Welcome to our series “Our Lakes and Flowing Waters”.
It may cover only a tiny fraction of our planet, but it contains astonishing life: freshwater. Most people only observe our local waterbodies from the surface and often conclude that the water appears turbid and seemingly lifeless. But this is a fallacy. Every pond, lake, quarry, maar, river and body of water is its own microcosm with astonishing life within. An entire, often self-contained ecosystem is hidden beneath the water's surface, featuring remarkable animals and plants.
Only 0.3% of the water on our planet is freshwater and thus drinking water. Many of our lakes and reservoirs are therefore vital drinking water protection areas. However, when examining these waterbodies in detail, one quickly notices that apart from the rivers, there are very few natural waterbodies in Germany. The Mecklenburg Lake District is one such example, approximately 15 to 20 thousand years old. It was formed during the Weichselian glaciation, the last ice age in Central Europe, and is embedded in a young moraine landscape shaped by ice sheets.
Not a single natural lake exists in Nordrhein-Westfalen. The waterbodies closest to a natural lake are the maars, the flooded volcanic craters of the Eifel. Many anthropogenic waterbodies in NRW are quarry lakes, gravel pits or reservoirs, each individual waterbody hosting its own life and, due to their often smaller size, remaining highly vulnerable.
Let us dive into our waters, which are so close by and yet remain so foreign to many people.
The food web.
Phytoplankton, zooplankton, small fish, large fish and apex predators determine the food web, both in the ocean and in our lakes. If the balance is maintained with a diverse variety of aquatic plants, the waterbody thrives. The normal lifespan of a lake is easily 30 thousand years. Over time, it becomes increasingly nutrient-rich until it eventually turns into a bog. The ecological community adapts, new species arrive, others disappear. Eventually, a bog remains, which in turn constitutes its own unique biotope.
Phytoplankton (such as algae), zooplankton (such as bryozoans, insect larvae and water fleas) and fish larvae provide the nutritional basis for all other species in the lake. Small fish such as young European perch, roach and rudd are in turn prey for apex predators such as wels catfish, northern pike and zander. Above all, the northern pike (Esox lucius) dominates our waters. It is an ambush predator with excellent camouflage: from stripes to spots, the skin reveals the age of the pike. The fewer the stripes and the more spots, the larger and older the pike is. Female pikes reach sexual maturity at four years, males at two years. But caution is advised: since females grow significantly larger than males – reaching up to 1.30 metres compared to adult males which only measure about 90 centimetres – the males must be careful during mating in spring. All too often, the female consumes the male afterwards.
Encountering an adult pike in our waters is a true experience. But here too, caution is required: pikes occupy territories and will defend them if in doubt. If the mouth is opened wide, the pike is issuing a warning. Then it is time to retreat, even for divers!
The perch.
Which diver does not know it: the European perch (Perca fluviatilis). It is a sometimes rather cheeky fish that is often not shy at all and even accompanies divers. It grows up to 40 centimetres in length and can reach an impressive age of up to 20 years. Its larvae live in open water feeding on zooplankton and return to the shallows near the banks as juveniles. Perch are cannibalistic, so one can often observe the larger individuals following the smaller ones. Their growth depends heavily on lake conditions. Under good conditions, a perch is fully grown at 8 to 10 years of age.
Hardly any waterbody can do without it. It is probably the most widely distributed and can be found almost everywhere. Since European perch have spines on their dorsal fins which they erect when threatened, pikes must always ingest them head-first. Once the pike has the perch sideways in its mouth, it must rotate it: often, this is precisely the opportunity for the perch to escape its predator once more.
Nature's wonder, the eel.
