Towards sustainable management of inland waters in Tanzania assessing the ecological integrity of river ecosystems in the upper Pangani river basin (Tanzania)

Dissertation presented in partial fulfilment of the requirements for the degree of Doctor of Science (PhD): Biology) December 2020River ecosystems encompass river channels and its floodplains and form a diverse mosaic of habitats upon which countless species of animals and plants depend for survival. They provide a plethora of services for humans including a source of clean water for domestic and industrial uses, a source of food, a means of waste disposal, transportation, power production, and sites for the pursuit of leisure activities. Yet, they belong to the most threatened ecosystems on earth. Major threats to river ecosystems include habitat degradation, water pollution, flow modification, overexploitation, and invasion by exotic species. This is especially true for (sub) tropical developing countries where intensification of land-use for agriculture and poor disposal of untreated waste have markedly degraded rivers and associated floodplain ecosystems. Nevertheless, a proper understanding of ecosystem functioning and biological diversity is lacking. In this Ph.D., we contribute to bridging this knowledge gap. We investigate different factors that explain biodiversity and ecosystem quality in (afro) tropical river systems and associated temporary pool ecosystems in northeastern-Tanzania by using macroinvertebrates as biological indicators and collecting environmental and biological data at various spatial-temporal scales. Firstly, we assessed how seasonality (i.e., wet and dry seasons) influences macroinvertebrate community structure and water quality conditions (Chapters 1 and 2). An extensive repeated-sampling survey was conducted to measure water quality, macroinvertebrates, and other presumed important environmental variables in the two sub-catchments of the Upper Pangani River Basin (UPRB). We found evidence that water quality conditions and macroinvertebrate assemblages differ between seasons and that these differences are associated with high flow velocity, and runoff carrying sediment and nutrient loads from the catchment area to the river systems during the rainy season. Moreover, our results revealed that chlorophyll-a, oxygen and phosphorous (dry season), nitrogen and turbidity (wet season), and substrate composition and agricultural land-use (both seasons) are important determinants for the variation in macroinvertebrates assemblages between sites. We also attempted to identify indicator taxa linked to specific water quality conditions and found that families Hydropsychidae (Trichoptera), Potamonautidae (Decapoda), Baetidae (Ephemeroptera), and Heptageniidae (Ephemeroptera) showed to be indicator taxa of good water quality conditions, while Hirudinea (Annelida) and Chironomidae (Diptera) appeared to be indicator taxa of poor water quality conditions (Chapter 1). Secondly, we focused on the impact of land-use at different spatial scales on river quality (Chapters 1 and 2). To quantify this we used three different spatial methods of land-use estimation; (i) land-use of the entire watershed area above the monitoring site, (ii) a circular buffer around a monitoring site, and (iii) a circular buffer immediately upstream of a monitoring site, with circular buffers varying from 100m to 2km. The land-use percentage compositions in the dry and wet seasons were quantified using Landsat-8 satellite images with a maximum mapping resolution of 30m. We found that physico-chemical water quality and macroinvertebrate assemblages responded differently to land-uses at different scales in both dry and wet seasons. Nevertheless, the relationships were not always straightforward and clearly scale-dependent, suggesting that the spatial estimate used, and the spatial scale considered can strongly confound the conclusions (Chapter 2). Land-use of the entire watershed area upstream of the monitoring site better explained variation in physico-chemical water quality and macroinvertebrate indices whereas macroinvertebrate abundances showed strong links with more local land-use patterns within 100m and 2km radii. In Chapter 3, we added the main constraint that is not always included in studies of river systems i.e., connectivity and spatial autocorrelation among sites. For this, we use a spatially explicit analysis framework (spatial stream network (SSN) models) to test to what extent dendritic stream network structure affects spatial patterns of benthic macroinvertebrates and water chemistry at the catchment scale. We showed that spatial patterns and spatial autocorrelation exist in stream water chemistry and macroinvertebrate indices at both fine- and broad- spatial scale comprising both flow-connected and flow-unconnected spatial relationships. And that SSN models managed to make good predictions of water chemistry concentrations and macroinvertebrate indices at unsampled sites with estimates of uncertainty. The results highlight the value of SSN models and stress the need to specify spatial dependencies representing the dendritic network structure of river ecosystems. Finally, we assessed to what extent the seasonal connectivity of the river with temporary wetlands in the surrounding landscape is a crucial determinant of aquatic communities and environmental conditions in the floodplain wetlands. This was achieved by comparing environmental conditions and diversity and composition of macroinvertebrate communities from river connected pools with endorheic pools (Chapter 4). Macroinvertebrate communities from the two habitats were clearly differentiated and spatial taxon turnover was the main determinant of variation in community composition among pools. Hydrological connectivity facilitated the migration of fish to the river connected pools which structured the invertebrate community assemblages through selective predation, particularly of large prey such as large branchiopod crustaceans. Based on our dataset we identified indicator taxa for the different habitat types and found no specific fauna unique to river connected pools. Overall, our results suggest monitoring of river systems in wet and dry seasons given the fact that different selective filters limit invertebrate assemblages in both seasons. We recommend the creation of intact riparian buffer zones of at least 60 m from each side of the riverbank to help alleviate some of the observed negative effects of the land-use activities on the river systems. In addition, conservation and management schemes of temporary pools should focus on both river connected and endorheic pools to support high regional diversity. More importantly, SSN models should be used to support river basin management in the region in a rapid and cost-effective way

