Phytoplankton
Key points:
- Phytoplankton are microscopic, photosynthetic organisms found in water, forming the base of aquatic food webs and producing oxygen.
- Phytoplankton respond quickly to environmental changes and are key indicators of estuary health.
- Excess growth can cause ‘algal blooms’ that reduce oxygen, harm wildlife, and some phytoplankton species may produce toxins.
- We monitor phytoplankton in our estuaries to track their abundance and species, helping manage water quality and protect ecosystems and public health.
What are phytoplankton?
Phytoplankton are microscopic plant-like organisms found in aquatic environments, including freshwater, estuarine, and marine ecosystems. The term includes cyanobacteria which are photosynthetic bacteria and microalgae. Phytoplankton derive their energy through photosynthesis (releasing oxygen as a by-product) during the day, and respiration (releasing carbon dioxide) at night. They uptake nutrients, principally nitrogen and phosphorus, from the water to grow. Different species have different nutrient requirements, and some cyanobacteria can directly “fix” nitrogen from the atmosphere providing a competitive advantage where nitrogen is limiting.
Key characteristics:
- Most are single-celled, but some can form groups or long strands (colonies or filaments).
- Cell size typically ranges from a few micrometers (approximately the width of a human red blood cell) to hundreds of micrometers (approximately the width of a human hair).
- They are primary producers in aquatic food webs and are a critical food source for zooplankton, shellfish and small fish.
- They are important for oxygen production and carbon cycling.
- Includes various groups such as:
- Diatoms (Bacillariophyta).
- Green algae (Chlorophyta).
- Cyanobacteria.
- Dinoflagellates.
- Chrysophytes and others.
Why is it important to monitor phytoplankton?
Monitoring phytoplankton in estuaries is essential for understanding and managing the health of these dynamic and productive ecosystems.
Phytoplankton respond rapidly to changes in nutrient availability, salinity, light, and temperature, making them reliable indicators of eutrophication (excess nutrients), pollution, and the impacts of climate change. Shifts in their abundance and community composition (which species are present) can reflect changes in water quality and help inform strategies to manage or prevent environmental degradation and protect public health.
When are phytoplankton a problem?
When phytoplankton growth remains within natural bounds, these organisms are an important part of the ecosystem. They form the foundation of aquatic food webs, supporting fish, shellfish and other marine life, while also producing oxygen and helping to regulate carbon and nutrient cycles.
However, phytoplankton can become detrimental to the environment when they grow excessively and accumulate high biomass, unbalancing the ecosystem. Excessive nutrient inputs, particularly nitrogen and phosphorus from agricultural runoff or urban wastewater, combined with favourable conditions such as high temperatures, abundant sunlight, and stagnant water, can promote rapid growth and lead to algal blooms. The term algal bloom commonly describes the resulting dense layers of phytoplankton on the water surface and within the water column.
Algal blooms are often recognised by changes in water colour – commonly green, blue-green, red, or brown – depending on the type of phytoplankton. One or more species may dominate the phytoplankton community during a bloom. Bloom events differ in duration, lasting from a few days to several weeks, and they can cover anything from a small area to an entire river or estuary.
Nuisance phytoplankton are species that don’t produce toxins but can disrupt ecosystems when in high densities. While not directly toxic, these blooms can discolour the water, release foul odours, reduce light availability for seagrasses, and contribute to fish kills by depleting oxygen levels. This happens at night when the phytoplankton use oxygen for respiration. Some diatom species may even clog fish gills, causing additional stress or mortality.
While most phytoplankton species are not considered toxic, more than 300 have been identified as capable of producing various types of biotoxins. We use the term potentially harmful to describe phytoplankton species that may produce toxins, posing risks to aquatic fauna (such as fish) and human health. Blooms of phytoplankton species that are known to produce toxins are often referred to as harmful algal bloom (HAB) events. Environmental factors such as light, temperature, nutrient availability, and salinity can influence both the likelihood and intensity of toxin production in some phytoplankton strains.
How do we monitor and analyse phytoplankton?
The Phytoplankton Ecology Unit, within the department’s Aquatic Science Branch, specialises in identifying phytoplankton species and assessing their abundance to inform algal bloom responses and water quality management across WA.
Fortnightly or monthly samples are collected by our water quality monitoring teams and sent to the Phytoplankton Ecology Unit. These samples are usually taken from the full depth of the water column because phytoplankton can move up and down and are not always on the surface. A small 1 mL portion is placed in a special counting tray (Sedgwick-Rafter (SR) chamber) and observed under a microscope (sometimes with ultraviolet light to help identify certain species). All phytoplankton present in the SR chamber are identified and counted to measure both their density and the overall composition of the phytoplankton community.
Abundant and potentially harmful species are evaluated against relevant health and environmental guidelines to assess for any possible ecological impacts and public health risks. Both potentially harmful and non-harmful species naturally occur within phytoplankton communities. When phytoplankton exceed established guideline values (measured as density or biovolume depending on the species), any associated risks are communicated to relevant water management and government authorities to take appropriate actions. These actions may include producing specific health warning advice and directing the placement of warning signs in affected areas.
In addition to routine monitoring, grab samples are collected on ad-hoc basis in response to visible surface scums, benthic algal mats (dense layers that grow on the bottom of waterways), or fish kill events. Typically, only the dominant or potentially harmful species are analysed in this type of sample.
Routine integrated water quality and phytoplankton monitoring has been in place across a number of estuaries in the South West and South Coast regions and continues to this day as part of Healthy Estuaries WA. Shorter term monitoring programs have been used to characterise the water quality and phytoplankton assemblages in Mid West estuaries and along the South Coast as far as Esperance.
After phytoplankton are identified by the Aquatic Science Branch’s Phytoplankton Ecology Unit, all data (from routine sampling program or statewide ad-hoc investigations) are stored in the department’s Algae of West Australian River and Estuaries (AWARE) database. Powerful data visualisation tools have been developed to explore and report on the phytoplankton assemblages, which are also combined with water quality data to understand and report on estuary condition.
Examples of some phytoplankton species in our estuaries:
