What is phytoplankton
The word phytoplankton means planktonic plants, including all those organisms having a photoautotrophic metabolism, being able to convert inorganic nutrients into organic matter, using solar energy. Algae and cyanobacteria are the most important members of phytoplankton: trying to understand "why" or "how" some of them are able to live in certain types of pelagic freshwater environments and others are not, is the very basic aim of our studies. The microscopic observation of a phytoplankton sample can reveal a surprising world, made by organisms very different in size, shape and morphology.
Environmental requirements and controlling factors
The variability of size and shapes in phytoplankton cells is of great importance for the adaptation of the organisms to the environmental conditions. There are strong relationships between the cell morphology and the ability of the organism to take up nutrients from water or to harvest light. Moreover, the speed of sedimentation of the cell towards the bottom of the lake depends on the shape, as well as the ability to escape the predation by herbivorous zooplankton.
The small species, like the diatom Stephanodiscus parvus, are very efficient in taking up nutrients from water, thanks to their high surface respect to cell volume.
The cells of Asterionella formosa, a diatom building typical star-like colonies, with their elongated shape and the relatively small volume, expose to the underwater light a very high portion of the cell surface.
The diatoms are particularly able in building structures for contrasting sedimentation: the ribbon-like colonies of Fragilaria crotonensis (left) and the "wings" of Tabellaria fenestrata (right) are really good examples!
Another mechanism against sedimentation, giving even the possibility of moving: the spines of the chrysophyte Mallomonas caudata, by which this alga rows into the water column.
The large dimension of the dinoflagellate Ceratium hirundinella can create some difficulties to the herbivorous predators. This species can even move through the water column, thanks to its flagella.
Large colonies are also a good solution against predation (in the picture, the cyanobacterium Aphanothece smithii).
Role in the environment
Phytoplankton is “the grass” of the surface waters of lakes and seas: through its photosynthetic activity it provides the organic biomass sustaining the upper levels of trophic web in aquatic ecosystems. It has been estimated that about 95% of the organic carbon produced in lakes and seas is due to phytoplankton photosynthesis. Moreover, phytoplankton plays an important role in the biogeochemical cycles of carbon, nitrogen, silica and, through the photosynthetic process, is responsible for the modification of the oxygen concentration, pH and alkalinity of the surface waters. In case of blooms, phytoplankton can give peculiar colours and odours to the waters of lakes and ponds: a phytoplankton bloom usually takes place when, under stable physical conditions, one species find a set of environmental conditions favouring its fast growth, allowing it to overcome other species.
Why study phytoplankton?
“I believe that phytoplankton has much to teach us about the way this world works and the lessons we may learn should be as widely applied as possible.” (G.P. Harris. 1986. Phytoplankton Ecology: Structure, Function and Fluctuations. Chapman & Hall). What Harris wants to tell us, is that the phytoplanktonic world can be a good model for the comprehension of many general ecological principles. Classic topics of ecology, such as competition, adaptation of the organisms to environment, pattern and rate of colonization of a new environment, ecosystem response to disturbance, many kind of interaction between organism and its environment, ecological succession, can be easily studied through phytoplankton. Basically, it is a matter of scales: the life cycle of a phytoplanktonic cell takes few days, a population develops in a few weeks and important changes at community level occur across a single year. Therefore, considering that phytoplankton responds quickly to environmental changes, using these organisms, the evolution of an aquatic ecosystem can be analysed at a reasonable time scale. The effect of frequency or intensity of an allochtonous disturbance on the biodiversity can be quite easily investigated by studying a phytoplankton assemblage. Because of the close relationship between structure and function, changes of the dominant morphotypes inside a phytoplankton assemblage can immediately give an indication on the environmental variability. Moreover, phytoplanktonic organisms colonize a huge of aquatic environments, even many different in their basic features: this allow ecologists to study the adaptation mechanisms of the organisms. Last, but not least, many phytoplanktonic species have a good value as biological indicators: therefore, the species composition and its variability across space and time can be useful in classifying the quality of an aquatic ecosystem and in recognizing alterations due to, for instance, eutrophication, acidification and pollution by metals.