Phytoplankton species vary in size, shape, evolutionary and phylogenetic position, energy and nutrient demands, and many other features. However, even if they do not float freely and involuntarily in open water, independent of shores and bottom, they persist in suspension and are liable to passive movement by wind and current.
The size and shape of phytoplankton species, among other properties, determine their adaptabilities to aquatic environments. Algae obtain the nutrients they require from the surrounding medium via the cell surface, and then transport them to sites of use within the cell. Small size and large surface area therefore assist conversion to active biomass. On the other hand, larger size can deter grazing. Size, shape and composition also greatly influence sinking properties and light harvesting abilities. Nevertheless, phytoplanktonic organisms are no bigger than the smallest turbulent eddies generated in dissipating input kinetic energy.
Size of phytoplankton varies from <1mm (Synechococcus) to >1mm (Gloeotrichia colonies); aggregates of Microcystis may even be larger. During the last few decades, numerous terms (such as nanoplankton, micro-algae, ultraplankton, etc.) have been developed in order to categorise phytoplankton species by their size. Confusion can be demonstrated by the point that the upper size of nanoplankton has been considered by different authors to range between 10 and 80 um (Reynolds 1984). Rationalisation of the categories proposed by Sommer (1994) is adopted below:
- Picoplankton 0.2 um - 2 um: Synechococcus, Nanochloris, Chlorella
- Nanoplankton 2 um - 20 um: Rhodomonas, many Chlorococcales, small chrysophytes
- Microplankton 20 um - 200 um: Asterionella, Ceratium, Sphaerocystis, Snowella, filamentous blue–greens
- Mesoplankton 200 um - 2 mm: Gloeotrichia, Aphanizomenon clusters, Aulacoseira, chain-forming Pennales
- Macroplankton 2 mm - 2 cm: Extremely large Microcystis colonies
Methods of studying phytoplankton frequently depend on size. The term ‘net-plankton’ refers to those species which can be collected in plankton nets. These are usually the larger sizes. Picophytoplankton can only be quantitatively studied using microscopes equipped with epifluorescence. The standard technique for studying larger species is inverted microscopy, introduced by Utermöhl (1931).
The ‘size’ of individual phytoplankton species can be expressed in different ways. The greatest axial linear dimension (GALD [um]) comes from direct measurements. Volume (V [um3]) and surface area (SA [um2]) are usually approximated to the nearest geometric equivalent. Sometimes, for non-spherical forms, size is expressed as the diameter of the spherical equivalent volume (DSE [um]).
The shapes of most phytoplankton species can be compared with simple geometrical forms such as rods, ellipses, spheres, cones or parallelepipeds, or to combinations of these for compound shapes (Ceratium, Staurastrum). Spines and other surface structures are common, as is mucilage investment. Morphological properties greatly influence physiological performance (e.g., light harvesting), or the behaviour of the cell in limnetic habitats.
References:
Reynolds, C.S. (1984) The Ecology of Freshwater Phytoplankton. Cambridge University Press, Cambridge, 384 pp.
Sommer, U. (1994) Planktologie. Springer-Verlag, Berlin, 274 pp.
Utermöhl, H. (1931) Neue Wege in der quantitativen Erfassung des Planktons. Verhandlungen der Internationale Vereinigung für Limnologie, 5, 567–96.
