Here’s what you need to know:
1. “Algae” is a broad biological category, encompassing all organisms that get their energy from the sun, except for true plants, which diverged from algal ancestors about 470 million years ago. The algae alive today include everything from giant seaweeds to single-celled phytoplankton and pond scum.
2. Algae are the essential, first level of most aquatic food chains. Without algae there would be no fish, shrimp, clams, coral, sea turtles, dolphins, etc.
3. Algae also make a large portion of the oxygen we breathe, and are important in the global cycling of other chemicals necessary for life, like carbon dioxide, nutrients, etc.
4. SOMETIMES algae are a nuisance. Excessive algal productivity is called eutrophication, which rhymes with nutrification and stems from pollution by excessive nutrients- nitrogen and phosphorus from fertilizer or wastewater. If the eutrophic overgrowth of algae causes harm to people or the environment it’s called a HAB (harmful algal bloom).
5. Like nutrients, high amounts of light and heat also stimulate algal growth, and can favor more toxic forms of algae. An ancient and often toxic group of algae, the cyanobacteria (aka blue green algae), flourish in waters too warm for most other algae, like dark-bottomed reflecting pools. Cyanobacteria especially flourish when phosphorus levels in the water are high. I think the algae in the reflecting pool now are mostly a type of non-toxic green algae called Desmodesmus, but there’s more than one type of algae in the pool, and the dominant species are likely to shift over time, possibly to more harmful cyanobacteria like the Microcystis aeruginosa we often get in Florida.
6. The best way to keep algae from getting out of hand is to make sure you’re not adding any extra nutrients to the waterbody you’re managing. The other best way is to make sure the aquatic ecosystem has living components that can remove nutrients and/or remove excess algae.
a. True plants like cattails, lily pads, and pond weeds compete with algae for nutrients and light and can help lower nutrient levels to the point where algae will no longer grow in excess.
b. Algal grazers like tiny crustaceans and snails can help suppress algae, and filter feeders like clams and mussels can sieve vast quantities of microscopic algae particles out of the water
7. For small, artificial bodies of water, like swimming pools or fountains, algae can be suppressed with toxic chemicals like chlorine, copper sulfate, or hydrogen peroxide. However, these compounds kill desirable aquatic life, too, and are not appropriate for large scales or natural environments. The Department of the Interior says the hydrogen peroxide they are trying to kill the reflecting pond algae with now is non-toxic, but that's an exaggeration. A high dose of hydrogen peroxide will kill anything. If you decide to control algae chemically, you’re losing your chance to control the algae biologically (like with plants, grazers, or filter feeders) because the chemical controls will kill the biological controls. Also, chemical suppression of algae does nothing to remove nutrients from the water, so the fuel for the next algae bloom remains and the algae will come back with a vengeance as soon as the algae-killing chemicals wear off. This creates a kind of chemical-addiction cycle that is great for pool and pond management companies, but isn’t great for customers, or nature.
8. In my opinion, anything bigger than a swimming pool should be managed as a true aquatic ecosystem, with plants, grazers, filter feeders, and all the other living things that keep algae in check naturally. Trying to maintain a huge reflecting pool in a lifeless state is a pretty dumb idea, in my opinion, dooming the managers to never-ending, expensive algal vacuuming and toxic chemical treatments.
9. Some of the algal bloom problems in the reflecting pool are because of high nutrient levels in the water it’s filled with. The water comes from the Potomac River Estuary via a little reservoir called the tidal basin, which exchanges with the low-salinity estuary at high tide. Like the Chesapeake Bay, which it is a part of, the Potomac is quite high in nutrients from a variety of human sources. So it is unsurprising that a stagnant pool filled with its water is prone to algae blooms.
10. Sometimes in the past the reflecting pool was filled with potable water from the city’s drinking water supply. That water was lower in nutrients and would initially be less prone to algae blooms than the Potomac water from the tidal basin. BUT that was an expensive use of precious drinking water, and it wasn’t just a one-time use, either, because water would be lost to evaporation and leaks. Plus, water changes would eventually be needed because the initially clean water would get contaminated from fertilizer rich runoff, bird poop, etc. and after a while the nutrient levels would be just as high as Potomac water, anyway. So, I think the Obama era re-plumbing of the reflecting pool with dirtier but cheaper to flush-through tidal basin water was an OK idea.
11. I think the BEST way to do the reflecting pool would be more like a natural lake than a cement bottomed pool. The center would have to be dug quite deep to remain as open water, but to keep the water quality good the borders of the pond would have to be shallow wetlands vegetated with cattails, lily pads, etc. Instead of just existing to reflect the monuments around it, this “green” reflecting pool would perform multiple ecological functions, providing plant and animal habitat and naturally removing excess nutrients from the urban runoff passing through it.