Macroinvertebrates

this book chapter is published in Ecology to Conservation Management ,a book called Fundamentals of Tropical Freshwater Wetlands page number 307 to 336This chapter introduces the diversity and community composition of macroinvertebrates occurring in wetlands with emphasis on the permanent and temporary wetlands in the Afrotropical region of the world. The chapter explores factors shaping the composition of macroinvertebrate communities of the permanent and temporary habitats, and the structuring role of dispersal mechanisms. The diversity and composition of macroinvertebrates is contrasted between different regions of the world and between permanent and temporary wetland types. Furthermore, the role of macroinvertebrates as indicators of habitat quality, ecosystem functions, and services provided by macroinvertebrates in wetlands is explored. Finally, the threats to macroinvertebrates in wetlands are highlighted

Seasonal variation in benthic macroinvertebrate assemblages and water quality in an Afrotropical river catchment, northeastern Tanzania

This research article published by Elsevier, 2020Population growth and economic development have resulted in increased water demands, threatening freshwater resources. In riverine ecosystems, continuous monitoring of the river quality is needed to follow up on their ecological condition in the light of water pollution and habitat degradation. However, in many parts of the world, such monitoring is lacking, and ecological indicators have not been defined. In this study, we assessed seasonal variation in benthic macroinvertebrate assemblages in a tropical river catchment in northeastern Tanzania, which currently experiencing an increase in agricultural activities. We examined the potential of in-stream environmental variables and land-use patterns to predict the river macroinvertebrate assemblages, and also identified indicator taxa linked to specific water quality conditions. Macroinvertebrate abundance, taxon richness and TARISS (Tanzania River Scoring System) score were higher in the dry season most likely due to higher surface runoff from agricultural land and poorer water quality in the wet season. In the wet season macro invertebrates seem to be limited by chlorophyll-a, oxygen and phosphorous while in the dry season, when water flow is lower, nitrogen and turbidity become important. Substrate composition was important in both seasons. Given the fact that different selective filters limit macroinvertebrate assemblages in both seasons, a complete picture of water quality can only be established by monitoring in both seasons. Riparian buffer zones may help to alleviate some of the observed negative effects of agricultural activities on the river system in the wet season while limiting irrigation return flows may increase water quality in the dry season

Fish predation affects invertebrate community structure of tropical temporary ponds, with downstream effects on phytoplankton that are obscured by pesticide pollution

