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Deoxygenation of the deep water of Lake Victoria, East Africa
Abstract
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... In the northern part of Lake Victoria, for instance, haplochromines declined drastically between 1982 and 1986, coinciding with the upsurge in L. niloticus biomass, and started recovering during the late 1990s following a reduction in the predator biomass (Fig. 6). However, the timing of haplochromine decline and resurgence is also consistent with the period of severe eutrophication and anoxia (Hecky et al. 1994) and the general improvement in water quality (higher oxygen levels, higher visibility, and weaker stratification; Sitoki et al. 2010;Marshall et al. 2013;van Rijssel et al. 2016), respectively. It is difficult to attribute the changes in haplochromines to a single cause, but the trends in Lakes Nabugabo and Kyoga, which are clearly associated with L. niloticus dynamics (Chapman et al. 2003;Ogutu-Ohwayo et al. 2013), suggest that predation played a major role in the northern portion of the lake. ...
... The emergence of these stressors at the same time has made the separation of primary and secondary drivers of changes in cichlid assemblages, especially haplochromines, a challenging task. Nutrient loading in Lake Victoria has been ongoing since the 1920s, primarily from land use changes in the catchment (Hecky 1993;Hecky et al. 1994). However, the impacts of eutrophication became manifest especially after 1980 with the onset of blue-green algal blooms, anoxia, reduced water transparency, and even fish kills (Hecky 1993;Mugidde 1993;Sitoki et al. 2010;Hecky et al. 2010;van Rijssel et al. 2016). ...
... Ogutu-Ohwayo and Hecky (1991) suggested that the development of the deep water anoxia changed habitat conditions for these detritivores and exposed them to extreme predation by L. niloticus. The development of deep water anoxia is not doubted (Hecky et al. 1994); however, the conditions may not have favoured L. niloticus, considering that the species is quite sensitive to hypoxia (Schofield and Chapman 2000). Although Wanink et al. (2001) found some L. niloticus in deep waters with dissolved oxygen (DO) concentration less than 2 mgL À1 , including areas where DO was almost zero, their numbers were comparatively lower. ...
Lakes Victoria, Kyoga, and Nabugabo (“the Lake Victoria region”) are remarkable for hosting one of the largest assemblages of cichlid fishes among the African inland lakes. Here, we review the role and severity of anthropogenic and environmental stressors on the cichlid communities in the Lake Victoria region to understand the mechanisms leading to the persistence and resurgence of some of the cichlid fishes. Our review suggests that (1) the native Oreochromis species populations primarily collapsed due to overfishing and that the introduced species and habitat change suppressed their ability to recover; (2) without primary triggers associated with change in the environment and habitat conditions, particularly eutrophication and associated anoxia and reduced water transparency, Nile perch (Lates niloticus) predation alone may not have caused the massive loss of species diversity; and (3) the resurgence of haplochromine cichlids is due to a combination of general improvement in the environment and reduction in L. niloticus abundance, with additionally possibly some rapid ecological adaptations. We conclude that environmental stressors will likely continue to shape the ecosystems in which the remaining endemic cichlid fish diversity continue to evolve, clearly involving genetic exchange between species. If water clarity can be improved again, it is possible to maintain a diverse assemblage of endemic species.
... In recognition of these urgent needs for sustained data collection on the AGL, we propose a long-term, networked and harmonized approach. In regard to the various environmental and anthropogenic threats on AGL Hecky et al., 1994Hecky et al., , 2010Kolding, 1995;Kolding et al., 2008;Lung'ayia et al., 2001;Mgaya and Mahongo, 2017;Njiru et al., 2012;Odada and Olago, 2006;Ogutu-Ohwayo et al., 1997), the proposed multi-lakes monitoring is designed to support an ecosystem approach to management. Six main themes are relevant to the harmonized multi-lake monitoring strategy: climate, limnology, fisheries, land-use, biodiversity and socio-economy/human environment ( Fig. 3 and Table 2): ...
... Bakibinga-Ibembe et al., 2011Mchau et al., 2019Witte et al., 2012;Cocquyt et al., 2021Ochumba 1987, 1990Gabagambi and Skorping, 2018, Gabagambi et al., 2020Balagizi et al., 2018bSchmid et al,, 2005;Barenbold et al., 2020. Haambiya et al., 2015Kolding, 1995;Van der Knaap, 2013;Nyamweya et al., 2020; Obiero et al., 2015 Bootsma andWest, 2001:;Bakibinga-Ibembe et al., 2011Deirmendjian et al., 2021: Hecky et al., 2010Lung'ayia et al., 2001Hecky et al., 1994 Jane et al., 2021;Njiru et al., 2012Sayer et al., 2018Mgaya and Mahongo, 2017 Obiero et al.,2020;Ogello et al., 2013;Ogutu-Ohwayo et al., 1997; van Zwieten et al., 2016 Mgaya andMahongo, 2017;Ofulla et al., 2010Owino and Ryan, 2007Kanangire et al., 2018Odada and Olago, 2006;Scheren et al., 2001Kashindye et al., 2015 Mwamburi et al., 2020 Heather-Clark and de Jong, 2007; NEMA, 2012;Verheyen et ...
