Tiny Plankton, Big Threat To Puget Sound Food Chain

SEATTLE, WA — If you took a plane ride over central Puget Sound right now, you might notice a swirling tomato-soup colored fuzz floating on the water. That fuzz is a plankton bloom, and the countless tiny organisms in it pose a serious threat to the food chain, from the tiniest krill up to salmon and orcas.

State Department of Ecology scientists used a seaplane this week to measure the bloom, and saw that it stretches from Tacoma up to Edmonds. The Noctiluca plankton bloom has become common in Puget Sound in spring, but it arrived much earlier than expected this year, likely due to warm, sunny weather. The swarm of millimeter-long Noctiluca organisms, while not toxic to humans, devour vital nutrients in Puget Sound, creating a leak in the food chain, state Department of Ecology ecologist Christopher Krembs said.

This type of bloom starts with nitrogen. Most of the nitrogen in Puget Sound cycles in from the Pacific Ocean or rivers. But in sunny, calm conditions, nitrogen from land sources — including sewage treatment plants — can hang around on the sea surface, allowing rapid growth of plankton, including Noctiluca.

Noctiluca feast on diatoms, a crucial food for small crustaceans and filter feeders that are chock-full of lipids. In large numbers, Noctiluca eat all the lipid-rich diatoms and other nutritious organic material. That leaves creatures like herring and salmon deprived.

"The currency here is lipids," Krembs said, referring to fish that need the fats lipids provide to survive in cold water. "At the base of this is diatoms, food that supports what we consider commercially-interesting organisms. When Noctiluca feed off the diatoms ... they're cannibalizing the food chain."

The bloom is peaking just as juvenile Chinook salmon are leaving freshwater and entering Puget Sound. Once in the saltwater, the salmon rely on diatoms and their lipids to be passed up the food chain.

Without those nutrients, "They're just goners," Krembs said.

That's a big deal for orcas. The Southern Resident orcas as a whole need to eat about 1,400 Chinook per day to survive. The Chinook population is already under stress due pollution and dams in the Snake and Columbia rivers.

The wisps of tomato soup plankton you see in Puget Sound today are actually the Noctiluca in the process of dying. Once dead, they just kind of float in the water totally useless. They also emanate lots of ammonia, even enough to kill some smaller fish, Krembs said.

The Noctiluca are similar to the sound's jellyfish, which have exploded in numbers recently. Both creatures hog nutrients and are inedible for other marine animals in the food chain.

These plankton blooms have not always happened. The state only knows of two instances of a major bloom between World War II and the 1990s. In the late 1990s, the blooms started happening almost every year. The blooms have been especially strong over the last two years, Krembs said.

Weather plays an important role in keeping the Noctiluca population down. Rain in the spring helps circulate water, preventing nitrogen buildup. The first two weeks of May, however, was the driest in more than 70 years, with no rain over the first 13 days.

Even with recent rain, Krembs thinks the Noctiluca bloom could last another month or so.

"It's such a small organism, but to see it on such a large scale, it drives home the point how it can actually change the food web dynamics," he said.

Watch this video from the state Department of Ecology about the plankton bloom going on now: