Hypoxia

How do marine sediments affect oxygen and nutrient levels in the water?

Nitrogen and phosphorus enter marine sediments either by diffusion from the water column or as part of organic particles that settle on the surface. Once nitrogen is in the sediment it can either be buried, be converted to nitrogen gas by bacteria (a process known as denitrification) or re-enter the water column. Similarly, phosphorus can be buried in the sediments, absorb onto iron oxides, or diffuse back out into the water column. By measuring the mass balance of nutrients entering and leaving the sediment, researchers can estimate some

The search goes on for a set of definitions and thresholds to represent low-oxygen concentrations that threaten various aquatic creatures. Over the years, ecologists have relocated, reshaped and revised the word “hypoxia” to describe these conditions. In part four of our series "Oxygen for life" we look at how scientists determine whether oxygen levels are low enough to be considered harmful to sea life. 

Scientists are reporting a decline in oxygen-rich waters throughout the world. Causes for the decline vary from place to place but may involve climate change and increasing discharges of tainted water. In Puget Sound, low oxygen levels can occur naturally or due to eutrophication from human-caused pollution. In this five-part series, we describe the critical nature of oxygen to Puget Sound sea life. Scientists are finding that changes in oxygen levels can lead to physiological adjustments, shifts in predator-prey relationships and other repercussions throughout the food web.

As observed in Hood Canal, low-oxygen conditions can upend the lives of Dungeness crabs trying to stay alive. Levels of dissolved oxygen can alter predator-prey relationships for a multitude of species, affecting populations throughout the food web. Part two of our series "Oxygen for life" examines a crab case study.

In time, lower dissolved oxygen worsened by climate change could increase the abundance of rare species in Puget Sound while putting populations of more common species into a tailspin. Part three of our series "Oxygen for life" looks at how warmer waters will gradually make it harder for many sea creatures to breathe. 

How do excess nutrients trigger low oxygen conditions in Puget Sound and what do those conditions mean for the species that live here?