this research article was published by ElsivierAquatic biota of tropical temporary ponds typically experience a wide range of stressors that can drive the structure and dynamics of natural communities. Particularly in regions with intense agricultural activity, aquatic biota may not only experience predation pressure but also stress from pesticides that inadvertently enter the ponds. We increasingly understand how these different sources of stress affect classic model taxa under controlled laboratory conditions, but how predators and pesticides may jointly affect pond invertebrate communities is still unclear, particularly for tropical systems. Here, we conducted an outdoor mesocosm experiment to study how fish predation combined with exposure to an environmentally relevant concentration of the commonly used insecticide cypermethrin (0.8 ng/L) affects the structure of invertebrate communities, and its potential effects on leaf litter decomposition and invertebrate grazing efficiency as measures of ecosystem functioning. A total of seven invertebrate taxa were recorded in the mesocosm communities. Fish predation effectively lowered the number of invertebrate taxa, with fish mesocosms being dominated by high densities of rotifers, associated with lower phytoplankton levels, but only when communities were not simultaneously exposed to cypermethrin. In contrast, cypermethrin exposure did not affect invertebrate community structure, and neither fish predation nor cypermethrin exposure affected our measures of ecosystem functioning. These findings suggest that predation by killifish can strongly affect invertebrate community structure of tropical temporary ponds, and that downstream effects on phytoplankton biomass can be mediated by exposure to cypermethrin. More broadly, we contend that a deeper understanding of (tropical) temporary pond ecology is necessary to effectively manage these increasingly polluted systems

Large branchiopod occurrence and community structure in relation to land-use types in temporary ponds of northern Tanzania

This research article was published by Springer Nature Switzerland AG., 2023Large branchiopods are a key component of the fauna of temporary ponds and play an important role in the functioning of these vulnerable ecosystems. Owing to the establishment of new settlements and agricultural expansion, temporary ponds in Tanzania are disappearing at an alarming rate whilst little is known about their diversity and ecology. We contrasted temporary ponds from a protected area with those in communal lands to detect associations between land-use types and large branchiopod community structure. Six large branchiopod species were collected, five of which have been previously reported from Southern Africa, whilst one turned out to be new to science: Streptocephalus manyarensis n.sp. Kafula and Brendonck (2023). The clam shrimp Cyzicus sp., fairy shrimps Streptocephalus lamellifer Thiele (1900) and S. bourquinii Hamer and Appleton (1993) were the most abundant and widely occurring. Variation in large branchiopod community structure was explained by the presence of Nothobranchius killifish and orthophosphate concentration. The large branchiopod community structure was different in settlement and protected areas. Our study on the occurrence and structure of large branchiopod communities in relation to land-use types serves as a base for formulation of guidelines and management tools to regulate land-use practices adjacent to temporary pond ecosystems

Where does land use matter most? Contrasting land use effects on river quality at different spatial scales

Understanding the influence of land-use activities on river quality has been a key focus of river monitoring programs worldwide. However, defining which land-use spatial scale is relevant remains elusive. In this study, therefore, we contrasted the influence of land use on river quality using three types of land-use estimators, namely circular buffers around a monitoring site, circular buffers upstream of the monitoring site and the entire watershed area upstream of the monitoring site. The land-use percentage compositions within the Usa-Kikuletwa River catchment in northeastern Tanzania were quantified using Landsat-8 satellite images with a maximum mapping resolution of 30 m. Redundancy analysis models and generalized linear models were used to evaluate the influence of land use on macroinvertebrate assemblages and physico-chemical water quality at different spatial scales in the dry and wet seasons. Overall, a substantial fraction of variation in physico-chemical water quality, macroinvertebrate taxon richness, Chao-1 and TARISS (Tanzania River Scoring System) score could be explained by land use of the entire watershed area upstream of the monitoring site in the dry and wet seasons. However, macroinvertebrate abundances showed strong links with more local land-use patterns within 100 m and 2 km radii. Circular buffers upstream of monitoring sites were more informative for macroinvertebrate assemblages than circular buffers around the monitoring sites. However, the latter did correlate well with physico-chemical water quality variables. Land-use variables correlated across spatial scales (i.e., 100 m up to 2 km radii), but not with the land use in the entire watershed area above the monitoring site. Our results indicate that physico-chemical water quality variables and macroinvertebrates may respond differently to land-uses at different scales. More importantly, our results illustrate that the choice regarding spatial land-use metrics can bias conclusions of environmental impact studies in river systems.status: publishe

Where does land use matter most? Contrasting land use effects on river quality at different spatial scales

