Benthos, plankton, macrophytes - who's who in an aquatic ecosystem
In limnology and water assessment, words keep coming up: plankton, benthos, macrophytes, periphyton. They sound scholarly, but they simply mean groups of organisms divided according to where and how they live in the water. This short guide sorts out who is who - and why it matters for quality indicators.
Plankton - suspended in the water column
Organisms floating in the water, carried by its movement:
- phytoplankton - microscopic algae (including cyanobacteria); their biomass is described by chlorophyll-a, and an excess means blooms,
- zooplankton - tiny animals (e.g. rotifers, planktonic crustaceans) feeding on phytoplankton.
Benthos - life on the bottom
Organisms bound to the bottom. In water assessment the key ones are the benthic macroinvertebrates (insect larvae, molluscs, crustaceans), which are an excellent bioindicator - see bioindication with macroinvertebrates.
Periphyton - coatings on substrates
A thin layer of algae and micro-organisms growing on stones, plants and other surfaces. It includes, among others, diatoms (phytobenthos), used in the assessment of rivers.
Macrophytes - aquatic plants
Larger plants: submerged, with floating leaves, and emergent. Their composition and cover are a biological indicator of status - see mapping macrophytes.
Nekton - active swimmers
Organisms that move independently of the current - above all fish (ichthyofauna), also an element of status assessment (fish monitoring).
| Group | Where it lives | Examples | Role in indicators |
|---|---|---|---|
| Phytoplankton | suspended in the water column | single-celled algae, including cyanobacteria | biomass described by chlorophyll-a; an excess is a bloom |
| Zooplankton | suspended in the water column | rotifers, planktonic crustaceans (copepods, cladocerans) | feeds on phytoplankton; a link in the food web |
| Benthos | on and in the bottom | macroinvertebrates (insect larvae, molluscs, crustaceans) | bioindicator in biotic indices - see bioindication |
| Periphyton (phytobenthos) | coatings on stones, plants and the bottom | diatoms and other attached algae | used in assessing the status of rivers (diatom indices) |
| Macrophytes | rooted or free-floating higher plants | submerged, floating-leaved, emergent | composition and cover are a biological indicator - see macrophyte mapping |
| Nekton | swims actively, independently of the current | fish (ichthyofauna) | an element of status assessment - see fish monitoring |
The distinction plankton vs benthos vs periphyton is not a matter of species but of way of life: the same algal taxon may be planktonic (suspended) or periphytic (attached) - what decides is where and how it lives at a given moment, not the name of the group.
What this division is for
Because different groups respond differently to conditions and reflect them on different time scales - which is why the assessment of ecological status relies on several of them at once (see elements of assessment).
In practice
Biological data is usually lists of species and abundances - which can also be kept systematically. In LimnoLog you record them as custom indicators, alongside physico-chemical measurements, in a single project.
Calculate it yourself
- 🧮 Biodiversity indices calculatorFrom a list of taxa and abundances, calculate Shannon-Wiener, Simpson and Pielou evenness.
See it in the LimnoLog app
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