Plankton
Plankton are microscopic organisms that spend part or all of their lives in the water column, passively carried by ocean currents. The term includes two trophic levels of organisms. Phytoplankton are microscopic marine plants and zooplankton are microscopic marine animals. Plankton are graded into 2 size groups: picoplankton (0.1 to 1.0 microns) (1 micron =1 mm divided by 1000) and megaplankton (>1.0 mm).
	Coscorodiscus sp.	Ceratium sp.
Phytoplankton are unicellular photosynthetic plants that exist as single motile and non-motile cells or as chains of cells and are important ecologically and economically. Small-scale distribution of species is primarily determined by physical and chemical environmental factors. They form the basis of most marine food webs and have significant influence on nutrient cycling and hence water quality. Phytoplankton generally go unnoticed until a 'bloom' occurs, when physical conditions concentrate cells to very high levels or increased light and/or nutrient availability allows the dense growth of a single species resulting in water discolouration and a decline in ecosystem health may occur. Nutrient and light availability are "bottom-up' controls on phytoplankton productivity and biomass. To date 145 phytoplankton species have been reported from Moreton Bay, including diatoms (Bacillariophyta), dinoflagellates (Dinophyta) and cyanobacteria (Cyanophyta), as well as other small flagellate groups (Cryptophyta, Chlorophyta and Euglenophyta). These groups are typical of variable salinity estuarine environments. Zooplankton are non-photosynthetic protist or animal plankton which have heterotrophic nutrition (i.e., they require carbon already fixed into organic molecules). Although capable of swimming movement their large-scale distribution is dictated by water motion. Zooplankton includes protists (unicellular organism), microscopic animals (copepods), jellyfish and larvae of fish and shellfish, which spend only a part of their life in the water column as plankton. Zooplankton graze on phytoplankton providing a 'top-down' control on phytoplankton biomass, an interaction that varies according to the dominance of the controlling bottom-up mechanisms of light and nutrients.
Phytoplankton: microscopic floating plants Zooplankton: microscopic floating animals
The diversity and biomass of phytoplankton of Moreton Bay are broadly related to water quality. Low community diversity has been found in relation to poor water quality (high nutrients and turbidity) in the Bremer River and Bramble Bay and high community diversity was related to clean oceanic waters of Rous Channel. Neritic diatoms frequently form chains and their elaborate siliceous skeletons are used for species identification. Diatoms represent 95% of the Moreton Bay phytoplankton population. Because they form siliceous skeletons, diatoms have a nutritional requirement for silica. Highest diatom abundances have been recorded in Bramble Bay (1.5 x 106 cells L-1), during a bloom dominated by estuarine species Skeletonema costatum and Thalassiosira rotula. Diurnal cell concentrations within Moreton Bay decrease by 10 to 70% during the night. Dinoflagellates are the second most dominant phytoplankton in Moreton Bay. Approximately 2% of the worldwide dinoflagellate species are toxic and are responsible for toxic 'red tides' which can result in shellfish toxicity as well as shellfish and mammal (including human) mortality. Dinophysis caudata is a toxic dinoflagellate responsible for diarrhetic shellfish poisoning and has been identified in Moreton Bay since the 1940's (Wood, E.J.F., 1954, Aust. J. Fresh. W. Mar. Res. 5). Dinophysis caudata occurs year round, primarily in the north-eastern regions of the Bay and is of potential concern because blooms are associated with sewage derived nutrients. Noctiluca scintillans is a bioluminescent heterotrophic dinoflagellate and is a voracious obligate grazer of other plankton species. Grazing impacts of Noctilica scintillans on zooplankton and fish eggs are of concern when blooms occur. The other flagellate groups found in Moreton Bay appear to be less common but probably go unrecognised. These are the very small soft-bodied or unarmoured species, non-thecate and non-calcareous forms of Chromophyta and Chlorophyta . Diatoms, dinoflagellates and other flagellates Noctiluca scintillans Dinophysis Thalassiosira Skeletonema sp. Chromatophyta sp.