Multiple stressors cause rapid ecosystem change in Lake Victoria

Abstract

SUMMARY1. Lake Victoria endured multiple stresses over the past century including population growth, increased cultivation of land, meteorological variability, resource extraction, intensive fishing, introduction of exotic species and more recently climate warming. These stressors became manifest through a fundamental and rapid change in the fish community and fishery in the early 1980s and visible eutrophication. However, the relation of these two phenomena and the possible interaction of the multiple stressors have been difficult to establish because of the temporally fragmented nature of the environmental data. 2. Comprehensive limnological observations from the 1960s were repeated in the 1990s and established the eutrophication of the lake, but these do not provide insight to the time course of when changes in trophic state occurred. Comprehensive fishery catch data from 1965 to the present provide a time course of the change in community composition and yield but cannot be correlated in time with discontinuous and sparse limnological data to determine possible cause-effect relationships. 3. Palaeolimnologic studies were conducted on three cores, two offshore and one nearshore, to establish a time course for the eutrophication of the lake that can be related to time-based data on the fishery. In the 1920s, the cores recorded an increase in nitrogen content of the sediments, but there was no significant response in the paleo-productivity indicators of biogenic Si deposition and change d 13 C of deposited organic matter.Phosphorus deposition began to increase in the 1940s in all three cores after which biogenic Si deposition increased steadily over time. Responses in d 13 C of organic matter begin in the 1960s at the coring sites. In the 1970s, the d 13 C of organic matter at the nearshore site increased nearly 3& in a 10-year period likely as a response to a dramatic increase in internal P loading caused by spreading anoxia. 4. Nile perch, the large predatory fish introduced in 1954, had become established through much of the lake at low abundances by the 1970s. In 1980, the catch of this fish began to increase, and by the end of the decade, the Lake Victoria fishery was the largest lake fishery in the world; and Nile perch dominated the catch. While catches of some other fishes also increased, the endemic haplochromines suffered a catastrophic decline in abundance and loss of biodiversity. 5. The detailed chronostratigraphies for these sediment cores established that the major changes in the trophic condition of the lake were accomplished prior to the change in the fish community and that the increased primary productivity of the lake likely contributed to the increased fish catches after 1980. The increased algal abundance also would have greatly reduced visibility and facilitated the emergence of Nile perch as the dominant top predator. 6. Thematic implications: multiple stresses were present in Lake Victoria over several decades, but transition to a new ecosystem state with a transformed food web and high...

Changing ecology of Lake Victoria cichlids and their environment: evidence from C13 and N15 analyses

“…Based on paleolimnological analyses, changes in lower food web organisms began as early as the 1940s but accelerated dramatically through the 1960s and 1970s (Verschuren et al, 2002;Hecky et al, 2010). From the 1980s onwards, several studies have shown increased nitrogen and phosphorus loadings in the lake coinciding with decreased water transparency and oxygen levels (Mugidde, 1993;Hecky et al, 1994Hecky et al, , 2010Seehausen et al, 1997a;Verschuren et al, 2002). The eutrophication is thought to be caused by agricultural malpractices, urbanization and deforestation, although recent studies have suggested that climatic variability and wind stress played a crucial role as well (Kolding et al, 2008;Hecky et al, 2010;van Rijssel, 2014).…”

“…Based on paleolimnological analyses, changes in lower food web organisms began as early as the 1940s but accelerated dramatically through the 1960s and 1970s (Verschuren et al, 2002;Hecky et al, 2010). From the 1980s onwards, several studies have shown increased nitrogen and phosphorus loadings in the lake coinciding with decreased water transparency and oxygen levels (Mugidde, 1993;Hecky et al, 1994Hecky et al, , 2010Seehausen et al, 1997a;Verschuren et al, 2002).…”

“…Lake Victoria has suffered from severe eutrophication in the past decades, and the shallow, inshore habitats especially have high algal biomasses and a high carbon demand by photosynthesis (Ramlal et al, 2001;Hecky et al, 2010). Based on paleolimnological analyses, changes in lower food web organisms began as early as the 1940s but accelerated dramatically through the 1960s and 1970s (Verschuren et al, 2002;Hecky et al, 2010).…”

“…From the 1980s onwards, several studies have shown increased nitrogen and phosphorus loadings in the lake coinciding with decreased water transparency and oxygen levels (Mugidde, 1993;Hecky et al, 1994Hecky et al, , 2010Seehausen et al, 1997a;Verschuren et al, 2002). The eutrophication is thought to be caused by agricultural malpractices, urbanization and deforestation, although recent studies have suggested that climatic variability and wind stress played a crucial role as well (Kolding et al, 2008;Hecky et al, 2010;van Rijssel, 2014). Most of the studies reporting eutrophication focussed on the northern part of the lake (Hecky, 1993;Mugidde, 1993;Mugidde et al, 2003) and because of the lack of regular and consistent measurements of biological productivity, paleolimnological analysis was used to provide more continuous analysis of historical changes in the ecosystem.…”

Belowground Influence of Rhizobium Inoculant and Water Hyacinth Composts on Yellow Bean Infested by Aphis fabae and Colletotrichum lindemuthianum under Field Conditions

“…Beans complement the shortage of animal protein in East Africa, especially in the prevailing situation whereby fish production has been greatly impeded by the disastrous spread of water hyacinth in Lake Victoria (Ntiba et al, 2001;Hecky et al, 2010). The leguminous crop is also very important in agro-ecosystems, as it symbiotically fixes nitrogen through endophytic Rhizobium species (Bala et al, 2011;Devi et al, 2013).…”