Green algae

 Identifying green algae. Okay, now at last we're really talking about the real green algae. Aren't we? On the whole, we're all stumped when we try to be scientific about identifying the species of real green algae (Chlorophytes) that we find in the aquarium. It reassured me to read Diana Walstad telling how she gave up on algal taxonomy after she'd had a biologist examine some "green mat" algae. "Under the microscope, the algae turned out to be a conglomerate of many separate species. The two dominant genera, identified by their filamentous branching pattern and characteristic spores, were Oedogonium and Pithophora... they appeared to be a mixture of several species." Cyanobacteria types appeared in her mat as small blue-green bulbs attached to the green algae filaments, and additionally she found that there were small populations of diatoms. Thus, Diana Walstad decided to stick to common descriptive names to describe the dominant type of algae in the mixed biofilm.
 
I'm sure that under higher magnification many fungi and bacteria would also have been found co-existing with Diana Walstad's algae, because this uncategorizable mix is characteristic of the biofilm.
 
Algal meadows. Each species in this mixed community has its particular requirements and its characteristic range of tolerances. And when resources are constrained to a limiting-point, various types of algae can be assumed to have varying competitive abilities. If you watch a freshwater pond in the temperate zones through the season, you'll usually see a progression of algal types. Conditions like warmth and daylight hours are changing, which tip the balance in favor of one type of algae over another.
 
You've probably noticed during the lifespan of your own aquaria that at first one species of algae is prominent enough to rate as a problem, but then it takes a minor role, as a different species succeeds it. Not only are conditions of light spectrum and nutrients evolving in the aquarium, but the algae are also carrying on chemical warfare in their competition for resources.
 
Algal succession and allelochemicals. Many species of algae produce allelochemicals, which suppress the growth of other algae. Investigators found that allelopathic chemicals released by the filamentous alga Pithophora tended to suppress other algae, especially the single-celled pelagic algae responsible for "green water." In Ecology of the Planted Aquarium, Diana Walstad reported a 1969 study, in which aquaria with 24-hour lighting turned green in a week, but not identical tanks containing active Pithophora.
 
Though the toxins of a successful alga may help suppress its competitors, directly or indirectly they may also be encouraging the species that will take that alga's place. Dying algae can release phytotoxins, an effect recognized in drinking water by unappealing tastes produced by otherwise acceptable algae after chlorination. This is a minor problem in water treatment systems, but it can be a major problem in the narrow confines of an aquarium. After you've done some successful algae killing, water changes are in order.
 
So, algal species tend to ebb and flow in whatever niches aquaria offer. Algal species follow one another in sequences of blooms, all responding to the resources you make available to them and to their own interactions, such as competition for a space in the light, or chemical warfare ("allelopathy").
 
The niches change too, as plantings and bacterial communities mature, and the bioload changes. And resources for algae also shift under the pressure of grazing patterns that are established by the particular group of algae-eaters you've set loose upon them. Each algae-eater has a characteristic repertory of algae it favors. Young fresh growths, even of preferred algae, seem to be favored over older mature growths. Some algae being faster and better adapted than others to re-colonize a cleaned surface, the result of all these effects taken together is an algal meadow, a mosaic patchwork of algae that develops on undisturbed surfaces, such the rear glass of the tank.
 
Testing algal succession. Once in a while it's worth scraping the algae off a side wall to watch this process unfold again from the start. You'll see that the first growth is normally a quite uniform blanket of one pioneering species, probably the familiar one on the front glass where you clean every couple of weeks. If you are curious to see the most diverse growth of algae on your side glass pane, try this: take a plastic pocket comb and snap off the end guards. Now scrape a set of closely parallel "furrows" down the algal meadow on the glass, in a single pass. Leave the rest undisturbed. You'll need a magnifier to watch the developments along these linear "edges." There's a parallel here with some experimental sustainable logging techniques in tropical forests ...but that would be taking us a long way from tropical fish. "Succession" is the ecological term for these phenomena. For non-aquarists, familiar examples of succession occur when unplowed pasture reverts in stages to woodland, then forest.
 
"Good" filamentous algae. A non-invasive (in the aquarium, though perhaps not in a pond) filamentous algae (I'm calling it Pithophora but is it Cladophora ægropila?) that was given to me almost a decade ago seems to help clear green water and to compete with more noxious algae, maybe by scavenging the available phosphate that algae require in an uninterrupted supply, or maybe by subtler allelopathic chemistry. This algae has elongated narrow cells linked into branching chains that are wool-like and coarse (not slimy) to the touch, characteristics that give it common names like "Cottonball Algae" and "Green Wool Algae." At The Krib it's referred to as Hair or Thread Algae. Baensch's translators are alone in dubbing it "Green Bunch Algae.
 
Usually it grows on the bottom, attached at a few points to rock or gravel, but trapped oxygen bubbles may make it buoyant. In ponds it may form dense surface mats. Its color may vary from lime green to dark olive greenish brown.
 
Marimo. Similar algae-balls (Marimo) from Lake Akan in Japan are treated with a curiosity and respect that algae seldom inspire. From 2008 something very like them has been coming onto the aquarium market.