The results revealed that temperature of the surface water was in general high in all selected sites, but with significant variation between the sites. These results are in accordance with Lowe-McConnell (1987) who pointed out that tropical regions are characterized by high temperatures with relatively little variations. Temperature values recorded in the sampling sites corroborated also with the values found by Schmid et al. (2005), Bisimwa (2009) and Hyangya et al. (2021) in the same lake.
However, the results indicated that fish cages had got significant impact on the decrease of water temperature at the sites. Therefore, temperature is known to have a significant effect on the biological functions of the aquatic organisms (Colman et al. 1992, Boyd 1998, Frantzy 2017). In most of tropical water systems, species grow best at temperature between 20°C and 32 oC and water temperatures generally remain in this range year-round (Lowe-McConnell 1987, Boyd 1998). Therefore, in respect to the present findings with regards to the temperature conditions, though the detected fish cages’ impact on temperature, based on the recorded ranges, the sampling sites could be considered to provide permissible conditions for the biological functions of farmed fish.
However, previous studies conducted in many water bodies reported that the vegetation clearance of the water banks was one of the factors triggering the variation water temperature in the littoral zones (Jennings et al. 2003, Hyangya et al. 2021). The above statement could corroborate with present study; where the water banks around the fish cage sites were undergoing serious vegetation clearance during the study period, and in turn showing the influence of anthropogenic activities in the Lake water quality.
One of the major indicators of the water quality, after the dissolved gases (DO and CO2), is the ionic composition of water, of which most important measure is the pH (Colman et al. 1992).
The present results revealed that fish cages have got significant impact on the water pH, among others; showing a decrease of water pH values at the sites with fish cages than in the control site. The pH is known to influence the physiological functions of fish and other aquatic lives (Boyd and Tucker 1992). According to Boyd &Tucker (1998) and Ali et al. (2000) pH range for diverse fish production is between 6.5 and 9; and any variation beyond acceptable range could be fatal to many aquatic organisms (Furhan Iqbal et al. 2004). However, in the present study, despite of the detected impact of fish cages on the water pH, the rates recorded in the sampling sites varied within the favourable range (Boyd and Tucker 1992, 1998, Ali et al. 2000), showing the cage sites are still suitable for fish production.
Therefore, the present pH values corroborate with those reported in previous studies (Muvundja et al. 2014) in the same lake. The later stated that in Lake Kivu, pH values have a high fluctuation in the coastal waters resulting from large amounts of alkaline carbonates and other substances contained in the waters. In addition, Loka et al. (2012) reported that the pH of freshwater varies between 3 and 11 as a result of calcareous rocks. However, the present results vary from those of Kashindye et al. (2015) in Lake Victoria, Shirati bay, where the sites with the cages had recorded acidic pH of about 5.23 after comparing to the control sites with a pH value of about 8.81. These two results show that the impacts of cage culture may be felt differently from one ecosystem to another. Research by Degens et al. (1973), Schmid et al. (2005) and Hyangya et al. (2021), report electrical conductivity in Lake Kivu waters ranging from 1140 to 1200 µS.cm− 1. Records from this study show a minimum value of 928 to 977 µS.cm− 1 in the sites with the cages compared to the control site without the cage. The report of NaFIRRI and NARO (2018) in Uganda indicates that conductivity plays an important role in enhancing the immune system of animals. At low conductivity levels, farmed fish become more susceptible to diseases, while at high conductivity levels, above 1000 µS. cm− 1, fish have better immunity. This indicates that the farmed fish in the cages in Bukavu basin could develop diseases if the conductivity continues to decrease.
The content of Total Dissolved Solids was found to be higher in sites with fish cages than in the control site. This could be due to the fact that sites with cages receive waste food and fish faecal from the fish feeds. Accordingly, the results from the study of Environmental impact assessment on the fish cages in Bukavu basin revealed that 27.50 kg to 92.23 kg of fresh weight of sediment including waste food, fish faecal, organic and inorganic materials were deposited per day under the cages in Nguba and Ndendere bays, respectively (Lina et al. 2021, un published). Additionally, Mente et al. (2006) stated that accumulation of waste food and fish faecal material results in changes in the sediment under fish cages, characterized by high content of organic material. Therefore, Lake Kivu being a meromictic lake (Damas 1937), such rate of inputs from the fish cages can drastically increase the sediment load to the bottom of the bays and could quickly result into advert
Among the dissolved gases, the DO plays the most important role with regard to the water quality (Mbalassa et al. 2014). It is critical for aquatic organisms’ respiration (Colman et al. 1992). Therefore, the DO is among determining factors for the survival and the growth of aquatic organisms (Boyd and Tucker 1992, 1998; Colman et al. 1992). In the current, the content of DO did not show significant variations between the sampling sites. This could be to that fact that high concentration of nutrients at sites could allow the development of phytoplankton that can increase the photosynthetic rate in water, which in turn could increase the DO content. The results present releaved the significant impact of the fish cages the on decreasing water transparency around fish cages sites. However, according to Boyd and Tucker (1998) and Ali et al. 2000), light penetration varying from 30 cm to above 60 cm was acknowledged to be favorable for fish production. In this study, though fish cages impacted on water transparency, according to the above authors, the estimated values of water transparency were within the permissible limits. In addition, Blaber (2000) stated that low water transparency could also be associated with areas where there is an abundance of food. This could corroborate with the present study in the way that, high concentration of nutrients at fish cages sites could allow the development of phytoplankton along with zooplankton that can be eaten by young fish in and outside the cages.
