ESTIMATION OF PHOSPHORUS AND NITROGEN RELEASED BY THE BREEDING OF OREOCHROMIS NILOTICUS IN CAGES IN THE TOHO AND TODOUGBA LAGOONS(SOUTHER BENIN) BETWEEN 2017 AND 2020

The present study aim to estimate the ratio of aquaculture in the phosphorus and nitrogen concentrations determined in the Toho - Todougba lagoons. For this purpose, the two lagoons were subdivided into 7 stations for the determination of phosphorus and nitrogen concentrations in the water column. Production data from 2017 to 2019 were collected from the Direction of the Ficheries Production and from the literature. Data for 2020 were collected directly from fish farmers. Annual tilapia production was estimated by year and the amounts of phosphorus and nitrogen released from aquaculture are deduced based on the ratio of Montanhini Neto & Ostrensky (2013). The concentration of each of these nutrients was estimated by station and compared to the concentration determined by laboratory analysis of the water. This methodology shows that the amount of phosphorus and nitrogen released to the environment varies from 0.49 mg/L to 0.18 mg/L for phosphorus and from 1.53 mg/L to 0.58 mg/L for nitrogen. The lowest values are obtained in 2020 and differ significantly from the other years (p <0.05). The quantity of phosphorus discharged is higher at the high production stations (Tonon 0.20 mg/L and Lokohoue 0.11 mg/L). Some of this is stored in the sediment. The nitrogen generated by aquaculture is significantly lower than the average determined in water (p <0.05). However, the concentration determined is still related to the amount of organic matter released due to aquaculture. Although aquaculture is not the only source of nutrient release to water, strategies for aquaculture with less nutrient release should be determined.

The present study aim to estimate the ratio of aquaculture in the phosphorus and nitrogen concentrations determined in the Toho -Todougba lagoons. For this purpose, the two lagoons were subdivided into 7 stations for the determination of phosphorus and nitrogen concentrations in the water column. Production data from 2017 to 2019 were collected from the Direction of the Ficheries Production and from the literature. Data for 2020 were collected directly from fish farmers. Annual tilapia production was estimated by year and the amounts of phosphorus and nitrogen released from aquaculture are deduced based on the ratio of Montanhini Neto & Ostrensky (2013). The concentration of each of these nutrients was estimated by station and compared to the concentration determined by laboratory analysis of the water. This methodology shows that the amount of phosphorus and nitrogen released to the environment varies from 0.49 mg/L to 0.18 mg/L for phosphorus and from 1.53 mg/L to 0.58 mg/L for nitrogen. The lowest values are obtained in 2020 and differ significantly from the other years (p <0.05). The quantity of phosphorus discharged is higher at the high production stations (Tonon 0.20 mg/L and Lokohoué 0.11 mg/L). Some of this is stored in the sediment. The nitrogen generated by aquaculture is significantly lower than the average determined in water (p <0.05). However, the concentration determined is still related to the amount of organic matter released due to aquaculture. Although aquaculture is not the only source of nutrient release to water, strategies for aquaculture with less nutrient release should be determined.

…………………………………………………………………………………………………….... Introduction:-
Food security has become a global concern, and all of the United Nations institutions are focusing on it, especially with the current influence of the effects of climate change (Béné et al., 2015;Costello et al., 2020;FAO, 2020). As a result, all political and socio-economic strategies that can enable the availability of food resources for especially 423 vulnerable populations are encouraged throughout the world. The Republic of Benin is part of this same dynamic through its agricultural policies, funding and subsidies for agricultural projects and the development of agrientrepreneurship. In this dynamic towards agricultural development, the promotion of aquaculture is a fundamental pillar to facilitate access to quality fish products on the one hand and on the other hand to promote the development of youth entrepreneurship in this sector to face unemployment (PNDF_Aquaculture, 2019). The fish farming systems that has been expanding in Benin for nearly 10 years is cage fish farming. To this end, the Toho and Todougba lagoons are home to the large number of cages installed for aquaculture in Benin (DPH/MAEP, 2018)with a prospect of intensification over the years.
Unfortunately, the cage aquaculture system is a source of enrichment of water bodies in organic matter, phosphorus and nitrogen (Abery et al., 2005;Guo et al., 2009;MPO, 2015)which are factors leading to eutrophication when in excess (Carpenter, 2005; El-Sheekh et al., 2021). Eutrophication is a permanent stress to aquatic living resources (Vodougnon et al., 2018)with the corollary of biodiversity loss. Research results on the assessment of the ecological status of Toho and Todougba lagoons through trophic status indices revealed that the lagoons are in an advanced pollution trend and will be further affected if the trend continues (Achoh et al., 2021). It should be noted that the concentration of phosphorus (0.02 mg/L to 0.42 mg/L) and nitrogen (1.91 mg/L to 4.03 mg/L) (Achoh et al., 2021)determined in the lagoons is not solely due to fish farming since there are several other activities that interact with the environments. So, what is really the share of fish farming in the concentration of phosphorus and nitrogen in Toho and Todougba lagoons? The answer to this question is the subject of this study.

