What are algae?

Algae are one-celled or multicellular plant-like organisms that usually contain chlorophyll. There are both marine and freshwater algae, and algae are found almost everywhere on earth and are an important source of food for zooplankton, fish, and other lake organisms.

What makes algae grow?

Algae grow when they have the right conditions such as adequate nutrients (mostly phosphorus but nitrogen is important too), light, and water temperatures. The typical growing season for algae is much the same as your lawn and garden – the warmer, sunnier months from April through October.

In general, the more nutrient rich the lake, the more algae will grow. For lakes Union, Washington, and Sammamish, phosphorus is the nutrient that controls the amount of algae. The nutrients phosphorus and nitrogen are found in animal and human waste (sewage), in fertilizers, and even in rainwater. Too much phosphorus and nitrogen leads to too much nutrient-enrichment that leads to algal blooms.

What is an algae bloom?

An algae bloom is the visible appearance of millions of tiny plant-like organisms in the water. Algae are usually in very small numbers and thus not visible to the human eye. But when there is lots of sunshine and food (dissolved nutrients like nitrogen and phosphorous) they can reproduce very quickly and grow to such large numbers that we can see them in the water.

Green Algae Blooms

Accumulations of the green algae Oedogonium and Spirogyra have been observed at swimming beaches on Lakes Washington and Sammamish. Both of these green algae are very common, and grow as filaments often attached to rocks and the bottom substrate. Accumulations along the shoreline of these algae of often noted around Seafair (mid to late July) because the filaments have grown long enough to be broken off by wind and wave action. Filamentous green algae can interfere with boater access, recreation, and aesthetics. Though generally harmless, the decomposition of piles of these algae definitely degrades the immediate water quality and aesthetics as they can give off a very pungent odor similar to the smell of decomposing lawn clippings.

Green algae, or Chlorophytes, do not produce toxins like some cyanobacteria (blue-green algae).