Spatial and temporal variations of phytoplankton composition and biomass in Lake Bunyonyi, South-Western Uganda.

The purpose of this study was to examine the spatial and temporal variations of phytoplankton species composition and biomass in Lake Bunyonyi, South-Western Uganda. Samples were collected monthly from nine fixed stations in the lake from October 2019 to September 2020. Based on the morphological characterization, 52 different species of phytoplankton were recorded. These were dominated by cyanobacteria (21 species) and chlorophytes (15 species) followed by diatoms (10 species), euglenophytes (4 species), dinoflagellates (2 species), and cryptophytes (1 species). The biomass (Chl-a concentration) ranged from 0.019 ± 0.009 mg/L at Heissesero station to 0.045 ± 0.013 mg/L obtained at Nyombe station. On a temporal basis, the highest mean Chl-a concentration of 0.044 ± 0.03 mg/L was recorded in March 2020 while the least concentration of 0.015 ± 0.011 mg/L was obtained in September 2020. Significant differences existed in the Chl-a concentration values between stations and across sampling months. Chl-a concentration was significantly positively correlated with dissolved oxygen (DO), turbidity but negatively correlated with temperature. The Shannon-Wiener index and evenness put it clear that the distribution of phytoplankton species in the lake is inequitable. Besides, 94.2% of the phytoplankton species revealed had never been reported by the previous studies in the study area. The dominance of species cyanobacteria (such as Microcystis spp., Cylindrospermopsis raciborskii, Anabaenopsis sp., and Anabaena sp.) presents potential future challenges to water quality management. Therefore, the establishment of a strong and committed committee dubbed "Lake Bunyonyi Water Management Committee" to oversee the activities and avert potential water quality challenges is strongly recommended. The existence of some toxic phytoplankton species calls for regular monitoring and careful use of the lake and its food products.

Assessment of Ecological Risk of Heavy Metals Using Probabilistic Risk Assessment Model (AQUARISK) in Surface Sediments from Wami Estuary, Tanzania.

Total concentrations of As, Cd, Cr, Cu, Pb, and Zn in sediment samples obtained from Wami Estuary in Tanzania were used to generate contaminant probability density distributions and species sensitivity distributions using the AQUARISK model. Results of tier 1 assessment showed that As, Cd, Cr, Pb, and Zn were not of concern as their measured values and the 99th percentile of the fitted distributions were lower than the SQG low-trigger values. However, Cu was identified as of concern in this estuary. According to the Bur III distributional analysis of the exotoxicological data, the estimated percentage of species likely to be affected is 3.4, 79.4, 79.8, 99.9, 98.4, and 98.0 for As, Cd, Cr, Cu, Pb, and Zn, respectively. Lowering of the current median concentrations of metals (Cd, Cr, Cu, Pb, and Zn) is recommended as they exceeded modeled median target sediment concentration to achieve 95% or higher for species protection. With the ongoing increase in anthropogenic activities in the Wami River catchment, the environmental regulatory bodies may use the findings of the present study and augmented with AQUARISK to set discharge standards for various contaminants in order to minimize impacts to the receiving ecosystems.

Spatial and temporal variations of faecal indicator bacteria in Lake Bunyonyi, South-Western Uganda.

Background Microbial water quality serves to indicate health risks associated with the consumption of contaminated water. Nevertheless, little is known about the microbiological characteristics of water in Lake Bunyonyi. This study was therefore undertaken to examine the spatial and temporal variations of faecal indicator bacteria (FIB) in relation to physicochemical parameters in Lake Bunyonyi. Result The FIB concentration was consistently measured during sampling months and correlated with each other showing the presumed human faecal pollution in the lake. The highest concentration values for E. coli (64.7 ± 47.3 CFU/100 mL) and enterococci (24.6 ± 32.4 CFU/100 mL were obtained in the station close to the Mugyera trading centre. On a temporal basis, the maximum values were recorded during the rainy season in October 2019 (70.7 ± 56.5 CFU/100 mL for E. coli and 38.44 ± 31.8 CFU/100 mL for enterococci. FIB did not differ significantly among the study stations (p > 0.05) but showed significant temporal variations among the months (p < 0.05) with concentrations being significantly high in wet season than dry season (U = 794, p < 0.0001 for E. coli; U = 993.5, p = 0.008 for enterococci). Spearman's rank correlation revealed that FIB concentrations were significantly positively correlated with turbidity and DO concentration levels (p < 0.05). Approximately 97.2% of the water samples had E. coli and enterococci concentrations levels below USEPA threshold for recreational waters. Likewise, 98.1 and 90.7% of samples recorded E. coli and enterococci counts exceeding the UNBS, APHA, WHO and EU threshold values for drinking water. Conclusion The FIB counts show that the Lake Bunyonyi water is bacteriologically unsuitable for drinking unless it is treated since the FIB pose health risks to consumers. Besides, the water can be used for recreational purposes.

Differences in trophic resources and niches of two juvenile predatory species in three Pangani estuarine zones, Tanzania: stomach contents and stable isotope approaches.

