algae

a highly diverse group of
plant-like organisms that grow in water.

Algae in the Slate River near the Crested Butte Recreation Bridge on September 8, 2020. Photo Credit: Allison Del Gizzi, CCWC Staff

WHAT ARE ALGAE?

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You have probably seen it, maybe even slipped on it the last time you went fly fishing, but have you ever wondered about the slimy stuff growing on the bottom of our streams and rivers? That slippery slime is probably some kind of algae, and it’s part of the natural ecology of our rivers and streams.

Algae in Washington Gulch near the recreation path on September 8, 2020. Credit: Allison Del Gizzi, CCWC Staff

Algae is a term used to refer to a highly diverse group of plants that grow in water. Freshwater algae come in a variety of shapes, sizes, and colors, and are an essential component of all freshwater ecosystems.
 
In most of the rivers and streams throughout the Upper Gunnison Watershed, algae can be seen growing on rocks, logs, and other underwater surfaces.  If you were to visit a particular fishing spot or boat ramp consistently, you might notice that the algae population varies week-to-week, month-to-month, and even year-to-year.

DO OUR STREAMS HAVE TOO MUCH ALGAE?

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Exactly what algae communities’ impact on environmental health is and methods to determine which factors are most heavily controlling their distribution are developing topics both locally and nationally.

Recent sampling conducted by CCWC (with the support of partners) indicated that algae concentrations in Crested Butte's local watersheds are within the state’s current standard.

Algae in Woods Creek on September 9, 2020. Photo Credit: Allison Del Gizzi, CCWC Staff

The most effective way to use that data to implement beneficial management practices is also an emerging subject.

For more about algae in our rivers look for CCWC’s upcoming Washington Gulch Watershed Plan, expected to be completed in Spring 2021.

What Affects Algal Growth?

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The observable increasing and decreasing of algae populations is a result of many different influences. The following factors can heavily impact algal growth:

Climate and Hydrology
Seasonal temperatures, climate driven events, and the way water moves through an ecosystem affect algal growth and development in various ways. In high alpine environments, spring snowmelt typically scours algae from river bottoms allowing for a fresh start in the summer. However, years with a smaller snowpack see weaker spring runoff and flows fail to fully clean streambeds, resulting in larger amounts of algae the following summer. Other climatic events such as drought and fire can also alter algae growth patterns.

Snowmelt Driven River System. Photo Credit: NASA, Monday, January 28, 2019

Vegetation Coverage
Algae, like other plants, need light to survive. Shade can play a major role in algae production since it limits algal growth. The amount of shade over a stream depends on many factors such as elevation, stream size, native vegetation type, and vegetation health. A stream surrounded by degraded vegetation or no vegetation could be expected to see a rise in algal growth due to increased access to sunlight.

Riverbed Material
Riverbed materials vary in different environments and can include bedrock, cobble, sand, silt, and wood. The size of riverbed material present in a stream determines the amount of surface area available, controls access to light, and influences invertebrate communities, therefore affecting algae’s ability to grow and reproduce.

Algae growing on a rocky bottom in the Slate River, August 27, 2020. Photo Credit: Allison Del Gizzi, CCWC Staff

Aquatic Life
Algae are at the base of aquatic food webs, which means they are usually being eaten by other organisms. Some invertebrates, commonly referred to as “bugs”, consume algae. Generally, fish eat the bugs that consume the algae. These relationships between algae, bugs, and fish influence algae growth and distribution.

Photo Courtesy of US Fish and Wildlife Service

You may be an angler, or know one, who looks for slippery reaches to fish in. Such thinking is well-founded, as a slippery reach could suggest that numerous fish are eating bugs rapidly, preventing the bugs from consuming algae and, therefore, making the stream slippery.
 
Water Management Practices
The way water is managed by humans affects flow rates which can heavily impact algae growth. Man-made alterations like diversions and dams can have a particularly noticeable effect. If stream flow is reduced due to a diversion, algal growth immediately downstream may increase. If a flow is reduced to zero, the algal community could be killed in that stretch of river.

Nutrients
When algae have access to high quantities of nutrients, they can grow faster and become more abundant. Nutrients occur naturally in any stream system. For example, when leaves fall off of trees in autumn, they decompose and add nutrients to rivers and streams. Human inputs such as agriculture, stormwater, and wastewater runoff also deliver nutrients to stream systems.

Sediment-laden water from a tributary. Photo Credit: NASA, Thursday, June 19, 2014

Water Chemistry
All aquatic life, including algae, can be impacted by water chemistry. Many species have a range of water chemistry conditions that they do well in. For example, if a particular stream had a high concentration of arsenic, many species of algae and bugs would have difficulty growing and surviving.

Aluminum pond in Redwell Basin August 6, 2013.

All of the factors described above control algae growth and distribution simultaneously and to varying degrees. Determining which factors are most heavily influencing algal growth at any one time in any one location can be challenging, to say the least.