Keywords: Water quality, Species and food webs, Algae, Nutrient pollution, Harmful algal blooms, Phytoplankton

Diverse communities of microscopic organisms called phytoplankton make up the base of the aquatic food web. In that role, they are essential to the tiny animals that eat them, but phytoplankton are not dependent on others. Thanks to chlorophyl, these tiny organisms can generate their own energy from nutrients and sunlight. Despite their critical importance to a great diversity of sea life in Puget Sound, phytoplankton can also contribute to low-oxygen conditions, and some can be harmful in other ways.

While nitrogen and sunlight can activate plankton populations and invigorate the food web, particularly in spring and summer, it turns out that not all plankton are equally beneficial, and some are considered harmful. Water conditions —including temperature, minerals, and salinity — play a role in determining which types of plankton will win in the Battle of the Species.

Spring phytoplankton growth in Puget Sound is normally dominated by diatoms, a stunningly beautiful variety of planktonic species with a cell wall made of silica, the raw material for glass. Diatoms are considered to be a foundation of the food web, providing energy-rich lipids for a variety of zooplankton — from tiny shrimplike crustaceans, such as krill and copepods, to the larval stages of fish, crabs and shellfish.

As spring turns to summer, Puget Sound sees a decline in river flows, the primary source of silica. Without silica, the diatom population can be overwhelmed by other phytoplankton, such as dinoflagellates that do not need silica and can thrive in the presence of excess nitrogen. Dinoflagellates, which include a variety of toxic species, are characterized by their tail-like flagella that help them move through the water.

A microscopic image of a single, round phytoplankton with a tail-like appendage against a blue background.

The tail-like flagella of Noctiluca scintillans helps this common phytoplankton move through the water. This species produces extensive plankton blooms and can devour all sorts of other plankton reducing the food supply for other species. Photos: ProyectoAgua (CC BY-NC-SA 2.0)

A common dinoflagellate, Noctiluca scintillans, also known as the “sea sparkle,” produces extensive plankton blooms that look like orange paint spilled upon the water. Although the organism produces no significant toxins, it can devour all sorts of other plankton — from diatoms to copepods to fish eggs — reducing the food supply for other species. At the same time, Noctiluca are not eaten by many species, making them somewhat of a dead end in the food web.

Red-orange water near the shoreline of beach next to small jetty and passenger ferry.

Noctiluca scintillans can create large blooms that look like orange paint in the water as seen here near the ferry terminal in Edmonds, Washington. Photo: Washington State Department of Ecology/ photo submitted by Jeri Cusimano (CC BY-NC 2.0)

Other dinoflagellates are able to take advantage of the stratified conditions in Puget Sound by using their flagella to swim down to find nitrogen then return to the surface for sunlight. Among the wide variety of dinoflagellates found in Puget Sound, a few are known for their toxicity in humans. They include Alexandrium, which causes paralytic shellfish poisoning, and Dinophysis, responsible for diarrhetic shellfish poisoning.

In the battle for survival among plankton, the species that thrive above others are those best adapted for the specific circumstances — including physical conditions, such as temperature and salinity; the levels of macronutrients, such as nitrogen and phosphorus; and the presence of micronutrients, including iron and copper.

Researchers are engaged in sorting out the specific conditions that give rise to extensive blooms of particular species. For dinoflagellates in general, a variety of studies suggest that higher water temperatures and increased ratios of nitrogen to other nutrients may be key factors in their growth.

Climate change, including warmer waters, is likely to encourage an ongoing shift in plankton species in Puget Sound. At the same time, an overall warming trend may melt the winter snowpack earlier in the year, diminishing summer streamflows that help flush the nitrogen-rich surface waters out to sea. Ultimately, increases in nitrogen coming into Puget Sound could contribute to major alterations in the food web.

This article was funded in part by King County in conjunction with a series of online workshops exploring Puget Sound water quality.

About the author: Christopher Dunagan is a senior writer at the Puget Sound Institute.

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