The eel is an absolutely fascinating fish and a true globetrotter. Its life begins in the Sargasso Sea, off the American coast in the North Atlantic. This is no joke: all European and American eels originate from this area. The European parent eels (Anguilla anguilla) have swum the long distance from European rivers across the Atlantic to this location, spanning thousands of kilometres, to mate and subsequently die. The next generation of eels then begins a new life cycle. For between 5 and 7 thousand kilometres, the larvae drift and swim with the Gulf Stream towards Europe over a period of years. There, they metamorphose into so-called glass eels, but continue to live in shallow coastal waters for a while. Then, the eels leave the saltwater and migrate into the rivers, acquiring their typical dark coloration, and, if not caught beforehand, reach the river basins where they will spend the majority of their lives.
After many years, towards the end of their lives, the eels migrate back to the sea and swim back to the Sargasso Sea to mate. The cycle begins anew.
Eels can live up to 80 years and, in damp weather, can leave the water and glide over land – a true wonder of nature!
The zander.
The zander (Sander lucioperca), like the European perch, belongs to the percid family and is a brackish and freshwater fish. It colonises almost all rivers and large lakes in Europe; but the lagoons of the Baltic Sea are also its home. It hunts primarily in turbid, open water for small fish. It is very well adapted to brackish water and thus to poor visibility. As a diver, one will typically only spot it over its nests in spring; the rest of the year, the zander spends in open water, hunting fish or resting on the deep bottom. Parental care, which a zander pair shares, is highly energy-consuming after a long winter without prey; therefore, divers should not approach too closely to zander in spring, as they defend their nests quite aggressively.
The wels catfish, the ancient beast.
There are police officers who draw their service weapon when a wels catfish (Silurus glanis) does something completely natural: guarding its spawn. Admittedly, it does look somewhat eerie, and its size, with a "normal" two metres, is also impressive to intimidating. And yet, it is one of the most fascinating apex predators in our lakes and rivers: a true survivor. It copes with the most adverse conditions, can live up to 80 years, and reaches lengths of up to three metres. It continues to grow throughout its long life.
The best time to observe wels catfish underwater is in spring. After mating, the male catfish guard the nests, which are mostly located close to the shore in warm surface water. After a few weeks, the phase is over, and the fish retreat to deeper regions, or they simply lie (like the catfish in our picture) sleeping near the surface in the bank vegetation. Catfish are mostly nocturnal and suck their prey into their large mouths using negative pressure. This happens so quickly and forcefully that it is audible to divers as a bang underwater.
As opportunists, selectivity is not particularly important for the catfish, so a water bird can certainly be taken if the opportunity arises; one is also familiar with the anecdote of the consumed dachshund.
To kill a wels catfish with a firearm (as occurred in the summer of 2025) because it was defending its nest and bit a few swimmers is, in our view, completely excessive! Naturally, a catfish (like many animals and some humans) is entitled to robustly defend its offspring. The small teeth in its powerful mouth act like rasps and can cause skin abrasions, but nothing more. Thus, recreational pleasure stands against biodiversity. For ElasmOcean, the evaluation is clear: let us simply respect nature. Temporarily closing off the shoreline area for a few days would have sufficed until the catfish retreated to deeper waters with the larvae after hatching. It is its only habitat – for us, this specific area is truly not essential for survival during these few days, with all due understanding for leisure and fun!
Plants in our lakes.
They are oxygen suppliers, food, purifiers and shelter; they provide cover, nourishment and habitat: plant communities in lakes and rivers. Native plants such as various pondweeds (in our picture: dense forests of curly-leaf pondweed) or milfoil, mare's tail, buttercup, rigid hornwort and charophyte meadows shape the landscape in summer and autumn. Towards winter, many of them decay, but their roots survive and sprout again in spring, providing shelter and food.
They are essential for any lake. The species correspond to the nutrient content: milfoil occurs in nutrient-rich lakes, whereas the mare's tail can readily thrive in nutrient-poor mountain lakes such as the Samaranger See at the Fernpass in Austria.
For all of them, the rule is: plants are opportunists. Wherever an opportunity arises for a species, it grows and displaces other plants if possible. Depending on the season, a lake can produce completely different plant communities, and even individual years can differ significantly, depending on the weather conditions.
There are true wonders in our lakes.