BACKGROUND: Estuaries are primary habitats that serve as feeding and nursery grounds for most juvenile marine fish. However, estuaries have been used as fishing grounds by the artisanal fishers in Tanzania. The slow-growing predatory fish at juvenile and sub-adult stages are among the most frequently caught species that functionally enhance multiple linkages of energy pathways within the food web. Stomach contents and stable isotopes (δ13C and δ15N) were used to describe the nutritional sources and trophic niches between the co-existing benthic, predatory species, Carangoides chrysophrys and Epinephelus malabaricus in the Pangani estuary, Tanzania. RESULTS: The findings indicated significant inter-specific variations in dietary composition (PERMANOVA, p = 0.001, pseudo-F = 15.81). The prey-specific index of relative importance (%PSIRI) indicated that juvenile shrimps (%PSIRI = 51.4) and Teleostei (%PSIRI = 26.5) were the main diets of C. chrysophrys while brachyura (%PSIRI = 38.8), juvenile shrimps (%PSIRI = 25.6) and Teleostei (%PSIRI = 23.3) were important diets of E. malabaricus. The isotope mixing models indicated that the predatory fish species accumulate nutrients derived from similar autotrophic sources, microphytobenthos, seagrass and macro-algae via consumption of small fish, including clupeids and mugilids. Yet, they significantly showed different isotopic niche width with varying degree of niche overlap across the longitudinal estuary gradient. This situation was justified by the presence of basal food sources among the estuarine zones that isotopically were different. CONCLUSION: The reliance of both predators on clupeids and mugilid preys that are trophically linked with estuarine and marine basal food sources, is an indication of low estuarine food webs' connectivity to the fresh water related food web. This situation is most likely threatening the stability of the estuarine food web structure. Management strategies and plans in place should be cautiously implemented to ensure the balanced anthropogenic freshwater use in the catchment and fishing activities, for the maintenance of the Pangani estuarine ecosystem health.

Temporal and spatial distribution of trace metals in the Rufiji delta mangrove, Tanzania.

Spatial and temporal distribution of trace metals and their cycling is a key issue for understanding the ongoing biogeochemical processes in coastal environments. Sediment cores were collected from six different sampling locations from the Rufiji delta mangrove forests in southeastern coastal Tanzania that are perceived to be impacted by urban development and agricultural activities in the catchment, and pollution in upstream sections of the Rufiji River. The chronology and sediment accumulation rates at these sampling sites were derived based on the distribution of 210Pbexcess method. The trace metals (As, Cd, Cr, Cu, Ni, Pb, and Zn) were sequentially extracted as per the BCR method and analyzed. The results indicate that the mass accumulation rates range from 0.40 g cm-2 year-1 (cores NR3 and NR4) to 1.75 g cm-2 year-1 (core SR1). Trace metals in the cores are mainly associated with the residual phase and their abundances in sediments are ranked as Cr > Zn > Ni > Cu > Pb > Cd. The results imply that trace metals in the Rufiji delta mangroves are mainly of crustal origin, and they are less sensitive to weathering. Further, these metals are least available for uptake by plants and they pose limited threat to the biota.

Polycyclic aromatic hydrocarbon (PAH) contamination of surface sediments and oysters from the inter-tidal areas of Dar es Salaam, Tanzania.

Surface sediment and oyster samples from the inter-tidal areas of Dar es Salaam were analyzed for 23 polycyclic aromatic hydrocarbons (PAHs) including the 16 compounds prioritized by US-EPA using GC/MS. The total concentration of PAHs in the sediment ranged from 78 to 25,000 ng/g dry weight, while oyster concentrations ranged from 170 to 650 ng/g dry weight. Hazards due to sediment contamination were assessed using Equilibrium Partitioning Sediment Benchmarks and Threshold Effect Levels. Diagnostic indices and principle component analysis were used to identify possible sources. Interestingly, no correlation between sediment and oyster concentrations at the same sites was found. This is supported by completely different contamination patterns, suggesting different sources for both matrices. Hazard assessment revealed possible effects at six out of eight sites on the benthic communities and oyster populations. The contribution of PAH intake via oyster consumption to carcinogenic risks in humans seems to be low.

Metals in Nile perch (Lates niloticus) and suspended particulate matter from Lake Victoria, Tanzania.

A study was conducted to assess the levels of pollutant metals in suspended particulate matter and Nile perch from Lake Victoria. The metals in particulate matter were determined to ascertain their concentrations at the base of the food chain. Nile perch samples were collected in September 2003 from five major fish processing factories at the shores of Lake Victoria in Mwanza and Musoma. The concentrations of total Hg, Pb, Cd, and Cu were generally low in particulate matter and in most locations were close to or below their limits of detection. The concentrations of Zn were high in suspended particulate matter, the highest being 219.4 +/- 153.0 microg L(-1) found in particulate matter from Nungwe Bay in the southern part of Lake Victoria. Nile perch generally contained low levels of heavy metals; the range for Pb was <0.01-0.08 microg g(-1) ww, Cd was <0.001-0.04 microg g(-1) ww, Cu was 0.01-0.97 microg g(-1) ww, and Zn was <0.01-18.94 microg g(-1) ww. The concentration of total mercury ranged between 31.0 and 684.2 ng g(-1) ww; generally, it was below the Food and Agriculture Organization of the United Nations/World Health Organization (1000 ng total Hg g(-1) ww for piscivorous fish species) maximum allowable level. Indeed, all Nile perch samples that weighed less than 10 kg had less than 200 ng total Hg g(-1) ww and therefore are safe for regular consumption by at-risk groups such as children and pregnant women. Levels of mercury and other heavy metals in Nile perch at present is, therefore, not a severe environmental issue; however, urgent regulatory measures should be taken to minimize metal input into the lake to maintain the current levels in the fish.