Study area
The Toho and Todougba lagoons are located between 6° 23' and 6° 27' N and 2° 07' and 2° 13' E. They have an area of 914.259346 hectares or 9142593.46 square meters and an average depth of 3.2 meters with an estimated volume of 29256299.072 cubic meters or 29256299072 liters. They form a lagoon complex with other lagoons in the basin, notably Ahouangan, Dathi and Djonou lagoons. The Toho and Todougba lagoons have become entrenched ecosystems fed mainly by rainwater and runoff. Despite the variation of the physico-chemical quality of the water during the year, the Toho and Todougba lagoons are increasingly attractive for the development of cage fish farming and this since the first trials in 2013. The number of cages installed on these lagoons is growing every year.
In addition to cage aquaculture, market gardening, the hotel and restaurant industry and tourism are among the activities that interact with these environments and that are likely to influence their functioning. There is also fishing which is practiced there with the use of non-regulatory gear by the majority of anglers as a way of resilience in the face of declining catches.
To conduct this study, seven (07) stations were selected taking into account the presence or absence of cages, the distribution of cages on the complex and accessibility. The characteristics of each of these stations are presented in Table 1    Extreme zone of the right arm of the complex, in contact with the full flood with a low aquaculture production and receiving the waters having washed the plantations of palm groves and residual waters of hotels and restaurants. 2019 2020 13 10 Lokohoué An area of high aquaculture production that receives a moderate amount of runoff directly and is adjacent to a floating restaurant on the water.

96
Pont Area of exit of the water circulation receiving any type of waste coming from the locality of Pahou. It is located about 100 meters from the new market of Pahou under construction. It also houses cages for aquaculture.

60
Tonon Area housing the farm with more cages in operation on the water, an area of high aquaculture production and not directly receiving runoff.

180
Tchiakpè As the intersection zone between the right and left arms, there is no aquaculture production. However, it is a transit zone for people and goods coming from or going to the center of Pahou towards the village Tchiakpècodji. 0 0 Savi I Medium aquaculture production area receiving water that has washed out the palm plantations. There is a weak aquaculture production at this level. There is a private school and a hotel on the edge of this resort.

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In addition, at each study station, water samples are collected and brought back to the laboratory for determination of phosphorus and nitrogen concentrations in the water column by spectrophotometry. At these stations, the number of operational cages for tilapia rearing was counted in 2019 (Aïzonou et al., 2019)and in 2020. This allowed the estimation of the biomass produced per study station and the deduction of the amount of phosphorus and nitrogen released at each station.
Next, an estimate was made of the amount of phosphorus and nitrogen that would be produced by a set of 100 cages. This estimate was necessary because of the upcoming installation of an aquaculture village by the Territorial Agency for Agricultural Development Pole 7 (ATDA 7) of the Ministry of Agriculture, Livestock and Fisheries.

Statistical analysis of the data
The data obtained were inserted into the Excel spreadsheet and the calculated average values of the quantity of phosphorus and nitrogen discharged were compared between years by the Mann Withney test (α = 0.05). After estimating the concentration of phosphorus and nitrogen discharged per study station, the resulting averages were compared between years. These estimated concentrations were compared to the concentration means determined in the laboratory for each station. The Mann-Whitney test was used for this purpose with (p-value = 0.05). The means per station were represented as a histogram. The tilapia yield per production cycle was revealed in 2019 (1033.89 Kg) by the work of Aïzonou et al., (2019). In this study, this same yield was maintained for the years 2017 and 2018 given the non-existence of these data from the Direction of Fisheries Production. The yield used for 2020 was determined following data collection from producers in 2020.

