Here you will find information on the aquatic plants of Lake Geneva
You can find out more about the aquatic plants of Lake Geneva in the following issues 41and 119 of our quarterly magazine Lake Geneva region as well as in our themed leaflets, our infographics or in our weekly column News in 60 seconds.
The underwater plants of Lake Geneva are often mistakenly called "algae", but are they really all algae?
There are several types of aquatic plants in the lake, the most important groups being :
The algae are usually single or multicellular organisms that are difficult or impossible to see with the naked eye. A magnifying glass or microscope is needed to identify them. Generally belonging to the "floating" organisms, they are part of what is known as plant "plankton" (or phytoplankton). There are also similar algae attached to rocks or sediments and to plants or animals.
Filamentous algae
When environmental conditions allow (temperature, nutrients, etc.), some of these unicellular algae form colonies of up to several metres in length, visible to the naked eye. Other macroscopic algae are multi-cellular, attached to a substrate or free in the water.
When the concentration of phytoplankton cells is very high, the water appears green, brown or red. [1] depending on the type of algae.
The main difference between algae and aquatic plants is the absence of tissues forming organs such as roots, enabling them to anchor themselves in the soil and draw nutrients from it. Algae do not produce flowers either, but reproduce vegetatively (by budding or fragmentation) or sexually (male and female gametes).
The aquatic plants (or hydrophytes) are vascular plants with seeds and flowers rooted in the seabed. They are an important part of the vegetation of coastal areas. Several species produce flowers that rise to the surface in summer and are therefore easy to observe. The seeds can then spread by air or water to colonise other sites. These plants have a true vascular system with stems and leaves, which is generally more flexible than terrestrial plants because they grow in the water that surrounds them.
Herbarium of aquatic plants
The characeae are a separate group of plants, considered to be macro-algae. They are the only living member of the charal family, with fossils dating back more than 400 million years. In terms of their cell structure and mode of reproduction, they are similar to algae, but they are also visible to the naked eye and have rhizoids to anchor themselves in the soil.
Characeae
The bryophytes or aquatic mosses are small plants with a stem and leaves. However, they have no real conducting vessels or root systems. They have rhizoids that allow them to anchor in sediment or adhere to riprap.
Bryophytes
These various submerged aquatic plants can locally form veritable meadows of varying densities, known as "meadows".
Lake Neuchâtel was temporarily (2020) invaded by a Cyanobacteria algae, commonly called "Blood of the Burgundians" in reference to the battle of Morat because it colours the water red. It is toxic, which has led to a local bathing ban.
The littoral zone of the lake, thanks in particular to the seagrass beds, is the richest in flora and fauna in the lake, but also the most fragile. It constitutes a complex biological system that performs essential functions for the benefit of the entire lake ecosystem. Far from being harmful, meadows serve as food, habitat, refuge, breeding ground and nursery for many animal species: invertebrates (molluscs, crustaceans, insect larvae, etc.), fish (perch, pike, roach, etc.) and water birds (coot, scaup, red-crested duck, etc.).
In addition, aquatic plants (essentially phytoplankton, although it accounts for only 1% of the planet's biomass of photosynthetic organisms) found in all the world's aquatic environments (rivers, lakes, oceans, seas, ponds, etc.) produce half or more of our planet's oxygen.
Lake Geneva is home to a wide variety of of aquatic plants which absorbs CO2 and releases O2 through photosynthesis (Alain Demierre)
Since the late 1960s, Lake Geneva has suffered from severe nutrient pollution. This excessive input, particularly of phosphates, has led to an explosion of algae and certain plants that prevent light from penetrating the water and whose decomposition absorbs much of the lake's oxygen. Since the 1970s, the diversity of aquatic plants has declined sharply. Many species have disappeared completely, and with them several species of fish and invertebrates.
Thanks in particular to the Association pour la Sauvegarde du Léman (ASL), which lobbied for a ban on phosphates in detergents and obliged wastewater treatment plants to treat wastewater, the level of phosphorus in the lake, the cause of eutrophication, has fallen from 90ug/L in 1980 to 20ug/L today. The improved transparency of the water now allows light to penetrate deeper, enabling plants to grow again on the lake bed. In recent years, characeae have reappeared in numbers in our lake and with them... the Pike!