To ensure the long-term sustainable use of African Great Lakes (AGL), and to better understand the functioning of these ecosystems, authorities, managers and scientists need regularly collected scientific data and information of key environmental indicators over multi-years to make informed decisions. Monitoring is regularly conducted at some sites across AGL; while at others sites, it is rare or conducted irregularly in response to sporadic funding or short-term projects/studies. Managers and scientists working on the AGL thus often lack critical long-term data to evaluate and gauge ongoing changes. Hence, we propose a multi-lake approach to harmonize data collection modalities for better understanding of regional and global environmental impacts on AGL. Climate variability has had strong impacts on all AGL in the recent past. Although these lakes have specific characteristics, their limnological cycles show many similarities. Because different anthropogenic pressures take place at the different AGL, harmonized multi-lake monitoring will provide comparable data to address the main drivers of concern (climate versus regional anthropogenic impact). To realize harmonized long-term multi-lake monitoring, the approach will need: (1) support of a wide community of researchers and managers; (2) political goodwill towards a common goal for such monitoring; and (3) sufficient capacity (e.g., institutional, financial, human and logistic resources) for its implementation. This paper presents an assessment of the state of monitoring the AGL and possible approaches to realize a long-term, multi-lake harmonized monitoring strategy. Key parameters are proposed. The support of national and regional authorities is necessary as each AGL crosses international boundaries.
... Among these was the upsurge of the introduced Nile perch during the 1980s (Pringle 2005;Goudswaard et al. 2008). At the same time, human-induced eutrophication resulted in decreased oxygen levels and an increase of water turbidity (Hecky et al. 1994;Seehausen et al. 1997a;Verschuren et al. 2002;Hecky et al. 2010). Concurrently, the abundance of macro-invertebrates such as insects, molluscs, shrimps and a small pelagic cyprinid increased (Kaufman 1992;Wanink 1999;Goudswaard et al. 2006). ...
... P < 0.001, Fig. 5). suggested that the increased gill surface is an adaptive response to the decreased dissolved oxygen levels in Lake Victoria (Hecky et al. 1994(Hecky et al. , 2010Wanink et al. 2001;Verschuren et al. 2002;van Rijssel et al. 2016). An increase of gill surface under hypoxic conditions is not uncommon in fish. ...
The haplochromine cichlids of Lake Victoria exhibit the fastest adaptive radiation of vertebrates known. Species identification of these lacustrine cichlids has been subject of debate for many years. Over the past 35 years, environmental perturbations have resulted in phenotypic change and possibly in hybridization, making species delimitation even more problematic. In this chapter, we document the effects of a changed environment on the ecomorphology of the zooplanktivore Haplochromis (Yssichromis) pyrrhocephalus over a 20-year period. We found that this species has extended its habitat to shallower waters and adjusted its diet to larger and more robust prey. Adaptive morphological responses to predation, to larger and tougher prey and to a hypoxic environment were found. Whether these morphological changes are the result of hybridization, phenotypic plasticity or evolutionary adaptation remains unclear. The morphological responses are sometimes so large that they transcend morphological species boundaries and even one of the limits of the genus Yssichromis. The extent of the morphological responses makes clear that nuptial colouration is indispensable for species distinction and the assessment of species richness. We review advanced molecular techniques that might make it possible to distinguish species genetically. These techniques may also reveal how adaptive responses might have played a role in the remarkable resurgence of these rapidly adapting cichlids.
... Up to 13 500 t of total phosphorus are deposited annually into Lake Victoria from the atmosphere, representing 55% of the total phosphorus input (Tamatamah et al. 2005). On Lake Victoria, the main source of water hyacinth was from rapid growth in sheltered bays, enhanced by relatively high nutrient levels, especially nitrogen and phosphorus (Hecky et al. 1994), that sustained its multiplication. Overland and river inflows plus atmospheric deposition contribute to the nutrient loading in Lake Victoria's shallow littoral zones (Hecky 1993; Tamatamah et al. 2005) where water hyacinth thrives (Twongo et al. 1991; Twongo 1996). ...
Water hyacinth invaded Lake Victoria in the 1980s and, by 1998, had attained peak coverage of approximately 2 000 ha in the Ugandan waters of the lake. Control interventions, especially via biological means, significantly reduced the weed’s coverage to non-nuisance levels (13.8 μg l−1, SE 1.6; SRP > 17.2 μg l−1, SE 3.2) in the water column. Control efforts, mainly the use of biological control agents on a large scale, manual and mechanical removal at strategic sites, plus a reduction in nutrient loading, should therefore target the identified hotspot bays of the lake.
