Hypoxia

Estuaries around the world including Puget Sound perform an amazing feat of continuous water mixing called estuarine exchange flow. 

Nitrogen is a chemical element that is essential for the growth of all life on earth. But too much nitrogen can lead to low dissolved oxygen and other problems such as toxic algal blooms that can harm or kill aquatic organisms. 

The following fact sheet provides an overview of low oxygen conditions in Puget Sound. It addresses some of the related causes and concerns that have been identified by scientists in the region. The overview was prepared in conjunction with a series of workshops on hypoxia and nutrient pollution presented by the University of Washington Puget Sound Institute. 

In many parts of Puget Sound, hypoxic waters are thought to be at least in part due to overgrowth of microscopic algae, which is triggered by excess nitrogen. That means it’s important to understand the dynamics of primary productivity – the rate at which those microscopic algae, known as phytoplankton, produce organic matter through photosynthesis and in this way provide the base of the food web. Researchers are investigating different types of phytoplankton and rates of primary productivity throughout the Salish Sea, and seeking to understand how primary productivity is likely to change as climate change alters patterns of coastal upwelling and freshwater flow into the Sound.

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.