Some HABs produce toxins which are harmful - if not deadly - to aquatic organisms.
Microalgae can be found in marine and freshwater ecosystems, and along with bacteria form the foundation of the aquatic food web. It’s also estimated that microalgae produces around half of the planet’s oxygen! However, under certain conditions, some algae can grow out of control and produce toxins that are harmful - if not lethal - to humans, fish, shellfish, marine mammals and birds.
Across the globe, Harmful Algae Blooms (HABs) have become more intense and frequent in recent years. Warmer sea temperatures is often cited as a key reason for this, as this warmer environment provides the ideal setting for a high germination rate. It’s also why HABs are more often seen in Summer months or during an El Niño event where hotter and drier conditions bring extreme droughts, which lead to warmer water temperatures. As such, Australia is no stranger to HABs.
Land based runoff (and in particular agricultural runoff) is also considered a large contributing factor to the current intensity and frequency of HABs. When rain falls on agricultural land, the nutrients from fertilisers and manure, for example, can runoff into streams and lakes, which can eventually end up in the ocean. And, these nutrients can soon lead to algae growth. Extreme weather events are also heightening the impact - for example in Australia when a long period of drought is followed by heavy rainfall.
The intensification of aquaculture has also received attention when it comes to the formation of HABs. If unmanaged, aquaculture can negatively impact the nutrient concentration of its surrounding environment. This is caused when fish stock is overfed and unused pellet feed drops to the ocean floor. As the pellets dissolve, nutrients are released into the water causing nutrient enrichment (eutrophication). This changes the chemical composition of the water and can aid in the formation of algal blooms.
When it comes to the impact of HABs on aquaculture the results can - and have proven to be - devastating.
Firstly HABs deplete the oxygen levels in the water in a number of ways:
- Algae consume O2 during the night which can reduce the oxygen content of the water
- The algae that dies off are decomposed by bacteria. In this process the bacteria also depletes the oxygen levels in the water
- In addition, certain species of algae in high numbers can block the fishes gills which hinders them in being able to take up oxygen from the water
For the fish, depletion of oxygen levels can cause stress or even mortality - especially salmonids like salmon which require a high oxygen content.
Secondly, some HABs produce toxins which are harmful - if not deadly - to aquatic organisms. In the case of shellfish - this can render the molluscs toxic (and not fit for human consumption) or can indeed lead to mortalities of both shellfish and finfish. And, the economic impact to the aquaculturist is of course detrimental.
Unfortunately, due to rising ocean temperatures (climate change), a higher CO2 content of the atmosphere and eutrophication, algae blooms are expected to become more frequent in the future. So how can aquaculturists mitigate HABs?
Join us next month for Part 2 of this blog post series and learn more about how compressed air is playing a pivotal role in the mitigation systems being used to prevent losses in fish due to HABs.