Algae concentrations in Lake Bolac have been monitored and publicised by Parks Victoria for the last three years. Algal numbers are divided into two categories: total algae, and the proportion that are recorded as toxic. The Victorian EPA has guidelines for safe water quality for recreational waters. The proscribed limits are 10 mm3/L total cyanobacteria algal concentration and 4 mm3/L for toxic cyanobacteria species.
The chart summarises measurements of samples from Lake Bolac. It shows that total algal counts have been well above the safety limits for periods of time, especially during the warmer months.
More information about algal blooms and health effects can be found on the health.vic site.
Types of algae and their growth
Diversity of micro-organisms
The overall health of a lake depends on having a functional community of diverse microscopic organisms that keep each other in balance. These organisms include algae and other primary producers that use the energy from sunlight to transform inorganic nutrients into organic compounds. Here are some common non-toxic micro-algae found in Lake Bolac. They include (clockwise from right) a Euglena, an unidentified, a diatom, Paestridium, and Botriococcus.
‘Algal bloom‘ is the phrase we use to describe a rapid increase in the amount of algae in a lake or river system. Algae are always in the water of natural lakes and rivers. Water is never completely empty of organisms unless it has been filtered and treated to purify it. Water in natural systems holds a whole ecosystem of producers, consumers and predators, most of which are microscopic. Most of the time there are dozens of different sorts of algae and other microorganisms in the water, but sometimes the conditions suit a particular type of algae. Their numbers build up and they ‘bloom’, often becoming visible in the water, colouring the water or creating a scum on the surface. All kinds of algae can bloom, and sometimes there is a succession of different algae that bloom over time, as lake water warms and different nutrients become available. The kind of algal bloom that we hear about most is the ‘nuisance algal bloom’. In Lake Bolac nuisance blooms have been invariably blue-green algae, otherwise known as cyanobacteria. In fact, cyanobacteria are really photosynthetic (light-using) bacteria, simple microscopic prokaryotes that have green and blue pigments to use sunlight. They have a simple structure and often occur in colonies. The first three images below are of blue-green algae including; Trichormus, Nodularia, and a blue-green algal scum from Winton Wetlands near Benalla. The final image is a floating colony of blue-green algae plus euglena.
Blue-green algae don't need ionic nitrogen for growth
Blue-green algae are different to other algae, in that their growth is not limited by the concentration of all nutrients. They are able to use nitrogen from the air (i.e. fix nitrogen, like legume bacteria do in the soil). This means that they can bloom when phosphorus concentrations are high, but nitrogen is low. This often happen in turbid or muddy systems where phosphorus is held on soil particles suspended in the water. This gives blue-green algae a competitive advantage in this situation to dominate over other species. Cyanobacteria like still conditions, as this allows them to float to the surface where light is abundant. They like warm conditions, since this allows them to grow quickly. They like well-mixed systems, since this brings nutrients from the sediments, up to where the light is. You might say that Lake Bolac is a perfect habitat for cyanobacteria.
Why do algal blooms end?
Blue-green algal blooms can die out. Things that cause their decline include:
a decline in the water nutrient concentration,
if herbivore numbers build up. Cyanobacteria are eaten by water-fleas Daphnia, ciliates and rotifers, amongst other things.
if the environmental conditions change, to include a decrease in water temperature or an increase in winds. Blue-green algae grow quickly when there is a thermocline layer of warm still water at the surface. Wind mixes the layers and reduces the ability of the toxic algae to outcompete other algal species.
Usually, when a water body blooms, the number of herbivore microorganisms build up and they eat all the algae, and the bloom ends. If there are too many carnivores (things such as fish and larger invertebrates), the microscopic herbivores can’t build up in numbers. If there are no water plants for the herbivores to hide in, they can’t build up in numbers. This is one of the reasons why well-vegetated wetlands rarely experience nuisance blooms. Even well-balanced, diverse systems can experience blooms, but these are more likely to be short-lived where there is high biodiversity and low nutrients.
The first image below is of a young copepod. The video is an adult copepod. If you look closely at the final image, you can see that that the ciliate herbivore has engulfed blue-green algae. It will digest the algae as food.