... A major shift in the ecological state of the lake came from the introduction in the 1950s of the Nile perch and later that of the Nile tilapia (Marshall, 2018). The impacts of, especially, the Nile perch took two decades to be clearly realised and coincided with a second major ecological impact from nutrient enrichment (Hecky et al., 1994). In the meanwhile, fisheries governace after independence in 1963 retained the structure from the colonial period while the fishery adapted from one based largely on species of endemic haplochromines to high-value Nile perch and Nile tilapia. ...
... With the rapid increase of Nile perch, Goudswaard et al., (2008) [23] stated that it could have led to the eutrophication of the lake by preying on phytoplanktivores thereby causing a high abundance of phytoplankton in the northern parts of the lake (Hecky, 1993; [26] Verschuren et al., 2002) [65] and also in the southern part (Cornelissen et al., 2014). From the 1980s onward, several studies have indicated increased nitrogen and phosphorus in the lake coinciding with decreased transparency and dissolved oxygen levels (Mugidde, 1993; [48] Hecky et al., 1994; [25] Seehausen et al., 1997a; [60] Verschuren et al., ...
Lake Victoria has been known for its species richness with over 500 species of haplochromine cichlids being reported. This however changed with the irruption of an introduced alien species Lates niloticus that saw to the near decimation of close to half of these endemic species. The introduction of the Nile perch led to this decline and even extinction due to its high predatory pressure on the relatively small sized haplochromines. Intense predation pressure on the detritivorous and pytoplantivorous haplochromine cichlids accelerated a phytoplankton boom and eutrophication that increased turbidity thus reducing light penetration and increasing deep water anoxia, contributing further to the decline of the haplochromine cichlids. Eutrophication which is also responsible for the declining fish stocks through fish mortalities was exacerbated by increased deforestation, cultivation, and influx of nutrients through runoff from farms that increased algal blooms decimating dissolved oxygen in deep waters. Other causes leading to the decline of the haplochromine cichlids include, but not limited to, introduction of Nile tilapia (Oreochromis niloticus), development of commercial fishery, venture into agriculture and global warming. A fishery that was once dominated by hundreds of species is now dominated by only three: Rastrineobola argentea, a native pelagic cyprinoid fish commonly known as omena or dagaa; the introduced Nile perch (Lates niloticus); and the Nile tilapia (Oreochromis niloticus). In recent years, in the continued deterioration of the lake's limnological conditions, there has been a resurgence of some of the declined haplochromine cichlid stocks. These resurging cichlids have different or additional morphological features that have allowed them to flourish in existence of the Nile perch and poor water quality conditions. This renewal is owed to certain factors including: the Nile perch fishery which has gravely reduced the dominating Nile perch populations thereby reducing predation pressure, the intense implementation of management practices after the decline of the cichlids, ecological equilibrium, and development of aquaculture and adaptive response of the haplochromines to predation, eutrophication, deep water anoxia, and dietary shift.
... These anthropogenic activities degrade water quality in the rivers draining into the bay. Water quality degradation is manifested in the form of reduced fish stocks, decreased biodiversity, dense algal blooms, increased sedimentation and nutrient loadings and anoxia in the water column (Hecky, 1993;Hecky et al., 1996Hecky et al., , 2010Ochumba & Kibaara, 1989;Sitoki et al., 2012). Cascading effects on the lake ecosystem have occurred as a result of these polluting activities, including the occurrence and persistence of cyanobacterial blooms reported within every part of the bay. ...
The production of phytoplankton (algal) toxins and their control is of concern because of the need to reduce their negative impacts on water quality and facilitate effective management of algal blooms. The present study was conducted between September 2017 to May 2018, focusing on Kisumu Bay in the Kenyan portion of Lake Victoria, in order to establish the magnitude of potential impacts on phytoplankton composition and microcystin following a prolonged presence of water hyacinth coverage between 2013 and 2018 within the gulf, with an estimated coverage range varying between 644 and 1224 ha. Triplicate samples of physico‐chemical parameters, nutrients, phytoplankton, chlorophyll‐a and algal toxins (N = 88) were collected at eleven sampling sites to determine their spatio‐temporal variability. The main identified algal taxa comprised Cyanophyceae, Bacillariophyceae, Chlorophyceae, Euglenophyceae, Zygnematophyceae and Dinophyceae. The most dominant algal species were Microcystis aeruginosa (25%), Merismopedia spp. (23%) and Anabaena flos‐aquae (16%). Enzyme‐linked immunosorbent assay (ELISA) technique was used to determine microcystin (MC) toxins in the water. Mean MC‐LR and MC‐YR concentrations were significantly correlated (R2 = 0.972), exceeding WHO standards at three sampling sites (Coca Cola, 2.84 ± 4.76; Kisumu pier, 1.78 ± 1.87; Midpoint, 1.44 ± 2.71 μg/L MC–LR). There were significant temporal variations (p < .05) in the SRP, TN, NO3‐N, NO2‐N, NH4‐N, SiO2‐Si, MC‐LR, MC‐YR, dissolved oxygen (DO), total dissolved and suspended solids (TDS; TSS), turbidity, electrical conductivity, Secchi depth, temperature and pH levels. The water depth, TP and DO also vary spatially. The nitrogen to phosphorus concentration ratios differed from the expected N:P ratio of 16:1, indicating a highly eutrophic status. The disproportionate ratio of total phosphorus and total nitrogen in the bay may be responsible for the enhanced cyanobacterial blooms it exhibits. The results of the present study provide useful information and data for formulating regulations for water quality management.