The flatleaf bladderwort (also known as Bremi's bladderwort, Utricularia bremii) is a carnivorous plant that particularly loves plankton-rich and clean lakes. It has no roots but instead possesses swimming bladders that also serve as trapping mechanisms: these small, bladder-like organs have a valve. When a small animal, such as a crustacean or a larva, approaches, it is sucked into the trapping bladder within milliseconds. The valve closes, and the animal is digested. After two hours, the bladder is ready for use again.
For flowering, the bladderwort drifts to the water's surface and extends its flower stalks above the surface, where they are pollinated by insects.
It is listed on the Red List and is highly endangered!
Elodea and Co. – Neozoa.
Neozoa are introduced, invasive species that disrupt our ecosystems and sometimes dominate them entirely.
Our picture shows Nuttall's waterweed (Elodea nuttallii, also known as western waterweed). It is one of the most widely distributed plants in our lakes and waterbodies; however, it is relatively new to our region. It was popular in aquariums in the mid-20th century and entered our waterbodies through the discarding of aquarium water and plants. It is an extremely invasive species that reproduces exclusively vegetatively. Waterfowl carry this species in their plumage and droppings from one lake to the next. Nuttall's waterweed is extremely dominant and often overgrows all other plants before they can permanently establish themselves in our quarry lakes. Its "sister", the Canadian waterweed (Elodea canadensis), is not quite as widespread but also dominates any vegetation once established in a waterbody.
Comparatively harmless neozoa and also an indicator of good water quality are freshwater jellyfish. They were presumably introduced from Asia at the end of the 19th century.
Probably the most well-known invader is the spinycheek crayfish (Faxonius limosus), which killed almost the entire European crayfish population.
The noble crayfish.
Spinycheek crayfish, signal crayfish, calico crayfish, marbled crayfish or red swamp crayfish: all of these are invasive species that have spread throughout Central Europe over the years. What they have in common is that they are carriers of the crayfish plague, to which they themselves are immune.
The tragedy began about 130 years ago. There are several European crayfish species: primarily the noble crayfish, the narrow-clawed crayfish and the stone crayfish. While the narrow-clawed crayfish is naturally distributed in the Balkans, the noble crayfish occurred in all Central European waters. In the mid-19th century, the rising bourgeoisie discovered the noble crayfish as a delicacy. A large portion of the native crayfish population ended up in the pot. In 1890, the spinycheek crayfish was introduced from America as a substitute. It quickly occupied the vacant niche in the ecosystem.
What people could not know back then: the American spinycheek crayfish carries a fungus on its body as a tenant to which it is immune, but which is lethal to our native crayfish. Even worse than the plague in the Middle Ages (from which it derives its misleading name), this fungus kills all noble crayfish. Around 130 years later, the spinycheek crayfish has completely conquered the European habitat. It was followed in the 20th century by other invasive crayfish species, some of which were introduced via the pond and aquarium trade.
But they still exist: the quiet little spots where the world is still (or rather, once again) in order for the noble crayfish. Only in isolated waters and in cold locations have noble crayfish and stone crayfish managed to survive in isolation to this day. These are isolated lakes, mostly nature reserves. In these areas, they have also been actively reintroduced, and in some lakes, such as the Plansee in Austria, native crayfish still occur naturally.
Highly recommended safety precautions for divers: before diving, inquire about any potentially endangered local crayfish populations. For example, noble crayfish are present in the Sundhäuser Seen near Nordhausen in the Harz region. If you have previously been in another body of water, completely drying your diving equipment before the dive is a must: dryness kills the spores of the crayfish plague fungus. If complete drying is not possible, the equipment should not be submersed for 21 days, as the spores die off without a host after about 19 days.
Better safe than sorry – even once can be too much.
Carp pest
What was a necessity in the Middle Ages is becoming a pest in the 21st century: the carp (Cyprinus spp.).
Monks transported carp from the Danube to our western regions centuries ago. Especially during Lent, they were a popular dish. Since carp are highly resilient to turbid water and poor living conditions, the animals survived in literal carp ponds.
