In highly agricultural areas where fertilizers are commonly used to support growth of plants there is also a problem with the fact that not all the fertilizer could be absorbed by treated plants. Because of rain or other weather related effects fertilizer gets access to rivers. Then it starts to be very concentrated when a river flows into lakes. In result condensation of manure is so big. Because of that it allows to grow unwanted algae. The effect of growth of these plants is very harmful for ecosystems of lakes and it affects significantly the quality of water.

Concept solution
I would like to propose the following idea that reduces the effect of unwanted growth of algae. In order to further understand my concept I would like to describe how algae grows and how fertilizers are linked with it. They are developed to support growth of plants and they contain lots of minerals (N, P, B, S , Ca , Mg etc.) They tremendously help plants to rise like for instance corn.

We are not able to easily verify how much of the manure is absorbed by plants. All of the unused fertilizer is being rinsed to rivers or ditches. We know that rivers are some sort of funnels and they collect contamination. The river stream flows into a lake or a sea. High concentration of mentioned substances supports growth of algae at most.

Description
We could build a “floating” greenhouse located on affected lake, with plants like sea weed or hydroponic growing plants, which could efficiently clean the water from fertilizers. Proposed system could be self-regulated. Thanks to that all the plants could grow to a certain height and then in automated way composed. Later it could be used also as a potential fuel for the greenhouse in order for example to keep the right temperature. In addition floating platform could bring light to the bottom of the lake in order to support growth of sea-weeds and also to allow develop natural plant in the lake. To do so we can use either LED or natural light tubes to bring luminosity. In addition to this if we already are in a situation when algae grow (green mold on the surface of a lake) then we will try to collect it in the shallow container(s). It will help to dry out and compost it like we do for the sea-weeds or hydroponics plants.

Drawbacks
Efficiency of this proposal need to be empirically verified, however since it could potentially operate all year long, could operate close to the coast (low cost to maintain), and also it is easy to move in my opinion there is a potential to reduce the algae bloom effects drastically.

Reference links:
https://www.usgs.gov/news/science-harmful-algae-blooms