- Algae can be toxic and non-toxic
- A lake typically has a significant number of 20–30 different types of algae at any given moment.
- 1-2 species dominate the mix depending on daylight hours, water temperature, nutrition, and predators.
Algae is often viewed as a managerial headache for those in the water industry, but this aquatic organism is a natural part of the ecosystem. The most well-known algae species are typically found in lakes, rivers, and ponds which live in the water column. Other types, such as periphytic algae, grow on different substrates, such as rocks, submersed aquatic plants and bottom sediments. In this guide, let’s take a deep dive into algae.
Algae can range from microscopic single-celled plants on the water’s surface to large seaweeds clinging to the seashore. All species are autotrophs, which means that they use sunlight and abiotic (non-living) dissolved chemicals to produce biotic (living) material via a process known as photosynthesis. They are non-flowering, typically aquatic plants that contain chlorophyll but do not have roots, leaves, stems or vascular tissue.
Photosynthesis is one of the most important processes on the planet. It relies on sunlight, so it can only occur during daylight hours. During photosynthesis, the algae, or other plant, the cell absorbs carbon dioxide, sunlight and water to create oxygen and energy in the form of glucose. But this is not the only way that plants can produce energy. In parallel with photosynthesis, plants also respire similarly to how we do as humans. During respiration, the algal cell absorbs oxygen and produces carbon dioxide every minute of the day. When it’s a sunny day, algae, and other plants, photosynthesise faster than they respire. On days like this, algae are considered a net oxygen as more oxygen is produced via photosynthesis than respiration. Cloudy days pose more challenges for aquatic ecosystems. On those days, little photosynthesis can take place, and algae respiration may cause the lake to become so short on oxygen that there are fish kills and even algae kill.
Not all algae are harmful, however, plants form the basis of the food chain in many environments. All other organisms, from bacteria to humans, are known as heterotrophs – organisms that eat other plants or animals for energy. A healthy algae population provides a vital foundation for thriving aquatic life and maintaining a healthy aquatic ecosystem.
An example of a good green algae food chain is:
Nutrients (e.g. phosphorous, nitrogen) > Algae (e.g. diatoms, green algae) > Zooplankton (e.g. daphnia) > Small fish > Piscivore fish.
On average, there are roughly 20-30 species of algae in a lake in a large quantity at any one time. Within this mix, 1-2 species are dominant depending on factors like the number of daylight hours, water temperature, amount and type of nutrients, and predators, amongst others.
First, let’s delve into algae that is not toxic. These are small-celled – typically under 100um – and are not visible to the naked human eye. Planktonic algae species like this are responsible for a tint on the water, green during the summer due to green algae and brown in the spring and fall due to diatoms. As they are planktonic, they are led by movement in the water. Remarkably, these species emit enzymes to avoid connecting with other individuals and prevent clumping.
Regarding cell biology, green algae typically have a soft cell wall, whereas diatoms have a rigid cell wall. Because they are tiny, green algae and diatoms are frequently eaten by filter-feeding daphnia and other zooplankton. With zooplankton regularly chowing down on good algae and diatom populations are continually eaten and so rarely form a standing “bloom” in natural summer conditions. If healthy populations of these species are present in a lake, it will retain high water clarity all summer and remain aesthetically appealing for all uses. Bottom line – healthy green algae and diatom populations prevent the development of harmful algal blooms.
Why are good green algae and diatom populations better for aquatic ecosystems than other species like cyanobacteria?
However, it’s not all good news. Good green algae and diatoms are slightly heavier than water, with a specific gravity of 1.03, so it is common for them to sink. Unfortunately, if they sink deeper than the thermocline, they die due to a lack of sunlight. In many lakes across the United States, this happens to a large portion of the good green algae and diatoms by mid-summer, enabling cyanobacteria to become dominant in the lake.
Cyanobacteria, also known as blue-green algae, are a type of bacteria that reside in water. These microscopic organisms are naturally found in bodies of water, but if left to proliferate, they can harm human health. Like other bacteria, they have a slimy wall with the chlorophyll in each cell that is spread throughout the entire cell. This differs from a true algae cell with a more rigid wall and chlorophyll in sacks known as chloroplasts.
