Phyto- and zooplankton

You will find planktonic organisms everywhere in the sea and in freshwaters (and also on VIRTUE discs)


The structure of a phytoplankton community shifts significantly due too environmental changes, caused by e.g. eutrophication, acidification and environmental toxins. Analysing these phytoplankton communities provides information about the effects of various types of pollutants. The information about total biovolume and biovolume per litre, of various algae groups and single species, can be combined with physical and chemical variables, as well as information about zooplankton and zoobenthos. Zooplankton and zoobenthos are in turn depending on the phytoplankton community composition of species, biomass and nutrients.

There are four main purposes of phytoplankton analysis.

  • Species analysis, or taxonomical inventory, is to gather information about biodiversity and indicator species who might be viewed upon as sources of difficulty. This can be toxin-producing cyanobacteria, diatoms who clog nets, slime-producing algae, species with troublesome mass growths that alter the taste and smell of water, species with a hetero- or mixotrophic lifestyle and finally species whose lifestyle indicate a certain environmental situation.
  • Analysing species quantity and biovolume to obtain quantitative dimensions for comparisons in time, place and between different lakes.
  • Analysis of the biovolume of various groups of algae. Special interest is tied to groups who generally have an indication value. These include some troublesome groups, nitrogen fixated groups, groups who characterizes acid environments and groups who are used as food for grazers (zooplankton, ciliates, zoobenthos).
  • Analysis of the total biovolume of phytoplankton. There are relation numbers for the total biovolume algae per litre in comparison to the content of chlorophyll a and the concentration of phosphorus. It is also possible to relate a highest biovolume during the growing season to empirically drawn limits for troublesome and very troublesome amounts of algae.

The phytoplankton community is commonly described with quantitative methodology, complemented with netted samples as a support variable. These samples are often necessary to get some enriched material where the identification of species are possible. The phytoplankton community composition and biovolume varies greatly over a year and is primary controlled by the waters mixing and the stratification conditions.


The yearly growth of plankton in lakes begins with pioneer stages of fast growing species in the spring and towards a late summer stage with large, slowly growing species. In many lakes, the increased amounts of diatoms in the spring is a first sign of increased nutrient levels in the water, something that occurs long before the algal blooms of the summer have time to be disruptive. Sampling throughout the whole growing season provides a considerably more reliable base to detect change or disturbance than one single sample every year. With only a single yearly sample the lake can be classified according to its water quality, but it will take many years before changes can be confirmed trough means and deviations. Compilation and change in the seasonal dynamics provides good information about the quality of the water. The highest value of the phytoplankton's biovolume during a year reflects the nutrient concentration, but intermediates that are caused by variations in weather does occur.

The largest yearly biovolume in nutrient-poor lakes often occur in the end of the spring circulation. In nutritious lakes, the maximum biovolume usually occur during the summer. The summer and late summer is commonly the most speciose (specious-rich) regardless of the lake's nutrient content.

A quantitative investigation on all lakes in a monitoring program where a good measure of species as well as the relative and absolute occurrence of different groups is required. For example, when describing an ecosystem's structure, comparison of quantity between lakes or time-series analysis.

The spatial distribution of phytoplankton in water varies widely. Shady parts of a lake usually contain less algae then bright areas. Shallow lakes of bays usually have a higher species richness and biovolume then deeper basins. The phytoplankton near reed or belts of underwater vegetation is highly mixed with fouling species which do not characterize in the flora of the open parts of a lake. Phytoplankton moves vertically during a day; on evenings and nights they descend and rise towards the surface again in the morning.

When the purpose is to examine special problems such as the occurrence of toxic- producing species, species which clog nets and water inlets, algae that causes smell and taste etc., sampling should be made in a way that is relevant for the type of problem.



Phytoplankton in the open sea accounts for the majority of the produced plant mass in the oceans. During the summer, there can be hundreds of thousands of individuals per litre of water on the surface. Phytoplankton is together with bacteria and benthic plants makes out the foundation of the marine food web with zooplankton, zoobenthos and fish as consumers.

Phytoplankton investigations are used in longer perspective to show any changes in the aquatic environment (between years). In a shorter perspective (a year) they can contribute to a description of the condition of the environment. Changes which can be observed are mostly natural and anthropogenic nutrient in the sea, which can affect the composition and quantity of phytoplankton. Furthermore, the purpose of the investigation is to describe succession and/or yearly cycles, i.e. typical seasonal variations in the presence and distribution of phytoplankton as well as to monitor the presence of toxic or otherwise harmful algae. The introduction of new phytoplankton species can be documented through phytoplankton analysis. The amount and species composition of phytoplankton can provide information about the origin of the water and about the local and regional addition of nutrient to the sea. It can also detect impending algal blooms, which can result in poisoning of animals and plants and/or oxygen deficiency in the areas bottom waters. Algal blooms can also lead to shifting of the ecological balance, which can bring changes of the composition of species in the sea. Not only to phytoplankton but also to zooplankton and fish.

As mentioned before, many phytoplankton species have a daily cycle in which they move vertically. For this reason, it is important that samples are taken throughout the entire depth of the euphotic zone, in order for samples from different stations and hours to be measurable.

Quantitative phytoplankton investigations are done through microscopic analysis, which also can cover biomass determination. The determination of phytoplankton species is time-demanding and requires an extensive taxonomic training and continuous further education in order to maintain the competence and knowledge about the constantly changing nomenclature.





Studies of zooplankton in lakes may be used to describe the condition and changes in the zooplankton community's species composition, relative occurrence of certain species (indicator species) which can indicate the condition of an aquatic area, and individual density and biomass of zooplankton in an aquatic area.

The composition of zooplankton communities is changed by a variety of environmental changes, such as eutrophication, acidification, intoxication and changes in the fish fauna's composition. For this reason, analysis of zooplankton communities provides information about various types of environmental degradation. Since the zooplankton community is affected by both the production of phytoplankton and the degradation of fish, the species composition and biomass is affected by several interacting factors. Thus, characteristic changes may occur while the underlying reasons are unclear. As the causalities of the zooplankton population regulations are more complex and less researched then for both zoobenthos and phytoplankton, the later are more commonly used for monitoring purposes. The main reason to include zooplankton in a surveillance program is their placement in the food web. Monitoring zooplankton provides an opportunity to estimate how interactions between trophic levels affects the tendency and extent of changes in the ecosystem. Information about biomass and species composition of zooplankton is often necessary in order to interpret changes in phytoplankton, zooplankton and fish communities.


The number of animal species in the oceans is unknown  and only about 2 percent of these live their whole life in the free water mass. The rest lives on the seafloor, most of them on hard substrates.

Zooplankton are an intermediate in the flow of matter and energy from primary producers such as phytoplankton to higher consumers like fish. Thus, knowledge of the production of zooplankton in an area can be a basis for assessing the possible production of fish. Eutrophication of the seas results in an increased production of phytoplankton, which in turn favours the zooplankton community. Any long-term eutrophication effects should therefore be detected in a surveillance program for zooplankton. The size of zooplankton varies widely and there is no sampling method that works for all fractions. Investigations are by traditions usually focused on the size fraction which is known as mesozooplankton (0.2-20 mm), which includes a large portion of the zooplankton which are the backbone of the Baltic sea's plankton fauna.