Seagrass beds provide refuge for many species, including Pike (Christian Witschard)
This large fish, emblematic of Lake Geneva, lays its eggs in the characeae to which it is closely linked. What's more, its hunting technique is based mainly on the hiding places provided by other aquatic plants such as periwinkle pondweed.
Pike egg-laying in characea. (Christian Witschard)
With the improvement in water quality, the diversity of aquatic plants is constantly increasing, and with it the diversity of fish fauna.
The meadows of Lake Geneva have vital functions for the lake ecosystem, which fully justify the attention that must be paid to protecting them.
As fishermen, you can take care of these meadows, and thus the fish, in particular by limiting mowing or anchoring your boats in the meadows.
The presence of areas of characea on the seabed is a bioindicator of good water quality (oligotrophic environment) because these algae are intolerant of excessive nutrient levels (especially phosphorus).
♣ Species present in our book of determination sheets (sold in our shop)
Microscopic algae generally have beautiful geometric structures. They can form large colonies when conditions allow. They are part of the plankton, the term for all the microscopic animals and plants that float in the lake and form the basis of the food chain. ♣
The periwinkle pondweed is one of the most common plants in our lake. It is easily recognised by its heart-shaped leaves which surround the stem. It forms dense weed beds that are much appreciated by the young fish that hide there (Perch, Roach, Pike, etc.). ♣
Flowering period: June-July
Pectinated pondweed can be recognised by its thread-like leaves. This species thrives in nutrient-laden water, which is why it dominated the lake in years when Lake Geneva was more polluted than it is today (1970-2000s). Since the quality of the lake's water has improved by being less saturated with nutrients, it has regressed, making way for species that prefer clearer, oxygenated waters. ♣
Flowering period: July-August
The submerged hornwort is a special plant with two types of leaves: the terminal leaves are very thin and very green, while the basal leaves are thicker, "forked" and more yellow-green. The submerged cornflower is unusual in that it has no roots. This plant can be found in certain harbours as it prefers eutrophic waters, i.e. waters rich in nutrients. ♣
Flowering period: June to August
Milfoil has a reddish stem (or green for young shoots) and branched leaves, finely divided like a feather. This plant is abundant and widespread throughout Lake Geneva. ♣
Flowering period: June to September
Elodea are exotic aquatic plants native to North America and frequently used in aquaria.
These plants appeared in Lake Geneva in the early 1990s as a result of the elimination of aquarium fish and plants from natural environments by humans. It is very important never to release an animal or plant into the wild.
Elodea are low plants that grow in whorls of 3. Nuttall's elodea can be distinguished from Canada elodea by its thinner leaves, which can bend. Canada Elodea, on the other hand, has rounder, wider leaves.
These plants flower very rarely in Lake Geneva. They can be seen in many harbours.
They are recognisable by their very fine, fragile structure and form dense mats on the seabed. There are several species of characeae in Lake Geneva, but they are difficult to distinguish with the naked eye.
Characeae are a spawning ground for many animals and a source of food, particularly for the Red-crested Pochard. Their return to the lake after several years of disappearance indicates an improvement in the transparency and oxygenation of the water. ♣
This magnificent pondweed is easily recognised by its long, pointed, translucent leaves. 
revealing the outline of the cells in the form of squares. The flower spikes are also very long. This species is abundant in Lake Geneva and provides a good habitat and spawning ground for invertebrates and fish.
Flowering period: July
Frizzy pondweed has long, oval leaves with finely toothed, wavy or frizzy margins, as its name suggests. The species is widespread in Lake Geneva, where it thrives in calm, nutrient-rich waters.
Flowering period: June-July
Dense pondweed has a densely leafy stem. The leaves are opposite, elongated, 1-2 cm long and surround the stem. It thrives in oligotrophic water, which is poor in nutrients. With the improved oxygenation and transparency of the water in Lake Geneva, this species is spreading.
Flowering period: July and August
The Marsh Zannichellie looks a little like the Pectinated pondweed, but the location and appearance of its flowers differentiate them. It also has filiform leaves, but the flowers grow at the base of the leaves, against the stems.
Fertilisation takes place underwater and the flowering period is from June to August.
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