Why do cyanobacteria pose a risk to human health? Cyanobacteria are found in many lakes and can disrupt a lake’s efficiency by producing potent toxins (cyanotoxins), taste, and odour in the lake water. The toxins can kill or harm humans that come into contact with the lake water and even pose a risk to human health several miles from the lake. Animals, particularly cows and dogs, are killed yearly by drinking water containing cyanotoxins. There is also a concern that some toxins in lake water are infiltrating drinking water plants and slowly accumulating in humans that drink the water.
How can you detect cyanotoxins? Although you will need a testing kit to prove their presence, generally, you can get a hint that cyanotoxins may be present by smelling the water. The taste and odour are usually a musty or fishy smell due to the presence of “MIB” and “geosmin” produced by cyanobacteria. While these toxins, MIB and geosmin – can cause complaints by consumers to the water treatment plant, they also present more severe health problems to humans.
When a lake has a cyanobacteria bloom, the toxins, taste and odour usually extend throughout the entire water column, from the surface to the bottom sediment. This is likely because although the cyanobacteria bloom proliferated in the upper water column where it received reliable sunlight, the dead cells are inedible by zooplankton and constantly sink from the upper water to the sediment. As a result, if there are cyanobacteria blooms near a water treatment plant intake, there is often little benefit to a water treatment plant using multiple intake gates at different elevations as the water at all depths will likely contain the cyanobacteria contaminants. What happens if an algaecide is applied to a lake to stop a cyanobacteria bloom? In that case, the cyanobacteria cells may lyse (be destroyed and split apart) and release toxins, taste, and odour compounds into the water, further exacerbating the problem. As a result, algaecides must be used cautiously and, preferably, prior to the expansion of the bloom.
What triggers cyanobacteria to bloom? Cyanobacteria typically become prevalent in a lake where there are abundant levels of nutrients, warm water, and long daylight hours. In conjunction, there is little effective mixing to re-suspend good green algae and diatoms to prevent them from sinking out of the sunlight, making a perfect storm for algal blooms to proliferate. Many species of cyanobacteria can store nitrogen and phosphorus nutrients for later use, adjust their buoyancy downward to get nutrients off the lake bottom and then come up to the surface for sunlight. Afterwards, they clump together for protection (forming harmful algae blooms, HABS), shade out, and kill good algae, emitting toxins, taste, and odour to kill or ward off predators. This process forms resting akinetes (spores) to lie on the lake’s bottom until the following year, when they all come up to the surface to take over the lake. At a size of over 100um, they are too large for the average daphnia to eat, so they often survive until it’s ready for summer again, proving a nightmare for water managers.
From an ecological perspective, cyanobacteria also prevent the flow of nutrients throughout the food chain to big healthy fish and instead create a problematic biochemical oxygen demand (BOD) load as bacteria slowly decompose them at the bottom of the lake:
Nutrients (e.g. nitrogen, phosphorus) > Cyanobacteria > BOD load at the lake’s sediment.
Often the cyanobacteria BOD load at the lake sediment triggers the bottom anoxic zone of the lake to move more rapidly and higher in stratified summer conditions. With this comes a multitude of problems with iron, manganese, phosphorus and sulphides. As a result, removing cyanobacteria blooms in the surface layers of the lake will typically improve the bottom layers of the lake too.
Filamentous algae are a form of non-toxic algae that link together to form mesh-like filaments. These colonies of microscopic plants usually grow on the surface of hard objects, such as on the bottom organic substrate or rocks in flowing waters. Filamentous algae play an essential role in the health of a freshwater ecosystem as they produce oxygen and food for the animals that live there; however, they can also cause problems such as stagnancy. Unlike other plants, filamentous algae do not have roots, so they get their nutrients from the surrounding water. When gases, such as dissolved oxygen (DO) created via photosynthesis, get trapped in the filaments, the buoyancy of the gas leads to the algal mat rising to the surface. This is common in shallow ponds and along near-shore areas in larger lakes and can appear like there are more algae than there is. Although filamentous algae can be unaesthetically-pleasing, it is not harmful to human health. Algae is a natural part of an aquatic ecosystem. Typically, the filamentous season seldom lasts a few weeks on each lake, between May and June each year.