Macroalgae and Phytoplankton
Both macroalgae and phytoplankton are naturally occurring in our estuaries, although the over-abundance of either suggests poor water quality. Also some species of phytoplankton can be harmful to humans and/or fish. The Department of Water and Environmental Regulation undertake routine identification of phytoplankton as part of our water quality monitoring programs, now supported in the Leschenault Estuary by the Regional Estuaries Initiative.
Phytoplankton cells can grow very rapidly when environmental conditions (including nutrients, temperature, salinity or flow) become favourable. A sudden and extreme increase in cell densities is known as a phytoplankton bloom. These events can be detrimental to the ecology, shading other aquatic vegetation (e.g. seagrasses) and may be responsible directly, or indirectly for fish deaths. Associated water discolouration and smells can reduce amenity of these waterways. Some phytoplanton can cause skin irritation to recreational users, while others are toxin-producing and classed as harmful. The Department of Health, Department of Water and Environmental Regulation and local government work together to assess, provide advice and respond to algal blooms.
Macroalgae is also commonly associated with seagrass habitats
The northern section of the estuary has high macrophyte biomass, often dominated by the brown algae Hormophysa, and the green algae, Chaetomorpha linum. Macroalgae is also commonly associated with seagrass habitats – either attached as epiphytes to the seagrass itself or as free-floating forms that accumulate in the seagrass beds. Macroalgae of the Leschenault has been surveyed multiple times since the 1980s, and while taxonomy of some species has changed, the diversity of brown, red and green macroalgae remains similar.
Residents have become increasingly concerned about the frequency of macroalgal blooms, which generally appear on the eastern banks of the estuary. Some green algae, termed ‘nuisance’ algae, has the ability to grow very rapidly when environmental conditions are suitable. Commonly they respond to nutrient enrichment, and can smother and shade seagrasses. A number of very large macroalgal blooms have been reported in recent years, usually occurring late spring into early summer. These blooms of Chaetomorpha and Ulva coincide with the period of warmer water and greater sunshine and are likely associated with rainfall events that flush nutrients into the estuary.
Diatoms are the most common group of phytoplankton in the Leschenault Estuary.
Diatoms are the most common group of phytoplankton in the Leschenault Estuary. They are distinguished microscopically by their ornate silica structure. Diatoms are autotrophs; this means they rely solely on photosynthesis for energy production. Diatoms are considered favourable as they are a critical food source to invertebrates and fish. Many of the diatoms in the Leschenault estuary are estuarine and marine species. Freshwater species can appear in the estuary with river flows from the catchment. Common species include Chaetoceros, Cylindrotheca, Skeletonema and Cyclotella.
Dinoflagellates and cryptophytes are the next most abundant of the phytoplankton groups. They are called mixotrophs as in addition to being able to photosynthesise, they are able to graze on other algae or protozoans (types of zooplankton). Both groups are also able to move or propel themselves in the water:
- dinoflagellates have a single whip like structure called a flagellum
- cryptophytes have two unequal flagella with flagellar hairs.
This mobility gives them a competitive advantage; for example, when photosynthesis is compromised by poor light conditions they can move to other areas. In high numbers (blooms) cryptophytes can discolour the water but are generally not toxic. Dinoflagellates are less favourable and are often associated with toxin-producing harmful algal blooms.
Composition of main phytoplankton groups
Toxins affect humans through the consumption of contaminated shellfish. Shellfish filter these toxic species out of the water. Toxins produced by harmful phytoplankton include amnesic shellfish poisoning (ASP), paralytic shellfish poisoning (PSP), neurotoxic shellfish poisoning (NSP) and diarrhetic shellfish poisoning (DSP). Note: The Department of Health recommends that only commercially harvested shellfish be consumed. Contact with some cyanobacteria species can also cause skin irritation. Other toxins (icthyotoxins) may directly cause large-scale fish deaths.
The Department of Water does not directly test for toxins produced by the phytoplankton (currently cost prohibitive and toxins may not always be present). Potentially toxin-producing species found in the Leschenault include Dinophysis acuminata, Gymnodinium Karenia complex, Karlodinium micrum, Prorocentrum minimum and Heterosigma akashiwo.
Phytoplankton cells are short-lived and once a bloom has run its course, the organic material sinks to the bottom. The sudden boom in decomposition (by oxygen-consuming bacteria) can use up the oxygen in the water column. Hypoxic (low oxygen) or anoxic (no oxygen) conditions are unfavourable to aquatic plants and animals and can result in mass mortalities of aquatic life.