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Freshwater biodiversity is declining dramatically, and the current biodiversity crisis requires defining bold goals and mobilizing substantial resources to meet the challenges. While the reasons are varied, both research and conservation... more
Freshwater biodiversity is declining dramatically, and the current biodiversity crisis requires defining bold goals and mobilizing substantial resources to meet the challenges. While the reasons are varied, both research and conservation of freshwater biodiversity lag far behind efforts in the terrestrial and marine realms. We identify fifteen pressing global needs to support informed global freshwater biodiversity stewardship. The proposed agenda aims to advance freshwater biodiversity research globally as a critical step in improving coordinated action towards its sustainable management and conservation.
The significance of large tropical lakes as environmental reservoirs of Vibrio cholerae in cholera endemic countries has yet to be established. By combining large scale plankton sampling, microbial culture and ultrasensitive molecular... more
The significance of large tropical lakes as environmental reservoirs of Vibrio cholerae in cholera endemic countries has yet to be established. By combining large scale plankton sampling, microbial culture and ultrasensitive molecular methods, namely Droplet Digital PCR (ddPCR) and targeted genomics, the presence of Vibrio cholerae was investigated in a 96,600 L volume of surface water collected on a 322 nautical mile (596 km) transect in Lake Tanganyika. V. cholerae was detected and identified in a large area of the lake. In contrast, toxigenic strains of V. cholerae O1 or O139 were not detected in plankton samples possibly in relation to environmental conditions of the lake ecosystem, namely very low salinity compared to marine brackish and coastal environments. This represents to our knowledge, the largest environmental study to determine the role of tropical lakes as a reservoir of V. cholerae.
In Lake Tanganyika, blooms of nitrogen-fixing (diazotrophic) cyanobacteria emerge, when the upper water column re-stratifies after a period of upwelling and convective mixing. During this seasonal transition, diazotrophic cyanobacteria... more
In Lake Tanganyika, blooms of nitrogen-fixing (diazotrophic) cyanobacteria emerge, when the upper water column re-stratifies after a period of upwelling and convective mixing. During this seasonal transition, diazotrophic cyanobacteria exploit the abundant phosphate and fix nitrogen after other phytoplankton taxa have consumed the available nitrate. However, it remains less clear, which mechanisms favour diazotrophic cyanobacteria under more heavily stratified conditions with lower levels of excess phosphate and persistent nitrate-depletion. Here, we collected profiles of physicochemical parameters, nutrients and photo-pigments, as well as the medium- to large-sized phytoplankton community during two lake-wide cruises to elucidate to what extent the abundance of diazotrophic cyanobacteria in Lake Tanganyika may be controlled by the nitrate resupply through the thermocline into the euphotic zone. At stations where nitrate was depleted, but phosphate remained available near the surfac...
Lake Tanganyika (LT) is the largest tropical freshwater lake, and the largest body of anoxic freshwater on Earth’s surface. LT’s mixed oxygenated surface waters float atop a permanently anoxic layer and host rich animal biodiversity.... more
Lake Tanganyika (LT) is the largest tropical freshwater lake, and the largest body of anoxic freshwater on Earth’s surface. LT’s mixed oxygenated surface waters float atop a permanently anoxic layer and host rich animal biodiversity. However, little is known about microorganisms inhabiting LT’s 1470 meter deep water column and their contributions to nutrient cycling, which affect ecosystem-level function and productivity. Here, we applied genome-resolved metagenomics and environmental analyses to link specific taxa to key biogeochemical processes across a vertical depth gradient in LT. We reconstructed 523 unique metagenome-assembled genomes (MAGs) from 34 bacterial and archaeal phyla, including many rarely observed in freshwater lakes. We identified sharp contrasts in community composition and metabolic potential with an abundance of typical freshwater taxa in oxygenated mixed upper layers, and Archaea and uncultured Candidate Phyla in deep anoxic waters. Genomic capacity for nitro...
