What feeds Lake Balaton?
Why is its color the way it is?
When do we say that a lake has good water quality?

Lake Balaton is not just the water itself: the lake, its shores and environment make up a whole living system of elements that are all connected through the relations of each individual creature living in and around it. The foundation comes from the sun’s energy and the nutrients from the plants: shells filter the algae, big fish feed on the small ones, then die and turn into nutrients for the others. Lake Balaton, like all natural water bodies, is filled with algae - tiny single-cell organisms floating in the water, visible only through a microscope’s lens. Like plants, most of these build up the material of their cells via photosynthesis, by binding the atmosphere’s carbon dioxide that has been diluted into the water. However, carbon-based compounds are not enough to form a living cell, they need other ingredients too. The most important of these are hydrogen and oxygen (easily accessible by breaking down water), as well as nitrogen and phosphorus (the most important plant nutrients).

The greatest source of nitrogen and phosphorus for the algae are diluted manure, fertilizers and communal sewage dumped or leached into the water by humans. These are absent in natural, good quality lakes. In the crystal clear lakes and creeks on mountains, algae are rare, the water is not tinted green, the only color comes from sediment if it’s stirred up. However, near agricultural areas or where sewage pipes are connected into lakes, the growth of algae populations can be unlimited for short periods - even resulting in exponential growth (just like viruses during a pandemic), and this has a great impact on the food web. On the one hand, the amount of available food sources skyrockets, on the other hand, the diluted oxygen in the water dwindles, especially at night. This might seem like a paradox, since algae emit oxygen during photosynthesis similar to plants, but they also breathe just like all living organisms, so they also consume oxygen. During the day, the former process is predominant, while during the night, the latter one. Additionally, the bacteria that decompose organic materials also take up their share of oxygen from the water, exacerbating the lack of oxygen. Without oxygen, the fish can start dying or the main source of food for fish, smaller arthropods can be affected as well.

At low oxygen levels the phosphorus which accumulated for decades in the sediment can be released into the water. This leads to a positive feedback loop through more intensive growth and reproduction of algae that can cause even larger problems.


What defines good water quality?

What do plants and algae in Lake Balaton feed on?

What is the significance of the plants’ carbon-dioxide binding ability? Why are plants important for the mitigation of climate change?

How do plants gain access to vital nitrogen, and what are the consequences of agricultural fertilizer overuse?

Algae blooms

Why is phosphorus important for plants?

Lake Balaton and its human use - a system of connections