at particular times of the year. The project so far has already produced preliminary results for Chinook salmon, Dungeness crabs and English sole.

Warmer water causes a higher rate of metabolism for most marine creatures, and that leads to a greater consumption of oxygen.

“We want to ask when and where are organisms present where the environmental conditions are dipping below (oxygen) thresholds,” said Tim Essington, a University of Washington ecologist discussing the project during an August video workshop — part of PSI’s Science of Puget Sound Water Quality series.

In this current study, the “critical oxygen level” for a species is defined as the level needed for an animal to maintain normal function and activities, not just stay alive, Essington said.

When dealing with concerns about oxygen, temperature is a key factor, he noted. Warmer water causes a higher rate of metabolism for most marine creatures, and that leads to a greater consumption of oxygen. Ironically, warmer water also holds less oxygen, so organisms face a double whammy — a greater need for oxygen when oxygen is less available. 

When it comes to oxygen needs, some animals are more sensitive to temperature change than others, Essington said, meaning their oxygen needs increase faster as the temperature rises. Others are less sensitive, so their oxygen needs remain fairly stable with temperature change. In any case, water temperature helps determine whether available oxygen levels drop below an animal’s critical needs, altering normal behaviors and increasing risks that could lead to death.

For this project, funded by King County, the researchers are using a measurement called the “metabolic index,” which conveniently accounts for the oxygen needs of a given species versus the amount of oxygen available, considering the temperature. 

In his work, Essington has analyzed data from more than 450 laboratory measurements of critical oxygen levels. With an approach that provides consistency among the data, he has calculated critical oxygen thresholds for more than 150 species. That information plays into the Salish Sea Model, which can theoretically pinpoint when and where environmental conditions fail to provide the required oxygen needs for a given species.

Stefano Mazzilli, a research scientist involved in water-quality modeling at the Puget Sound Institute, has used the Salish Sea Model to look for places in Puget Sound where the predicted oxygen levels fall below the needs of Chinook salmon, English sole and Dungeness crabs — the first species used to test this new approach.

“We are in a great position to start this work,” Mazzilli said during the recent workshop. “There has been a big push in Washington state in recent years to improve not only monitoring but also modeling of physical and biogeochemical change in Puget Sound.”

Since different species occupy different depths in the water column, the modeling exercise requires a focus on the range of depths where certain species reside, Mazzilli said. For Chinook salmon, which rarely travel into very deep water, the analysis focuses only on waters down to 100 meters (328 feet) from the surface. For English sole, a bottom fish, and Dungeness crab, which live on the bottom, the focus shifts to the lowest layer in Puget Sound for places where these animals reside. 

During his presentation, Mazzilli showed graphic plots addressing oxygen conditions over the course of a year at particular locations in Puget Sound. He included areas, such as southern Hood Canal, where the model predicts that oxygen needs would not be met during certain periods of time for one or more of the species.

For example, a plot of oxygen conditions in southern Hood Canal during the year 2014 shows a fairly steep decline in oxygen during the late summer months with a sudden drop off in October. Based on the model, during that time, slow-moving Dungeness crabs would experience oxygen levels below the critical threshold, a condition detrimental to their survival if they are unable to move into a safer area. 

An animated map of the entire Puget Sound, shown during Mazzilli’s presentation, spans a full year and uses the color red to show when and where a crab’s oxygen needs are not met. The animation shows that the first adverse conditions of the year appear in southern Hood Canal and Bellingham Bay in late August, followed by Penn Cove on Whidbey Island. One can observe how long each area remains in the danger zone for Dungeness crabs.

“Sequential days below the critical threshold are really important in terms of accumulated risk and opportunities for recovery,” Mazzilli said, showing a related static map with various colors depicting the number of days in the year that conditions fall below the oxygen threshold for the crabs. 

Another map reveals a more complicated concept: How the modeled condition in 2014 and its effect on the crabs compares to the reference condition — something akin to “natural conditions.” This is basically a modeled description of historical conditions at a time before humans influenced the oxygen levels in Puget Sound. (A Salish Sea Currents magazine article provides an extended description of natural conditions criteria).

A similar analysis was described for Chinook salmon with the additional complication of water depth. While Dungeness crabs stay on the bottom, Chinook may choose various depths, so their vulnerability becomes more variable.

In southern Hood Canal, with its seasonal low-oxygen condition, Chinook salmon would experience oxygen deprivation in the bottom layers, such as what happens with Dungeness crabs, Mazzilli said. The model predicts that the fish would be exposed to oxygen levels with “a high level of impact on their routine activities.” Alternatively, however, a fish would normally swim up to a higher level in the water column where low-oxygen conditions are not as severe. 

In mapping the vulnerabilities for more mobile species, one can average the oxygen levels at all depths for a specific area. In the case of Chinook, it turns out that there are no areas in Puget Sound where oxygen levels, as modeled, reach a critical condition, since the fish can move to a safer depth.

With the help of a graduate student over the next three years, Essington intends to expand the analysis of oxygen needs to other Puget Sound species.

“We are looking forward and would appreciate feedback and suggestions on how to assess risk, particularly for these mobile species,” Mazzilli said, adding that future work involves not only a focus on adults of various species but also on early life stages — often a more vulnerable period for many animals.