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Lake Victoria is known for its explosive speciationspeciation and recent time hybridizationhybridization, which is highly mediated by deterioration of water qualitywater quality. This chapter summarizes the knowledge on change of water... more
Lake Victoria is known for its explosive speciationspeciation and recent time hybridizationhybridization, which is highly mediated by deterioration of water qualitywater quality. This chapter summarizes the knowledge on change of water quality and environment of southern part of Lake Victoria, Tanzania. It analyses rainfallrainfall, air temperaturetemperature and water quality data spanning 30 years (1985 to 2015). It also investigates changes in physical-chemical data sampled during and after the Lake Victoria Environmental Management Project I (LVEMP I). The chapter reviews some of the significant water quality changes that have occurred for the past 50 years. The results indicate no significant changes in annual rainfall variability. Nevertheless, trends of air temperatureair temperature showed no clear patterns for Mwanza and Musoma, but trends of minimum and maximum air temperature in Bukoba increased significantly at annual rates of 0.19 {\textdegree}C and 0.14 {\textdegree}C, respectively. Water level in Lake Victoria has also declined significantly at an annual rate of about 5.5 cm from 1965 to 2004. These findings suggest that lake levels are determined by evapotranspiration rather than rainfall. It was also found that anthropogenicanthropogenic stressors are more important in explaining nutrientsnutrients loading while thermal stratificationstratification explains hypoxiahypoxia and reduction in water mixingmixing. It is concluded that the current blooms of harmful algaealgae and excess biomassbiomass in Lake Victoria will continue unabated unless nutrient loading, anoxiaanoxia and high rates of denitrification are curbed. Appropriate measures to improve land useland use management should therefore be taken, while deliberate dumping of industrial, municipal and agricultural wasteswastes into the lake should be controlled.
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Understanding genetic connectivity plays a crucial role in species conservation decisions, and genetic connectivity is an important component of modern fisheries management. In this study, we investigated the population genetics of four... more
Understanding genetic connectivity plays a crucial role in species conservation decisions, and genetic connectivity is an important component of modern fisheries management. In this study, we investigated the population genetics of four endemic Lates species of Lake Tanganyika (Lates stappersii, L. microlepis, L. mariae and L. angustifrons) using reduced-representation genomic sequencing methods. We find the four species to be strongly differentiated from one another (mean interspecific FST = 0.665), with no evidence for contemporary admixture. We also find evidence for strong genetic structure within L. mariae, with the majority of individuals from the most southern sampling site forming a genetic group that is distinct from the individuals at other sampling sites. We find evidence for much weaker structure within the other three species (L. stappersii, L. microlepis, and L. angustifrons). Our ability to detect this weak structure despite small and unbalanced sample sizes and impre...
The lake bottom along structural platforms in Lake Tanganyika, Africa, is carpeted with numerous large shell beds, known to be of late Holocene age, but of uncertain assemblage process. The shell beds may be the result of sedimentological... more
The lake bottom along structural platforms in Lake Tanganyika, Africa, is carpeted with numerous large shell beds, known to be of late Holocene age, but of uncertain assemblage process. The shell beds may be the result of sedimentological (physical) assembly processes, or biological processes, or both. Previous work focused on the distribution of shell-rich facies, and showed time averaging of the surficial shell bioclasts over the last ∼ 1600 calendar years BP. We focus on an extensive shell deposit along a deltaic platform in Kungwe Bay, Tanzania and examine time-averaging and taphonomy of Neothauma tanganyicense shells to constrain sedimentological and biological processes forming concentrations of shells. New radiocarbon dating indicates that Neothauma shells are time-averaged over the last ∼ 3000 calendar years. Younger shells predominate shallow-water and exhibit unimodal age distributions, while shells from deeper-water exhibit a broader age distribution. Taphonomic results i...
