Nitrogen and plankton: Do they hold the missing keys to the food web?

In a way, some of Puget Sound’s most serious ecological problems have been hiding in plain sight. I have been learning a lot lately about plankton, an incredibly diverse collection of microscopic organisms that drift through the water, forming the base of the food web.

Sources of nitrogen in Puget Sound (click to enlarge)
Graphic: Washington Department of Ecology

To put it simply, the right kinds of plankton help to create a healthy population of little fish that feed bigger fish that feed birds and marine mammals, including the endangered Southern Resident killer whales. On the other hand, the wrong kinds of plankton can disrupt the food web, stunt the growth of larger creatures and sometimes poison marine animals.

OK, that’s a bit of an oversimplification, but Puget Sound researchers are just beginning to understand the profound importance of a healthy planktonic community to support a large part of the food web. That’s one of the main points that I try to bring out in five stories published today in the Encyclopedia of Puget Sound. I am grateful to the many researchers who have shared their knowledge with me.

Average daily nitrogen coming in from rivers and wastewater treatment plants (1 kg = 2.2 pounds)
Graphic: Washington Department of Ecology

These stories tie together several major issues all related to nutrients — mainly nitrogen — that feed the marine phytoplankton, which use their chlorophyll to take energy from the sun as they grow and multiply. In the spring and summer, too much nitrogen can mean too much plankton growth. In turn, excess plankton can lead to low-oxygen conditions, ocean acidification and other significant problems.

The complex interplay of planktonic species with larger life forms in Puget Sound is still somewhat of a mystery to researchers trying to understand the food web. As part of the effort, the Washington Department of Ecology is working on a computer model to show how excess nitrogen can trigger low-oxygen conditions in the most vulnerable parts of the Salish Sea, such as southern Hood Canal and South Puget Sound.

Areas of Puget Sound listed as “impaired” for dissolved oxygen (click to enlarge)
Graphic: Washington Department of Ecology

Stormwater is often cited as the most serious problem facing Puget Sound, and we generally think of bacteria and toxic chemicals flowing into the waterway and causing all sorts of problems for the ecosystem. But stormwater also brings in nitrogen derived from fertilizers, animal wastes and atmospheric deposits from burning fossil fuels. Stormwater flows also pick up natural sources of nitrogen from plants and animals that end up in streams.

Sewage treatment plants are another major source of human nitrogen. Except for a few exceptions, not much has been done to reduce the release of nutrients from sewage-treatment plants, which provide not only nitrogen but also micronutrients such as vitamins and minerals. Some experts suspect that nutrients other than nitrogen help to determine which types of plankton will dominate at any given time.

I plan to follow and report on new scientific developments coming out of studies focused on the base of the food web. Meanwhile, I hope you will take time to read this package of related stories:

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