The study was carried out in the district of Douala IV in the city of Douala, located in the Littoral region. It lies between 4˚02'53" N and 9˚42'15" E and is bounded to the north by the Moungo department, to the south by the Wouri river, to the east by the Wouri river and to the west by the Fako department ( Figure 1 ). The characteristics of the study area are similar to those of the city of Douala as a whole. With an average altitude of 13 m, Douala is subject to an equatorial climate of the Guinean type and Cameroonian sub-type, characterised by two seasons: a nine-month rainy season (March-November) and a short dry season, from December to February [5] . Rainfall is abundant and regular, with annual values ranging from 2.596 mm to 5.328 mm. The air temperature in Douala is relatively high, with a monthly average of around 28˚C [7] .

During both seasons, observations were made of the following physico-chemical and biological parameters:

Floristic surveys of 100 m² each were carried out at 2 stations (neighbourhoods 105 and Sodiko Bipélé). Thus, 2 transects of 500 m with 6 quadrats of 100 m² each were marked out by wooden stakes fixed in the mud and surrounded by wire netting. To carry out this operation, a semi-open questionnaire was sent to 66 households, specifically those located two rows from each other and close to both banks of Lobé Creek, using a door-to-door strategy.

The pH, temperature, dissolved oxygen, electrical conductivity, suspended solids and total dissolved solids were measured twice a week using a HANNA HI 9829 multi-parameter. Depth was measured using a graduated stake.

The rate of increase was obtained after determining the rate of multiplication of the plants over a period of 50 days. To measure the rate of multiplication of E. crassipes, 6 quadras of 100 m² were set up in 2 transects, each with a surface area of 100 m², marked out by wooden stakes fixed in the mud and surrounded by wire netting. 5 plants of water hyacinth were introduced into each quadra and monitored weekly. This rate of increase was determined using the method described by [8] . Growth rate = (W2 − W1)/(T2 − T1) with W1 = number of plants (or biomass) at the start of the experiment at time T1, W2 = number of plants (or biomass) at the end of the experiment at time T2.

To analyze the nutrient content, in particular Total Nitrogen (TN), Total Phosphorus (TP) and Potassium in water hyacinth plants according to [9] have been use. To do this, a field trip enabled us to collect 100 g samples of fresh plant material. This sampling was carried out on the leaves, stems and roots of water hyacinth taken from each of the stations. They were then dried at a temperature of 105˚C in an oven for 24 hours to constant weight.

Total Nitrogen Content

The method used was the TNT Persulfate Digestion Method using the Hach DR3900 spectrophotometer. Firstly, the digestion reagent solution was prepared by dissolving sachets of TNT persulphate digestion reagent powder in distilled or deionised water. The sample was then prepared by measuring an appropriate quantity and transferring it to a TNT digestion vial (the blank prepared with distilled water). The digestion reagent solution was then added to each vial, which were then hermetically sealed. The vials were then placed in a digestion block set at 100˚C for 2 hours to allow digestion to take place. After the digestion period, the vials were cooled to room temperature. Finally, the contents of each vial were mixed by gentle inversion and an appropriate aliquot of the digested sample solution was transferred for analysis using the Hach DR3900.

Total Phosphorus Content

Phosphorus was determined using the “molybdovanadate” Method. 1 ml of molybdovanadate reagent was added to each water sample and to a control (distilled water). The orthophosphate molecules present reacted with the molybdate in an acid medium to form the phosphomolybdate complex. In the presence of vanadium, an acid called vanadomolybdophosphoric acid is formed and takes on a yellow colour. The intensity of the colour is proportional to the concentration of phosphates present in the medium. The reading was recorded on a Hach DR/3900 spectrophotometer and the values were displayed as orthophosphate ( $P{O}_4^{3-}$), expressed in mg/L. These values were then multiplied by 0.3261 to give P.

Potassium Content

To 25 ml of sample contained in a test tube, the contents of a potassium 1 and potassium 2 reagent capsule were added successively. The contents were then capped and turned several times to mix. As soon as the solution was clear, the contents of one capsule of potassium reagent 3 were added, and then shaken for 30 seconds. The solution obtained was poured into a 25 ml cell. Another cell (the blank) was filled with 25 ml of sample. Finally, the reading was taken with a DR/3900 spectrophotometer at a wavelength of 650 nm and the result was expressed as mg/l potassium.

The parameters studied were subjected to descriptive analysis using Microsoft Excel 2013. The results obtained were represented exclusively using graphs and curves.

The distribution of activities carried out around the watercourse shows that 9.38% of the population practice agriculture. The same applies to livestock farming. In addition, 6.25% of local residents fish, while 25% engage in other activities, such as mechanics, welding and trade. Finally, 50% do not engage in any activity whatsoever ( Figure 2 ).

Most people who farm do not improve the quality of their soil. Of those who do, 90.91% use no soil improvers at all. Of the remaining 9.09%, 6.06% use chemical fertilisers, while the rest (3.03%) use only natural products, including pig faeces, chicken droppings, household waste and ashes ( Figure 3 ).

People’s Perception of E. crassipes

According to the survey results, Eichhornia crassipes is gradually invading the creek’s watercourse and becoming a nuisance for the surrounding population; for 60.61% of households, Eichhornia crassipes is a nuisance plant. On the other hand, around 18.18% perceive it as useful for the landscape and the environment, while 21.21% have no interest in the plant ( Figure 4 ).