The introduction of Nile perch, Lates niloticus, into Lake Victoria resulted in a massive increase in fish catches, but with significant ecological costs. These included the near extirpation of the 500+ endemic haplochromine species and the eutrophication of the lake. A proposal to introduce Nile perch into the artificial Lake Kariba (Zambia/Zimbabwe) is of concern, therefore, because of potentially adverse impacts on its fisheries and biodiversity. It is very unlikely that Nile perch would improve the fisheries in Lake Kariba, as it did in lakes Victoria and Kyoga, because this lake is oligotrophic and much less productive. The principal fishery in Lake Kariba is already in difficulty, owing to overfishing, and Nile perch could reduce catches further. The biomass of other native fish species is very low and would be unlikely to support a significant Nile perch population. Lake Kariba has already experienced a major loss of biodiversity as river‐adapted fish species have not been able to adapt to lacustrine conditions. The non‐native Nile tilapia, Oreochromis niloticus, has also replaced the endemic tilapia species in the middle Zambezi system and the Nile perch could bring about a further loss of biodiversity. The ecological costs of its introduction into Lake Kariba would not be justified by an increase in fishery productivity, and the possible introduction of Nile perch should be forcefully rejected.
Lake Victoria, a lifeline for millions of people in East Africa, is affected by anthropogenic activities resulting in eutrophication and impacting the aquatic life and water quality. Therefore, understanding the ongoing changes in the catchment is critical for its restoration. In this context, catchment and lake sediments are important archives in tracing nutrient inputs and their dominant sources to establish causality with human activities and productivity shifts. In this study, we determine the 1) changes in concentrations of total organic carbon (TOC), black carbon (BC), total nitrogen (TN), C/N ratio, and phosphorous (P) fractions in catchment sediments and the open lake, 2) distribution of diatom population in the lake, and 3) land use and land cover changes in the catchment. The distribution of TOC, BC, TN, C/N, and P correlate while showing spatial and temporal variations. In particular, the steady increase in BC confirms atmospheric inputs from anthropogenic activities in the catchment. However, lake sediments show more variations than catchment-derived sediments in geochemical trends. Notably, the catchment has undergone dramatic land use changes since the 1960s (post-independence). This change is most evident in satellite records from 1985 to 2014, which indicate accelerated human activities. For example, urban growth (666-1022%) and agricultural expansion (23-48%) increased sharply at the expense of a decline in forest cover, grassland, and woodlands in the catchment. Cities like Kisumu and Homa Bay expanded, coinciding with rapid population growth and urbanization. Consequently, nutrient inputs have increased since the 1960s, and this change corresponds with the divergence of diatom communities in the lake. In addition, the transition to Nitzschia and cyanobacteria mark increasing cultural eutrophication in the lake. The geochemical trends and statistical data support our inference(s) and provide insights into urban development and agriculture practices, which propelled increased nutrients from the catchment and productivity shifts in the lake.
When environmental conditions change, microalgae can modulate the size of their macromolecular pools and their elemental composition in order to accommodate their nutritional and energetic demands. In this context, we evaluated how O2 levels and S starvation alter the resource partitioning in terms of organic and elemental composition in Chlamydomonas reinhardtii. The analysis of gene transcription, photosynthetic energy production and organic pool allocation allowed us to identify a two-phase cellular response related to external O2 concentration: (1) a short-term response where S starvation induced the expression of S starvation genes immediately after the start of the perturbation; (2) a long-term response that consisted of a drastic reduction of photosynthetic energy production and changes in C allocation following S deprivation, which was related to oxygen conditions. In anaerobic conditions, S starvation did not affect the pools of carbohydrates and lipids, whereas the protein pool decreased slightly over time. On the other hand, in aerobic conditions, S starvation modulated all organic pools: carbohydrates, lipids and proteins. We conclude that the energy levels produced by fermentation metabolism are not sufficient to sustain the acclimation process in C. reinhardtii; moreover, S availability affects C. reinhardtii more than do oxygen levels.
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