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Algae -- The Good, the Bad, and the Ugly: Algae and its Role in the Ecosystem

by Joe Cadieux on January 26, 2023

Welcome back to the wild world of pond biology brought to you by your friendly neighborhood Freshwater Biologist.

Here I’d like to explore algae’s role in the biosphere and under what conditions they can become such a nuisance … or even dangerous. 



Behold! The many and varied species of algae: the simple and extremely necessary wee organisms that birthed life as we know it on Earth. Algae is responsible for the majority of oxygen produced today and is the air and water filter that keeps most of our world habitable. So, no big thing…. Just the REASON YOU ALL CAN LIVE AND BREATHE ON EARTH!  Show some respect….. You tell ‘em, Aretha! 


The above statement is true, but probably deserves a bit longer explanation to all the uninitiated out there.  See, the oceans are big…really big. Like OMG, did you see the size of her butt….big (All props to Sir Mix-A-Lot, may his wisdom not be lost on the youths of today).  The greatest engine of turning carbon dioxide into nice fresh air are the free-floating algae of the seas.  The seas/oceans of the globe are massive, so there is and has always been (for the last elevendy-million years), a vast cornucopia of algae swimming around producing oxygen and feeding the base of the global food web. These algae are, by and large, very good.  It’s only when one (or a few) species becomes dominant that we run into problems.  In the oceans, these runaway algae blooms can produce Red Tides. A Red Tide occurs when a single species of algae over-populates and excretes a toxin into the water that kills a bunch of fish and is dangerous to most other creatures that come in contact with it. These runaway blooms are caused by warm water and abundant nutrient sources – usually the direct result of the stuff we unwittingly let wash into rivers and run to the ocean (fertilizers, effluent from wastewater treatment facilities, and general decomposed organic material - dead stuff mostly - to name a few). 

“Cool story, Bro…. but how does this relate to my pond? It’s a little smaller than the ocean,” you say snark-ily (snarkishly, snarkfully, snarktastically…..one of those).  

Well, my friends, the same ecological relationships that fostered the true beginnings of the world we know today are still operating in the koi pond in your back yard (and your ½ acre pond, and the creek that runs by your house, and the 1,000 acre lake up the road, and, and, and, etc).  Algae forms the base of the food chain, and any waterbody without a good diverse population is out of balance and prone to large blooms of single (or a few) species of algae. 

If you do not/cannot foster a diverse habitat in your waterbody, you run the risk of algae blooms. These blooms can slide by largely unnoticed, be an aesthetic nuisance, or create dangerous conditions for you, your pets, or the aquatic life under your care. 

Every pond must deal with the factors that can fuel algae blooms.  Primary among those parameters is nutrient load (Phosphorus and/or Nitrogen (mostly)) and elevated water temperature. 

Nutrient Load: Runoff and Internal Loading

Water acts as a sink or collection zone for nearly everything that may blow or wash through your yard.  This includes: dust, leaves, grass clippings, dissolved chemicals from lawn treatments, dead stuff, live stuff, and other organic/inorganic material that can travel through ground water or over land.  This is all generally called runoff and drift.  This material breaks down into food for the tiny creatures that make up the ground floor of your aquatic ecosystem.  This type of nutrient loading is episodic and usually leads to temporary blooms as your pond slowly accumulates organic material. 

Internal loading is a term used for nutrients that have set up shop in your pond for a long period of time. The accumulated goop of many years is a constant source of phosphorus and nitrogen. This muck (or organic soft sediment) is stored long term on the bottom of your pond or lake where oxygen is in its shortest supply.  This nutrient source is ever present and will be responsible for constant algae blooms and other high nutrient issues that come along (swimmer’s itch, leeches, duckweed, pond snails, weed growth, and ever-present algae issues).

High summertime water temperatures are another contributing factor to abundant algae growth. Warmer temps bring down water quality and oxygen carrying capacity of your waterbody.  As more organisms battle it out for available oxygen, the species of algae that can thrive in such conditions can come to dominate the microbial community. These low oxygen critters are the ones to look out for. Almost all of the detrimental effects brought on by poor water quality are due to these organisms that thrive in low oxygen aquatic environments. 

High nutrient levels and water temperatures in a low oxygen environment will lead to a stunted ecological cycle. The algae will grow rapidly, become overabundant and either float to the surface in matts or discolor the water (if it’s a planktonic jerk). This leaves room for the next generation to start at the bottom of the pond.  That floating matt then slowly dies off and breaks down into even more nutrients which help fuel further generations.  Repeat. Repeat. Repeat. Without chemical intervention (i.e., algaecide treatments galore), this will go on all summer, slowly concentrating the few species of algae until concentrations grow (potentially) to dangerous levels. 

Battling Low Oxygen Conditions:

The best way to mitigate the effects of the input of nutrients is to cycle them up the food chain and out of the pond.  If you concentrate a bunch of nitrogen and phosphorus into the cells of a 4-pound Largemouth Bass, they are not available for runaway algae blooms. This starts with a rocking micro-ecosystem fed by a high-oxygen environment. 

How do you save a warm old pond with a bunch of muck piled up on the bottom? 

I’m glad you asked…..

If only there were technology out there designed to increase dissolved oxygen in a wide variety of aquatic environments….




There are a wide variety of aeration systems (floating fountains/aerators, subsurface diffused systems, or even a correctly sized waterfall) available that fit this bill to a “T”. 

A good aeration system is ALWAYS the best/most effective ecological investment for your pond.  Whether it’s a good waterfall and filtering apparatus for your koi pond (Like this one: Bead Filter), an excellent aerating fountain (Like these: Kasco, Powerhouse), or a diffused aeration system (like these: EasyPro-Shallow water , Scott Aerator, Solar Powered, EasyPro-Deep water), there is no better purchase that you will ever make for your pond than good quality aeration. 

This largely solves the low-oxygen issue and will immediately change the ecosystem and diversity potential of your waterbody.  As a chiasmatic alien once told me….

Once the micro-ecology rebounds in your newly oxygenated bio-topia, you can work those problematic nutrients out of the pond.  Algae (and other microbes) consume the organic nutrients on the bottom of the pond, larger creatures (Zooplankton, small fish, insects) snack on those plankton and algae, larger fish and insects eat those critters, and so on, until you have a very large predator fish that you can hang on your wall, or is removed from the system for the dinner table (or to fertilize a tree, or to feed an eagle, or gets abducted by aliens). 


Algae in your pond is good and bad and ugly (sometimes); knowing what you have and whether it needs addressing is a tale not only told by the cottony mass floating on top of your pond. It a story of the components of the life that sustains us.

Long story longer: add oxygen to your pond to convert nutrient issues that would otherwise fuel consecutive algae blooms of potentially harmful or unsightly algae into a beautifully diverse habitat that will attract aliens….I mean big healthy fish and happy pond owners. 




Senior Biologist



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