Understanding the influence of land-use activities on river quality has been a key focus of river monitoring programs worldwide. However, defining which land-use spatial scale is relevant remains elusive. In this study, therefore, we contrasted the influence of land use on river quality using three types of land-use estimators, namely circular buffers around a monitoring site, circular buffers upstream of the monitoring site and the entire watershed area upstream of the monitoring site. The land-use percentage compositions within the Usa-Kikuletwa River catchment in northeastern Tanzania were quantified using Landsat-8 satellite images with a maximum mapping resolution of 30 m. Redundancy analysis models and generalized linear models were used to evaluate the influence of land use on macroinvertebrate assemblages and physico-chemical water quality at different spatial scales in the dry and wet seasons. Overall, a substantial fraction of variation in physico-chemical water quality, macroinvertebrate taxon richness, Chao-1 and TARISS (Tanzania River Scoring System) score could be explained by land use of the entire watershed area upstream of the monitoring site in the dry and wet seasons. However, macroinvertebrate abundances showed strong links with more local land-use patterns within 100 m and 2 km radii. Circular buffers upstream of monitoring sites were more informative for macroinvertebrate assemblages than circular buffers around the monitoring sites. However, the latter did correlate well with physico-chemical water quality variables. Land-use variables correlated across spatial scales (i.e., 100 m up to 2 km radii), but not with the land use in the entire watershed area above the monitoring site. Our results indicate that physico-chemical water quality variables and macroinvertebrates may respond differently to land-uses at different scales. More importantly, our results illustrate that the choice regarding spatial land-use metrics can bias conclusions of environmental impact studies in river system

Keep your natural enemies close – native predators can maintain low mosquito densities in temporary ponds in a malaria expansion area in Northern Tanzania

This research article published by Elsevier, 2021Mosquitoes and the diseases they transmit are a global public health threat that affects most human populations. Mosquito abundances are strongly linked to the number of suitable larval habitats available. However, it is still not well understood how different land uses impact larval habitat quality in different parts of the world. We investigated links between land use practices and abundance of mosquito larvae in temporary ponds located in different land uses in a malaria expansion zone in Tanzania. Temporary ponds are often cited as important sources of mosquitoes but are also of substantial conservation concern being home to a unique fauna and flora. Often, they also have high socio-economic importance. Overall, encountered mosquito larval abundances were very low, both for the collected Anopheles gambiae s.l. and Culex quinquefasciatus. Although temporary ponds are important mosquito larval habitats in other parts of Africa, currently they are unlikely to be an important factor affecting the prevalence of mosquito borne diseases in the Manyara study region. Larval abundances in temporary ponds were higher in agricultural areas where more eutrophic conditions were found and minimal in residential areas. Overall, even though temporary ponds are often modified as rice paddies in the rainy season, they were in a good ecological condition and hosted a diverse assemblage of aquatic predators that likely prevented larval mosquitoes from reaching high densities. Maintaining this good condition by preventing pesticide and nutrient input as much as possible, can be important to prevent pond degradation that is likely to make these habitats more suitable for mosquitoes in the future

Fish Diversity in Relation to Littoral Habitats in Three Basins of Lake Kivu (East Africa)

In total, 28 of the 29 fish species reported from the Lake Kivu basin occur in the littoral zone of the lake, but information about their structure, occurrence, and the habitats affecting their distribution is largely lacking. The lake’s inshore area is poorly heterogenous, with rock and macrophyte habitats representing the major habitats. The lack of heterogeneity in the habitats is probably an important factor influencing species richness, abundance, and the association between the species and habitats. We evaluated the fish diversity, abundance, and habitat parameters across 14 sites representing the major habitats of the lake, using data collected between April 2018 and October 2019. We calculated Hull and Jaccard indices and applied uni- and multivariate statistical approaches to the collected data. We identified 18 fish species in the lake. In the north, 17 species were found, with high abundance in rocky sites but low abundance in sandy habitats. In the south, 15 species were reported. We identified 12 species in the Ishungu Basin where the site with rock substrate had high abundance and 13 species in the Bukavu Basin with weak species richness and low abundance across degraded sites. Lamprichthys tanganicanus, a non-native species, was abundant in rocky (north) and macrophyte sites (south). Sand, rock, conductivity, depth, dissolved oxygen, and water transparency were significant parameters that could explain the fish distribution in the north while dissolved oxygen, vegetation cover, depth, and conductivity were significant in the south. A monitoring programme of the fish fauna in Lake Kivu is needed