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Rapid urbanization in developing countries has been accompanied by the spread of informal settlements, which is particularly prominent in sub-Saharan Africa. These settlements have become an important supplement to the inadequate formal... more
Rapid urbanization in developing countries has been accompanied by the spread of informal settlements, which is particularly prominent in sub-Saharan Africa. These settlements have become an important supplement to the inadequate formal housing supply in cities, and their spontaneously formed spatial patterns have important influences on sustainable development. In this study, qualitative and quantitative approaches were used to examine the morphological characteristics of informal settlements in Tanzania and the associated influences on urban development. Geographic spatial analyses, landscape pattern indices, and mathematical statistics, along with quick assessments, group discussions, and key informant interviews, were used to obtain detailed information on the spatial forms of informal settlements. The results indicate that the form of the settlements does not conform to the social, economic, or environmental characteristics of sustainable development. The disordered expansion o...
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Two endemic sardines in Lake Tanganyika, Limnothrissa miodon and Stolothrissa tanganicae, are important components of the lake's total annual fishery harvest. These two species along with four endemic Lates species represent the... more
Two endemic sardines in Lake Tanganyika, Limnothrissa miodon and Stolothrissa tanganicae, are important components of the lake's total annual fishery harvest. These two species along with four endemic Lates species represent the dominant species in Lake Tanganyika's pelagic fish community, in contrast to the complex pelagic communities in nearby Lake Malawi and Victoria. We use reduced representation genomic sequencing methods to gain a better understanding of possible genetic structure among and within populations of Lake Tanganyika's sardines. Samples were collected along the Tanzanian, Congolese, and Zambian shores, as well as from nearby Lake Kivu, where Limnothrissa was introduced in 1959. Our results reveal unexpected cryptic differentiation within both Stolothrissa and Limnothrissa. We resolve this genetic structure to be due to the presence of large sex-specific regions in the genomes of both species, but involving different polymorphic sites in each species. Add...
Maintaining sustainable fisheries requires understanding the influence of technological advances on catch efficiency. Fisheries using light sources for attraction could be widely impacted by the shift to light emitting diode (LED) light... more
Maintaining sustainable fisheries requires understanding the influence of technological advances on catch efficiency. Fisheries using light sources for attraction could be widely impacted by the shift to light emitting diode (LED) light systems. We studied the transition from kerosene lanterns to LED lamps in Lake Tanganyika, East Africa, examining factors that led to adoption as well as the impact of the new light sources on fish catch and composition. We used a combination of field experiments with catch assessments, fisher surveys, underwater light spectra measurements, and cost assessments to evaluate the impact of switching from kerosene to LED lamps. Overall, we found a very rapid rate of adoption of homemade outdoor LED light systems in Lake Tanganyika. Most of the batteries used to power these lamps were charged from the city power grid, rather than photovoltaic cells. The LED light spectra was distinct from the kerosene light and penetrated much deeper into the water column...
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Lake Tanganyika, an African Great Lake, is a complex tropical ecosystem that has been subjected to extreme climate-related changes in the last century, including seasonal changes in temperature and rainfall, decreased overall annual... more
Lake Tanganyika, an African Great Lake, is a complex tropical ecosystem that has been subjected to extreme climate-related changes in the last century, including seasonal changes in temperature and rainfall, decreased overall annual rainfall, and greater frequency of rainstorms. Atmospheric nitrogen (N) is an important component of the lake's N loading, but how long-term and seasonal changes in precipitation affect this loading still needs clarification. This study aimed to improve our understanding of the seasonal features of N deposition in the lake, by monitoring atmospheric N deposition concentrations and fluxes from March 2013 to February 2014. There was a significant temporal variation in wet N depositions in the study area. The distribution of the annual rainfall into major (March-May 299.8 mm) and minor (October-December 343.2 mm) rainy seasons translated into 20 and 30% of N deposition. In September and January-February, there was 10 and 12% precipitation, representing ...