Nevertheless, the common carp and the mirror carp are not native to our lakes. They belong in rivers, i.e., flowing waters and deltas. This applies even more clearly to grass carp and bighead carp. They originate from Asia and, with sizes of up to 1.5 metres, do not belong in our lakes at all. Nevertheless, they have been introduced for many years (mostly by angling clubs out of ignorance) as so-called "weed eaters".
The ecological damage is dramatic. Vegetation is literally eaten away, entire ecosystems are destroyed, and the bighead carp filters plankton from the lake in huge quantities, much like a miniature whale shark. The nutritional basis for all other species is thus lost. The common and mirror carp also turn many a lake upside down. This species can live up to 45 years and reach considerable sizes: many a carp measures over a meter in length and weighs up to 40 kilograms. This attracts recreational anglers who believe they must catch the largest fish. The carp is just what they need. According to German legal terms, any carp exceeding a weight of 2.5 kilograms must be harvested and killed. However, this usually does not happen because it could not then be caught again (and bragged about) in the future. The larger the carp, the prouder the angler who, after up to an hour of "drilling" (the repeated pulling on the hook in the mouth), briefly treats the carp, proudly photographs it in their arms and then releases it again. This is known as "Catch and release". In the view of some anglers, this represents "love of animals". We explicitly do not share this opinion: these animals suffer significant, seemingly endless torment and mortal fear, and they often die anyway from exhaustion after being released. The anglers no longer see this part – but divers find the dead animals on the lakebed.
Ignorance, before – after.
“I didn't know that!” is the standard response of most store managers in supermarkets when confronted with the fact that, contrary to the guidelines of parent corporations or franchisors, they continue to sell eel. Smoked eel repeatedly appears in refrigerated display cases. In addition to this ignorance of the staff, the excuse is often made that the contracted smokehouse offered it and delivered it together with other smoked goods.
Several retail groups such as Aldi Nord/Süd, Edeka/Netto, Kaufland/Lidl or Rewe/Penny are fulfilling their responsibility and have permanently delisted eel – for good reason.
The European eel (Anguilla anguilla) is considered highly threatened: the IUCN Red List assesses it as “Critically Endangered”. The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) lists it in Appendix II (trade only after a non-detriment finding). Since 2010, the import and export of European eels across European external borders has been entirely prohibited, and an export and import ban exists within the EU.
Eel cannot be bred in aquaculture. The already described, complex biology of this cosmopolitan animal prevents this. Not only is the meat popular in Europe as “smoked eel”, but the so-called “glass eel” (the juveniles illegally caught in European coastal waters) is also considered a delicacy in Asia.
What can each individual do to counter the threat to the eel? Keep your eyes open in the supermarket: if you spot eel in the chiller cabinet, kindly point out on-site that this animal has no business in retail anymore. Especially with retailers belonging to the companies mentioned above, a reference to the guidelines of the contractual partners is helpful. Our “after” photo was also taken in a shop belonging to these chains, whose merchant had no idea what they were doing. In the meantime, after repeated requests by ElasmOcean, this shop is also playing by the rules.
Where are you going?
One is inclined to ask this question repeatedly regarding our lakes and flowing waters. Much good is happening, but unfortunately, human egoism often triumphs. Recreational fun versus nature conservation, self-interest versus animal welfare, ignorance versus consideration for animal welfare.
Every single visitor to a lake can do something: think and perhaps decide in favour of nature. Let us simply leave our lakes in peace. They are a small but fascinating habitat with large and small living organisms that cannot escape. Precisely because this habitat is so small and thus unstable and fragile, we must protect our lakes significantly better.
Our series could only provide a brief insight into the exciting topic of “freshwater”. There would be much more to tell, which is why we offer workshops and presentations on this topic that address the beauty and necessary protection of our waters in an informative and detailed manner. Inquiries regarding our lecture series are welcome at kontaktelasmocean.org.
