A simple question but difficult answer. It is necessary to have some more information on the macrophytes, how many species are they? Are autochthonous or alien invasive species? There is a wide range of scenarios, hereafter some cases.

Case of alien invasive species:

The equilibrium of abundance and/or presence of species is altered by the arrival of alien invasive species.

Possible solutions: a) eradication is often a target hard to get, it is necessary to work on the whole area of presence of the species; b) continuous (every year for instance or with seasonal frequency) removal of the alien species; c) the continuous removal of the alien species could be very efficient and at low or no cost if they could be somehow exploited (fodder, fiber, etc..)

Case of autochthonous species, alteration of the food web:

Alteration by the disappearance of one or more autochthonous species.

Possible solutions: reintroduction of the species disappeared.

Case of autochthonous species, alteration of the abiotic factors:

Wide range of abiotic factors, nutrients, microelements, salinity, or something else, more information are needed.

I don't have an answer, but a cautionary tale. In Mexico, manatee was once introduced into an inland lake, with the purpose of controlling macrophytes. The failure was total. The lake was too cold for the tropical mammal, and the local people were frightened by the unusual, large animal: cold and bullets killed the manatees. Worst of all, manatees are classified as vulnerable species.

It is not possible to harvest manually or using machines. Look at this image

It would be very useful to have a brief description of the lagoon ecosystem and mainly of the human pressures that takes place in its surroundings.

Is it a natural or a modified ecosystem?

All this green color suggest the existence of eutrophication problems. There is an inlfux of waste waters from human settlements, agriculture, aquaculture...?

If we know the problem we can find the solution. Problems in nature are often linked to human activities, and there is where we must find the solutions.

There was a pineapple factory near the lagoon.

There was a pineapple factory near the lagoon. But it is closed 10 years ago.

There was a pineapple factory near the lagoon. But it is closed 10 years ago.

Matthieu, you have an interesting problem. From your second photo it looks like the free-floating water hyacinth, Eichhornia crassipes. In parts of its range it is fed to livestock and the flowers sold, but outside its native range it is invasive and considered a pest. However, it has many uses - as silage, poultry feed, even paper-making. In Cote d'Ivoire, it could perhaps be fed to cane rat ('cutting grass'), or to cattle. In neighbouring Ghana, the water surface of the Volta Lake was invaded by Water lettuce Pistia stratiotes, and floating masses of reeds, but these were generally disbursed by winds. Floating vegetation may cut off oxygen from the water beneath and harbour mosquito larvae and snail vectors of bilharzia, as well as displacing valuable water storage. The question is how to safely harvest it.

Thanks. The local population lives from this lagoon. I'd like to help them to save the system. Here is a picture of some fish found in this environment.

Water hyacinth, Eichhornia crassipes, is a very hardy plant but it does have numerous favorable uses. Some have been mentioned in previous answers. Also it has been used successfuly to treat water to remove excess nutrients. Mechanical removal ( by floating machines or by hand) would be the safest although not the fastest removal method. If herbicides are used the plants will die, sink to the bottom and decay which will add even more nutrients to the system and deplete oxygen as well during the decay process.

Water hyacinth: Removal by hand is the best option. The impact of humans slogging through the marsh is less than heavy machines. Labour i am sure is cheap in Cote'd Ivoire, as it is anywhere else in the developing world. That is how they control water hyacinth in India. Initially it is a lot of work, but once brought under control, then its just a smal crew that keeps periodically harvesting.

Control by insects may or may not work, but needs prior pilot studies for a few years, else the introduced insects can create a new problem for the local ecosytem.

What about the other species ? They may be native, like the rushes and sedges and pickerels in the first picture. Its possible that they may be taking over the lagoon, and the lagoon is transforming into a bog, a natural process that can be accelerated by sediments and fertilizers in runoff. Is that the case though ? I am of the impression that chemical fertilizers are not extensively used there. In the end, removal by hand together with soil conservation measures in the surrounding watershed, water quality study of inputs into the lagoon are probably the solutions.

while water hyacinth may clean the water, in most cases it significantly alters the aquatic ecosystem, by reducing light penetration for submerged plants, decreasing dissolved oxygen in water and has been associated with large fish kills. So it would indeed be good to contorl the hyacinth at least. Great photo of fish, i can see quite a few species. what are the small fish --catfish ?

Runoff from pineapple plantations (or other cleared areas) in the catchment, or from residues at the processing plant site and associated transportation systems could certainly have generated a large influx of nutrients that is still persistent and cycling within the lagoon. You don't need either past or ongoing use of fertilizers to generate a eutrophication event in a system like this--especially if the pineapple growing (or the development of the processing plant and roads) involved conversion from native forest, there'd have been a really large nutrient flux associated with it. I guess the positive side is that if the catchment is stabilized, and you can effectively harvest the hyacinth, you stand a chance of at least partly rehabilitating the nutrient status of the lagoon.

It could have been interesting to know the depth of the lagoon. From the picture the only 'deadly' plants are the water hyacinth and the grass (Vossia?) in front of it. These two will keep the on and off occurrence in the water body through succession while Pistia will be controlled efficiently with the water hyacinth.

The depth is about 4 m and the system is covered by more than ten macrophyte species. thanks.

Hi Matthieu. Your picture shows various cichlids and catfish - covering a range of feeding types and probably highly prized by local fishermen. As your other correspondents say, manual control of the water hyacinth and other macrophytes (with a suitable use for the product) is the best option, allowing more open water for pelagic and benthic fish species and for trade; don't introduce alien species like grass carp. Best wishes, Julian

Thanks

I agree with Julian, do not introduce alien species. In Italy I do not know one sucessfull case of introduced alien species.

Ok, thanks.

In the foreground of your photo it also looks like you have giant salvinia (Salvinia molesta).

Thank you so much for your reply. Yes, there are also other species like Salvinia molesta. 

Thanks