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Toxic metals that bioaccumulate and magnify along food chains are a concern to human health worldwide. This study determined heavy metal concentrations in three commercial fish species; viz: whitespotted rabbitfish (Siganus sutor),... more
Toxic metals that bioaccumulate and magnify along food chains are a concern to human health worldwide. This study determined heavy metal concentrations in three commercial fish species; viz: whitespotted rabbitfish (Siganus sutor), thumbprint emperor (Lethrinus harak), and the Indian Mackerel (Rastrelliger kanagurta) from Kunduchi fish market in Dar es Salaam, Tanzania, assessed potential risks to human health, and the suitability of their fins as a non-destructive monitoring organ. Metal concentrations in fish muscle differed significantly among fish species as a result of their diverse ecological needs, metabolism, feeding habits, and habitats. The results also indicated that fish fin may not be a very good non-destructive monitoring organ for most metals, but can be used selectively for some metals and fish species. The levels of metal intake (aluminum (Al), cadmium (Cd), copper (Cu), iron (Fe), lead (Pb) and zinc (Zn)) in fish muscles were below the FAO/WHO maximum levels for contaminants and toxins in foods for human consumption except for arsenic (As) which were higher than the recommended levels in Lethrinus harakand Rastrelliger kanagurta. It is recommended to reduce the amount of L. harakand R. kanagurta in daily meals of especially fishers and coastal communities who may eat fish up to three meals daily. Recommendation for further studies is provided.
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Research Interests: Geology and Geochemistry
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Identification of critical life-stage habitats is key to successful conservation efforts. Juveniles of some species show great flexibility in habitat use while other species rely heavily on a restricted number of juvenile habitats for... more
Identification of critical life-stage habitats is key to successful conservation efforts. Juveniles of some species show great flexibility in habitat use while other species rely heavily on a restricted number of juvenile habitats for protection and food. Considering the rapid degradation of coastal marine habitats worldwide, it is important to evaluate which species are more susceptible to loss of juvenile nursery habitats and how this differs across large biogeographic regions. Here we used a meta-analysis approach to investigate habitat use by juvenile reef fish species in tropical coastal ecosystems across the globe. Densities of juvenile fish species were compared among mangrove, seagrass and coral reef habitats. In the Caribbean, the majority of species showed significantly higher juvenile densities in mangroves as compared to seagrass beds and coral reefs, while for the Indo-Pacific region seagrass beds harbored the highest overall densities. Further analysis indicated that d...
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... [Taylor & Francis Online], [CSA] View all references; Sarvala et al., 200617. Sarvala, J., Langenberg, VT, Salonen, K., Chitamwebwa, D., Coulter, GW, Huttula, T., Kanyaru, R., Kotilainen, P., Makasa, L., Mulimbwa, N.... more
... [Taylor & Francis Online], [CSA] View all references; Sarvala et al., 200617. Sarvala, J., Langenberg, VT, Salonen, K., Chitamwebwa, D., Coulter, GW, Huttula, T., Kanyaru, R., Kotilainen, P., Makasa, L., Mulimbwa, N. and Mölsä, H. 2006. ...
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Ontogenetic niche shifts are taxonomically and ecologically widespread across the globe. Consequently, identifying the ecological mechanics that promote these shifts at diverse scales is central to an improved understanding of ecosystems... more
Ontogenetic niche shifts are taxonomically and ecologically widespread across the globe. Consequently, identifying the ecological mechanics that promote these shifts at diverse scales is central to an improved understanding of ecosystems generally. We evaluated multiple potential drivers of ontogenetic niche shifts (predation, growth, maturation, diet shifts, and food availability) for three fish species between connected coral reef and nearshore habitats. In all cases, neither diet compositional change nor sexual maturity functioned as apparent triggers for emigration from juvenile to adult habitats. Rather, the fitness advantages conferred on reef inhabitants (that is, enhanced growth rates) were primarily related to high prey availability on reefs. However, there exists a clear trade-off to this benefit as survival rates for small fishes were significantly reduced on reefs, thereby revealing the potential value of (and rationale behind high juvenile abundances in) nearshore habit...