Hydrobiology Services

Like all systems, aquatic environments can be described as systems made up of stocks and flows of matter and energy.

In the case of a watercourse, the stocks are mainly the inert elements of the sediments and all the plant and animal organisms that live there (Figure 1). Flows are all the elements transported by the water (chemical compounds in dissolved and particulate form and organisms) entering the area under consideration (import flows) and leaving it (export flows).

At any given moment, the “ecological status” of an aquatic system is the result of dynamic balances between these components: part of the energy and nutrient inputs is consumed by organisms, leading to an increase in biomass. The balance is restored by exports from the system (mortality, export of organisms through migration or external predators, for example). Under natural conditions, without human intervention and outside periods known as “catastrophes,” this balance is more or less maintained due to a near equality, on an annual scale, between inflows and outflows.

The result is the maintenance of a stock of organisms (bacterial, plant, and animal biomass), characterised by the diversity of the taxonomic groups that compose it. This characteristic is established in accordance with the diversity of energy and nutrient sources received by the system.

The diversity of resources corresponds to a diversity of “users” (diversity of plants in relation to mineral salt and solar energy inputs, for example). It is therefore easy to understand that by modifying, in particular, the inflows into aquatic systems (e.g., fertiliser or wastewater inputs), human activities are likely to disrupt these balances. This results in pollution from excess nutrients, which leads to the accumulation of organisms and/or inert elements (sludge, mineral and organic elements), most often associated with a decrease in population diversity.

Two other types of disturbance can affect aquatic environments: Toxic pollution (e.g., pesticide inputs), which results in a decrease in biomass and diversity, and physical pollution (temperature, rapid changes in flow, and dredging of sediments for aggregate extraction), the consequences of which vary depending on the agent causing the disturbance.

The changes brought about by human activities to the input flows of aquatic systems mainly affect the biomass and diversity of the populations living there. These are the two main criteria, one quantitative (biomass) and the other qualitative (diversity), on which biotic indices are based.

These indices are based on taxa designated as biological indicators, which, through their presence and abundance, reflect the events and disturbances they have experienced over a period corresponding to their life cycle (from a few weeks to several years).

Aquatic macroinvertebrates include insects, crustaceans, molluscs, worms, and other invertebrates visible to the naked eye (size > 0.5 mm) and whose life cycle takes place at least partially in the aquatic environment.

Samples are collected using a net with standardised dimensions and mesh size. The characteristics of the station and its environment are recorded. This information is used to refine the subsequent diagnosis by providing a better understanding of the context of the sampling point.

The samples are taken back to the laboratory where the organisms are sorted and identified under a binocular microscope using identification keys. The level of identification (genus, species, family, etc.) varies according to the group.

The index used to assess the ecological status of watercourses is the “IBGN equivalent” (Standardised Global Biological Index), which is in the process of being replaced by the “I2M2” (Multimetric Invertebrate Index).

Diatoms are microscopic unicellular plants that vary in size from 5 to 500 µm depending on the species.

Diatoms are most often collected by brushing the substrate.

One of the distinctive features of diatoms is that they have an external siliceous skeleton called a frustule. The characteristics of this skeleton (size, shape, ornamentation, etc.) form the basis of its classification and enable it to be identified by observation under an optical or electron microscope.

The index corresponding to them in the monitoring of the ecological status of watercourses is the IBD (Biological Diatom Index).

Bibliographical references

• NF T90-354 (April 2016). Water quality – Sampling, treatment and analysis of benthic diatoms in watercourses and canals.

• NF EN 15708 (January 2010). Water quality – Guide for the study, sampling, and laboratory analysis of phytobenthos in shallow watercourses.

• NF EN 13946 (April 2014). Water quality – Guide for routine sampling and pretreatment of benthic diatoms from rivers and water bodies.

• NF EN 14407 (April 2014). Water quality – Guide for the identification and enumeration of benthic diatom samples from rivers and lakes.

Macrophytes, as defined by the IBMR (Indice Biologique Macrophytique en Rivière) standard method, include all aquatic or amphibious plants visible to the naked eye or living in colonies that are also visible to the naked eye. This group includes higher plants, bryophytes (mosses and liverworts), lichens, macroalgae and, by extension, colonies of cyanobacteria, diatoms, bacteria and fungi.

The study of macrophytes in watercourses is interesting because they are good markers of mineral nutrient concentrations (nitrogen and phosphorus) and certain morphological characteristics of the environment.

They are collected by surveying a defined area and making an exhaustive list of the species present. These are generally predetermined on site and then identified in the laboratory, if necessary, by observation under a microscope or magnifying glass or binoculars.

Bibliographical references

• NF T90-395 (October 2003). Water quality – Determination of the macrophytic biological index in rivers (IBMR)

• NF EN 14184 (April 2014). Water quality – Guide for the study of aquatic macrophytes in watercourses