The physico-chemical parameters of the water in the creek vary from station to station. At Station 1, located upstream, conductivity and dissolved oxygen are the highest (conductivity 40.7 ± 1.83 µS/cm; dissolved oxygen 7.3 ± 0.14 mg/l) compared with Station 2, where they are low (conductivity 19.6 3 ± .11 µS/cm ; dissolved oxygen 5.8 ± 1.55 mg/l); on the other hand, pH and temperature are higher at Station 2 (pH 7, ±20.07; temperature 26.5 ± 1.13˚) than at Station 1 (temperature 24 ± 1.41; pH 6, ±30.14). Depth was greater at Station 2 (1.95 ± 0.21 m) ( Table 1 ).

With regard to the rate of growth, an accelerated rate of growth was observed, particularly at Station 1, with a total of 41 stems at quadra 2, 39 stems at quadra 1 and 37 stems at quadra 3 ( Figure 5(a) ). At Station 2, on the other hand, where stems are least represented, the number of plants varies from 33 to 39 stems, with 33 stems in quadra 6, 36 stems in quadra 4 and 39 stems in quadra 5 ( Figure 5(b) ). The average growth rate of E. crassipes was 0.69 feet/day at Station 1 and 0.63 feet/day a Station 2.

With an overall average of 5.65 ± 0.45 g/kg on Lobé Creek, the Total nitrogen content decreases from upstream to downstream. A value of 6.11 ± 1.5 g/kg was recorded at Station 1 and 5.20 ± 2.03 g/kg at Station 2 ( Figure 6 ).

Total phosphorus and potassium levels vary from station to station, with overall averages of 0.83 ± 0.44 g/kg for total phosphorus and 2.02 ± 0.5 g/kg for potassium. Unlike Station 1, Station 2 has the highest levels, with 0.88 ± 0.38 for total phosphorus and 2.61 ± 0.89 for Potassium ( Figure 6 ).

In the Sodiko and 105 neighbourhoods, according to the local people, E. crassipes has a considerable impact on the environment. Fishing is the least common activity (6.25%), while 25% of households carry out several other activities, including trading, mechanics, welding and hairdressing. Furthermore, half of the population surveyed does not carry out any activities at all. This situation could be explained by the fact that fishing is hampered by the proliferation of E. crassipes, which deprives people who live from fishing of their activity, forcing them to turn to other activities. In addition, these households (60.61%) perceive the water hyacinth as a harmful and dangerous plant for their environment, since it contributes considerably to the degradation of the aquatic ecosystem through the reduction in stocks of fish products and, at the same time, encourages the introduction of reptiles into their homes and the development of various diseases. On this subject, [10] concluded from a study that this plant encourages the development of diseases such as malaria and bilharzia.

In contrast to Station 1, depth, pH and temperature have higher values at Station 2, while conductivity, Total Dissolved Solids, Suspended Solids and Dissolved Oxygen are high at Station 1. The high temperatures recorded at Station 2 (26.5 ± 1.13˚C) are thought to be due both to the evapotranspiration of E. crassipes in the creek and to its ongoing degradation process, which releases heat. In this regard, according to [11] , the average water temperature of Lake Ravelobe invaded by Water Hyacinth was equal to 26.33 relative to the sampling period. In addition, the presence of silt formed by the decomposition of E. crassipes plants on the one hand, and by the accumulation of sediments on the other, could explain the shallow depth (1.6 ± 0.49) at Station 1. As for dissolved oxygen, the low dissolved oxygen values at the stations could be explained by the fact that decomposition of the E crassipes plants consumes most of the oxygen available at these sites; also, the plant cover formed by the water hyacinth would reduce oxygen exchange at the air-water interface and at the same time reduce light penetration. As for pH, it varies slightly at the two stations. Station 1 has a neutral pH (6.3 ± 0.14) and Station 2, an acid pH (7.2 ± 0.07); this would be due to the regular decomposition of the water hyacinth, the increase in biological activity, associated with external inputs which would lead to a considerable drop in oxygen and consequently an enrichment in CO₂ which would influence the acid-base balance and thus make the environment acidic.

At the end of this study, it emerged that E. crassipes is developing at an exponential rate in Lobé Creek, with a growth rate of 0.69 feet/day for Station 1 and 0.63 feet/day for Station 2, relative to the environmental conditions that would be favourable for its development. On this subject, according to studies carried out by [12] , the very low growth rate of water hyacinth in the Bangr-wéoogo urban park of 0.083 feet/day is due to the difference in season and spatial situation. Priso et al. (2016), on the subject of the growth rate of this plant, concluded that the observation of the growth rate of the water hyacinth shows that it is likely that almost the entire river Wouri and its tributaries will be invaded within a decade if nothing is done.

Micronutrient analysis of the leaves, stems and roots of E. crassipes shows that this plant, harvested in Lobé Creek, has high levels of nitrogen. This analysis shows that nitrogen levels are higher in Lobé Creek, particularly at Station 1 (6.11 ± 1.5 g/kg); this can be explained by the presence of human activities around the watercourse and by the process of enrichment of the watercourse by effluents. Furthermore, the phosphorus and potassium values are not very significant, with a maximum equal to Station 2 (0.88 ± 0.38 g/kg for phosphorus and 2.61 ± 0.89 g/kg for potassium). In view of the results obtained, water hyacinth could be a source of raw material for the manufacture of compost to enrich soils [13] . Furthermore, [14] , following a study in Vietnam, obtained results lower than those of this study, with N, P and K contents equal to 2.30 for nitrogen, 0.28 for phosphorus and 2.67 for potassium.