Estimation of phosphorus and nitrogen discharge per study station
The amount of phosphorus and nitrogen discharged per study station and the concentration in mg/L were estimated and compared with the concentrations determined for the same nutrients from the laboratory analyses.

Estimation of the concentration of phosphorus and nitrogen to be generated by the aquaculture village
The estimation of the amount of phosphorus and nitrogen that will be discharged and their concentration in relation to the ecosystem water for the aquaculture village consisting of 100 cages was done based on the current production data ( Table 5). The result is that 2325.14 kg of phosphorus and 7329.23 kg of nitrogen will be discharged from the aquaculture village per year for an estimated annual tilapia production of over 163 tons. In terms of concentration, the 100 cages will produce 0.25 mg of nitrogen per liter and 0.08 mg of phosphorus per liter of water.

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Comparing the estimated concentrations of phosphorus and nitrogen due to aquaculture with the concentrations determined after sampling the lagoon water by station, it appears that for some stations, the estimated quantities are significantly higher than those determined (Tonon and Lokohoué station for phosphorus). This is due to the fact that these stations have the highest number of cages installed and do not receive runoff directly compared to other stations. Therefore, the low amount determined compared to the estimated amount released by aquaculture at these stations is related to the sedimentation gradient which results in the estimated amount not being that found in the water column. Much of it is accumulated in the sediment (Holtan et al., 1988;Jin et al., 2006).
At the bridge station, where aquaculture is not as negligible, the estimated amount of phosphorus released into the environment is below that determined in the water column. This means that apart from aquaculture, there are other important sources of phosphorus input into the environment. It should be noted that this station is located in a sort of depression with respect to the surrounding locality and therefore receives all the runoff water with all its nutrient load. As a result, although there is a sedimentation gradient, the quantity determined in the water column is higher and reflects the importance of the quantity of phosphorus that this station receives through runoff. The Adjadji and Savi II stations, where the amount of aquaculture input is very low or even zero compared to the determined amount, should be noted that they are in contact with a vast palm grove and receive the leaching water. The quantities determined at these stations are higher than those of all other stations (concentrations determined and estimated together). This reflects the magnitude of the loading that these two extreme lagoon stations receive. Also, microbial activity tends to increase the amount of nitrogen in the water as a result of high organic matter discharge. This may also condition the elevated water N level relative to the estimated amount contributed by aquaculture. However, the input of organic matter from aquaculture is significant and thus contributes to this high concentration for what is determined. For stations where aquaculture does not exist, the nitrogen level is due to the organic load of the leachate received.
The present study considers tilapia cage culture, which represents about 90% of the total production. The remaining production (10%) covered by catfish farming Clarias gariepinus was not considered although it also has its share of phosphorus and nitrogen that it releases into the environment. This means that the estimated contribution from aquaculture should be higher if the contribution from catfish farming is considered.

Conclusion:-
The estimation of the amount of phosphorus and nitrogen released by aquaculture was done to critically look at the share of this activity in the current water load of Toho and Todougba lagoons. The estimated amount of phosphorus and nitrogen released by aquaculture into the environment from 2017 to 2020 is 0.43 mg/L, 0.49 mg/L, 0.41 mg/L and 0.18 mg/L for phosphorus and 1.36 mg/L, 1.53 mg/L, 1.28 mg/L and 0.58 mg/L for nitrogen respectively. Comparing the estimated concentration contributed by tilapia aquaculture to the determined concentration in the water column, it appears that for some stations, the determined averages are below the estimated ones (Tonon and Lokohoué stations) for phosphorus. At the same time, for all the extreme stations of the two lagoons (Adjadji, Pont, Savi II), the determined concentration is higher than the estimated one because of the importance of the runoff water they receive.As for nitrogen, the concentrations determined are higher than those estimated for all stations. Therefore, although the input of phosphorus and nitrogen to the environment exists outside of any aquaculture activities, it should be noted that aquaculture contributes an important part of the nutrient load of these lagoons. Thus, the higher the intensity of aquaculture, the greater the amount of phosphorus and nitrogen it releases. Under these conditions, what are the technical strategies for aquaculture production in cages that can significantly reduce the amount of phosphorus and nitrogen discharged without negatively affecting the production yield?