Phytoplankton

On a global scale, despite their small size, phytoplankton make up the majority of plant biomass in water and represent the planet's first lungs. In fact, 50% of the oxygen we breathe comes from aquatic environments. Floating freely in the 'euphotic' zone, i.e. where there is still light, these are photosynthetic organisms - which use the sun's energy to feed themselves - and so help to oxygenate the water.
Many species can form colonies, making them visible to the naked eye. These are commonly known as algae.

Be careful not to confuse macrophytes and seaweed!

When human activities introduce too many inorganic nutrients (such as phosphates) into the lake, algae proliferate rapidly (algal bloom). This proliferation creates an imbalance in the euphotic zone by preventing light from penetrating to the depths.

Main stages leading to eutrophication of an aquatic environment (source)

The macrophytesThe algae, aquatic plants growing at the bottom of the lake, are thus deprived of light. What's more, the decomposition of algae is a very oxygen-intensive process, causing asphyxiation of the environment over the long term. This asphyxiation, combined with the nutrient overload caused by the proliferation of phytoplankton, is known as eutrophication.

In short, phytoplankton is essential to life in aquatic environments and must be looked after.

In Lake Geneva, the reduction in phosphate inputs since the 1980s has led to a reduction in algal blooms and improved oxygenation of the water. This return to "normal" conditions has enabled aquatic plants to re-colonise shallow areas, forming new grass beds. Some fish, such as pike, have seen their populations increase as a result of this improvement in water quality.

Phytoplankton has impressive seasonal fluctuations; a litre of water can contain 30 to 50 times more plankton at the start of summer than in winter!

The quantity of phytoplankton is regulated by zooplankton. In spring, zooplankton consume a lot of phytoplankton, creating an imbalance. Then, having run out of food, the zooplankton disappear and the phytoplankton can develop again. Small fluctuations persist until summer, when equilibrium is reached.

Among the different types of phytoplankton, one taxon is particularly well known: Cyanobacteria or blue-green algae. Often cited for their potential toxicity in the event of an algal bloom, these are organisms that can photosynthesise under extreme conditions (pH, temperature, salinity, etc.). They are thought to be among the first to have produced oxygen and probably one of the first organisms on earth, having appeared over 3.8 billion years ago. Cyanobacteria (see below) have greatly contributed to making the planet habitable for animals.

Although tiny, phytoplankton is perhaps the living organism on which most responsibility rests.

Zooplankton

Zooplankton feed on phytoplankton and are eaten by fish (especially young fish, known as fry), as well as by the molluscs and crustaceans of Lake Geneva. It is therefore an essential link between autotrophic organisms, capable of producing their own organic matter from light energy (plants), and heterotrophic species, which feed on organic matter (animals). Zooplankton is made up of two groups: permanent and temporary.

Permanent zooplankton spend their entire lives as plankton (such as Daphnia), whereas temporary plankton are only considered zooplankton during the larval phase of their development. These include certain species of crustaceans or molluscs such as the Zebra Mussel.

Study of pathogenic organisms and the physiological and pathological conditions involved in abnormal mussel (Mytilus sp.) mortality Charles (2019)

In Lake Geneva, the total number of zooplankton species is around 200! Compared with the number of fish species in the lake - around twenty native species - this is an impressive figure! Unlike phytoplankton, zooplankton can survive at depth. When the sun sets over Lake Geneva, it rises to the surface to feed on phytoplankton, then descends to the depths during the day to hide from predators and protect itself from the heat. This is not the only technique used by zooplankton to avoid being eaten; many are practically transparent!

Zooplankton have an extremely high reproduction rate. Take the Daphnia for example. The female can reproduce every 4 to 5 days from the age of 11 days, laying up to fifty eggs a week. This is an effective strategy for maintaining the population, despite its short lifespan (30 days on average) and numerous predators. So there's a whole world going on, hidden from view... right there, beneath the surface!