Phytoplankton and seagrasses

There are other organisms that can accumulate and wash ashore in large masses (seagrasses) or can form blooms which give the water a distinctive colour (phytoplankton).



Phytoplankton are microscopic (5-200 µm), photosynthetic, single cells or groups of cells (colonies) that float or swim in aquatic environments. Phytoplankton blooms colour the seawater red, brown or green but with no obvious structures (eg filaments) visible. There are several groups of phytoplankton including diatoms, dinoflagellates and euglenoids which have formed blooms in South East Queensland. Some phytoplankton species produce toxins which have caused human deaths elsewhere in the world (eg Canada) and fish kills both in Australia and other continents.

Diatoms (Phylum Bacillariophyta)

Diatoms which are the most common group in the phytoplankton are composed of nonmotile single cells or a chain-like group of cells (colonies). Cells are symmetrical (either centric or bilateral), and have an outer skeleton made of silica and contain the brown pigment fucoxanthin. The centric diatoms Thalassiosira weisflogii and Melosira moniloformis have bloomed in local estuaries in response to increased nutrient loads. Surf diatoms (Anaulus australis) colour the surf brown along sand beaches in South East Queensland. These are natural blooms of diatoms which live in the sand during the night and surf the waves during the day.

Dinoflagellates (Phylum Dinophtya)

Dinoflagellates are mostly unicellular with two flagella, with some species known to form chain-like colonies. Most photosynthetic dinoflagellates contain the red pigment peridinin but some species contain either the brown pigment fucoxanthin or chlorophyll as the dominant pigment. Dinoflagellates are the most common bloom-forming phytoplankton in the marine environment with some species toxic to humans, fish and invertebrates.

Euglenoids (Phylum Euglenophyta)

Euglenoids are unicellular flagellates which contain chlorophyll as the dominant photosynthetic pigment. These blooms colour the water green. Eutrepiella blooms in coastal lakes in the South East Queensland and Euglena colours sand green on beaches along the Cooloola coast.


Seagrasses are the only flowering plants (Kingdom Plantae) adapted to living completely submerged in the marine environment. Seagrasses have retained the structures typical of land plants: roots, stems, leaves with veins containing vascular tissue, flowers and seeds. Unlike the majority of macroalgae which attach to rocks, seagrasses use roots and underground stems to anchor themselves into mud and sand. Following storm events, large masses of seagrass leaves can wash up and decompose on coastal beaches and shorelines.