Green Algae

Out on an early spring walk on March 4. The spring peepers were in full chorus. I couldn’t help including this short audio/video.

However, this post is about algae, not frogs. I stopped by my streamkeeper site and noticed some filamentous green algae on the rocks which is unusual. To understand why, I want to explain more about what I understand about algae.

Algae is a general term used to describe a collection of aqueous organisms which engage in photosynthesis, but don’t have structures we usually associate with plants, such as leaves, stems, or flowers. Not all of the organisms described as algae are even closely related to each other. These can include diatoms, filamentous algae, and even kelp. Cyanobacteria are often referred to as blue-green algae, although most scientists no longer classify them as algae.

The commonest algae in the mainstream of Pennypack Creek are diatoms. These are single-celled photsynthetic organisms characterized by a silicon-based cell wall which may form fantastic shapes. Click here to see pictures. Microscopically, they are beautiful. Macroscopically, they look like greenish-brown slime on the rocks in the stream. When there is heavy diatom growth, they may form large clumps which elongate downstream with the current like tendrils. However, these are still loose aggregates of individual diatoms. If you disturb the tendrils with a stick or your finger, they will break off in clumps. This picture of heavy diatom growth at my streamkeeper site was taken during a drought in fall 2016.

Filamentous green algae are also common in streams. Filamentous green algae are formed of chains of photosynthetic cells. The cell walls between cells adhere to each other, like in terrestrial plants, leading to a more definite structure. Simple filamentous green algae form long hair-like growths. If you disturb them with a twig, they don’t break easily. A common genus is Spirogyra, but there are other genera as well. This photo was taken near the base of the Rosebush Trail.

This photo shows more extensive growth of filamentous green algae in the mill race above my streamkeeper site.

Other filamentous green algae are branched. A common genus is Cladophora, but again there are others. Branched filamentous green algae may be more solid and plant-like with a more definite branched structure. Branched filamentous green algae can be seen covering this rock found at my Streamkeeper site.

Because they need plenty of sunlight for photosynthesis, green algae tend to grow in open areas with uninterrupted sun. Most of Pennypack Creek within the Trust property tends to be heavily shaded by tree canopy in the summer, although there are a few open spots. Therefore, little green algae is seen there during the summer. Our former executive director, David Robertson, also felt that the frequent summer floods tended to scour the green algae off the rocks. Diatoms are also scoured, but grow back quickly from the residual diatom slime. However, during early spring, particularly a warm one like this year, there is plenty of sun through the leafless canopy to allow the green algae to grow and persist, at least until the next storm flood.

So is this a good thing or a bad thing? It depends. Algae sit at the base of the food chain in the stream. Many aquatic macroinvertebrates depend on algae, particularly diatoms, for food. These macroinvertebrates, in turn, are food for fish, which in turn are eaten by mammals and birds, etc. A stream devoid of algae would be barren of life. On the other hand, streams can become overgrown with algae. In Pennypack Creek this particularly happens in times of drought. In part this is due to the slow water flow with less scouring of diatom clumps by the stream. However, there may also be an effect from the waste water treatment plant which is immediately upstream of the Trust property. While this treatment plant is apparently operating within federal guidelines, it may still be adding nitrogen and phosphorus to the steam water. Nitrogen and phosphorus are effectively fertilizers and stimulate the growth of algae. During drought, the waste water may be less diluted by natural water entering the stream, leading to higher nitrogen and phosphorus concentrations in the water.

What are the negative effects of algae overgrowth? Although algae are photosynthetic producing oxygen during the day, during the night they consume oxygen for metabolism and may lower oxygen concentrations transiently in the water. In stagnant water, heavy growth leads to accumulation of dead algae which are metabolized by bacteria, again consuming oxygen. Lower oxygen levels limit the diversity of organisms in the stream to only those which can tolerate these lower levels. In my observations at my Streamkeeper site over the past few years I have only seen occasional periods of algae overgrowth, either in the early spring when the canopy is open, or during summer droughts. Whether this is enough to affect wildlife, I don’t know, but will keep watching.


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