Therefore, lower values of water transparency observed in the fish cages sites could also be related to the intensive vegetation clearance observed along the littoral zone and lake catchment, among others. The runoff from the cultivation and uncovered flanks could drain subsequent sediment which in turn decreases water transparency in the lake. Maitland and Morgan (1997) stated that the clearing land also increases the runoff of surface water and the rate of soil erosion with subsequent silting in the waters draining such areas. This corroborate could with the present study where the sites with cages receive both runoff from cleared lands and uncovered flanks and a lot of untreated wastes from Bukavu town that is susceptible to reduce the water transparency in cage sites compared to the control site which is located far from inhabited areas and surrounded by a heavy cover of macrophytes.
The present results revealed that fish cages have triggered the increase of concentrations of NH4+ and NO2− in the water at the sites with cages compared to the control site. These results corroborate with Mente et al. (2006) who reported that the accumulation of waste food and fish faecal material results in changes in water quality under fish cages, characterized by accumulation of nitrogenous compounds, among others. The authors indicated further that around 50% of the nitrogen supplied with the food is wasted in dissolved form. In Lake Kivu, previous studies found out that the concentrations of NO2− in the littoral zone generally increase during rainy seasons (Sarmento 2008, Lina 2016, Mazambi et al. 2020); and this increase was related to the inputs from untreated waste water from overpopulated Bukavu Town and surrounding trading centres on the Lake Kivu watershed (Lina 2016, Hyangya et al. 2021); indicating the human activities influence on the lake water quality.
The present results indicate that SiO2 concentration was significantly higher in sites with cages than in the site control. However, currently silica nanoparticles (SiNPs) are one of the top five nano-materials implicated in nanotech-dependent consumer products in aquaculture industry (Park et al. 2011). This is in part, because of their various valuable applications such as controlling fish diseases, immune-modulator, and anti-toxins (Mahboub et al. 2020, Mahboub et al. 2021, Rashidian et al. 2021). Therefore, despite the wide application of SiNPs in aquaculture, Li et al. (2021) revealed that they can induce cytotoxicity, hepatotoxicity, immunotoxicity, and genotoxicity. In addition, Abdel-Latif et al. (2021) indicated severe variance in gill tissue architecture after exposing of O. niloticus to 100 mg L− 1 of SiO2NPs. Bashar et al. (2021) showed that concentrations of silica NP, above 2 mg/kg decreases growth performance and feed efficiency of O. niloticus. Likewise, Duan et al. (2013) documented the acute exposure of Zebrafish to different concentrations of SiNPs (25, 50, 100, 200 mg L− 1) for 96-h induced toxicity to the developing embryo. The later further revealed that the lower doses of SiNPs (25 and 50 mg L− 1) resulted in hyperactivity and besides persistent impacts on the behavior of Zebrafish larvae. Vidya et al. (2016) reported restlessness and gulping air at the surface post-acute exposure of O. mossambicus to SiNPs owing to impairment in the consumption of oxygen and consequently altered fish oxidative metabolism.
Notwithstanding the use of silica NP in aquaculture, Bartram and Ballance (1996) noted that in natural ecosystems, silica concentrations in groundwater and volcanic waters are generally higher. This also indicates that the fact that Lake Kivu is of volcanic origin (Haberyan & Hecky 1987), it would expect to have a large amount of silica.
The results revealed significant increase of Chl a concentration in the sites with fish cages than in the control site. The current results corroborate with that of Tibúrcio et al. (2021) who found the same results in in the Rosana Reservoir (Brazil). However, the authors stated further that chlorophyll-a represents an estimate of food availability in the ecosystem (Tibúrcio et al. 2021). Therefore, the high concentration of Chl a could be, according to Pergent et al. (1998) the result of the inputs from the important nutrient-rich food quantity given to the fish in cages. However, Simões et al. (2015) and Borges et al. (2010) stated that changes in nutrient loads in these systems due to food input alter phytoplankton composition and cause increasing issues in these environments.
In Lake Kivu, The increase in chl_a in the sites with cages prove that there are excessive blooms of phytoplankton related to the appropriate conditions prevailing, such as high level of nutrients (organic load) and stagnant hydrological conditions (Loka & al. 2012). A recent study focused on the spatial analysis of Chlorophyll a based on MERIS data from 2003 to 2004 from a cruise of Sarmento & al. (2006) studies in Bukavu basin (Kneubühler & al. 2007) specifies that concentrations are higher along the coastline, mainly on the shoreline. The strong and positive relationship of nutrients with chl_a is explained by the fact that most of the socio-economic activities discharge their waste directly or indirectly into the littoral zone of bays (Lina 2016) in the Bukavu basin.