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Discerning ecosystem change and food web dynamics underlying anthropogenic eutrophication and the introduction of non-native species is necessary for ensuring the long-term sustainability of fisheries and lake biodiversity. Previous studies of eutrophication in Lake Victoria, eastern Africa, have focused on the loss of endemic fish biodiversity over the past several decades, but changes in the plankton communities over this same time remain unclear. To fill this gap, we examined sediment cores from a eutrophic embayment, Mwanza Gulf, to determine the timing and magnitude of changes in the phytoplankton and zooplankton assemblages over the past century. Biogeochemical proxies indicate nutrient enrichment began around ~ 1920 CE and led to rapid increases in primary production, and our analysis of photosynthetic pigments revealed three zones: pre-eutrophication (prior to 1920 CE), onset of eutrophication with increases in all pigments (1920–1990 CE), and sustained eutrophication with cyanobacterial dominance (1990 CE–present). Cladoceran remains indicate an abrupt decline in biomass in ~ 1960 CE, in response to the cumulative effects of eutrophication and lake-level rise, preceding the collapse of haplochromine cichlids in the 1980s. Alona and Chydorus, typically benthic littoral taxa, have remained at relatively low abundances since the 1960s, whereas the abundance of Bosmina, typically a planktonic taxon, increased in the 1990s concurrently with the biomass recovery of haplochromine cichlid fishes. Overall, our results demonstrate substantial changes over the past century in the biomass structure and taxonomic composition of Mwanza Gulf phytoplankton and zooplankton communities, providing a historical food web perspective that can help understand the recent changes and inform future resource management decisions in the Lake Victoria ecosystem. [ABSTRACT FROM AUTHOR]

The Lake Victoria ecosystem is emblematic of the catastrophic effects that human activities, particularly cultural eutrophication, can have on freshwater biodiversity. However, little is known about the long-term spatial and temporal pattern of aquatic primary paleo-production (PP aq) and producer communities in Lake Victoria and how these patterns relate to past climate variability, landscape evolution, lake hydrology, mixing regimes, nutrient cycling, and biodiversity dynamics in the past 17 kyr. We use sediments from four well-dated cores along a transect from offshore to nearshore sites, and exploit XRF element scanning and hyperspectral imaging data, TC, TN, bSi, δ13C and δ15N, and sedimentary pigments to investigate paleolimnological variability and change. Our findings demonstrate that changes in PP aq and algal communities during the past 17 kyr were closely related to hydroclimatic changes, lake mixing, and nutrient availability. During the wetland phase (16.7–14.5 cal ka BP), PP aq levels remained generally low, while chromophytes and chlorophytes dominated the algal community. Following the rapid lake level rise (∼14.2 cal ka BP) during the early African Humid Period (AHP), PP aq levels steadily increased, accompanied by a shift towards cyanobacteria and chromophytes. During the Holocene, our results suggest repeated short-lived arid intervals (∼10.5, ∼9, 7.8–7.2, ∼4, and 3.2–3.0 cal ka BP) and two distinct periods of enhanced lake mixing associated with high PP aq and high diatom productivity: the first one between 11 and 9 cal ka BP, which coincided with the maximum of the AHP (high precipitation, high wind, enhanced mixing), and the second, less pronounced one, between 7 and 4 cal ka BP. Between these two periods (i.e. 9–7 cal ka BP) we observe reduced diatom productivity, relatively low PP aq , and high C/N ratios, suggesting conditions with more stable lake stratification, likely associated with reduced wind strength, and some nutrient limitation (N and P). Finally, the drier conditions around the end of the AHP (ca. 4 cal ka BP) and during the late Holocene were associated with decreasing lake mixing and increasing dominance of cyanobacteria. Given our reconstruction of PP aq over the past 17 kyr, we conclude that the levels in the 20th century are unprecedentedly high, consistent with the massive human-mediated impact on the Lake Victoria ecosystem including biodiversity loss. • Detailed reconstruction of aquatic primary production and its underlying drivers. • Primary production is linked to hydroclimatic changes, lake mixing, and nutrient availability. • The Holocene was marked by two distinct phases of increased lake mixing and higher primary production. • K and Rb/K profiles reflect hydroclimate oscillations. [ABSTRACT FROM AUTHOR]

Speciation rates vary considerably among lineages, and our understanding of what drives the rapid succession of speciation events within young adaptive radiations remains incomplete 1-11 . The cichlid fish family provides a notable example of such variation, with many slowly speciating lineages as well as several exceptionally large and rapid radiations 12 . Here, by reconstructing a large phylogeny of all currently described cichlid species, we show that explosive speciation is solely concentrated in species flocks of several large young lakes. Increases in the speciation rate are associated with the absence of top predators; however, this does not sufficiently explain explosive speciation. Across lake radiations, we observe a positive relationship between the speciation rate and enrichment of large insertion or deletion polymorphisms. Assembly of 100 cichlid genomes within the most rapidly speciating cichlid radiation, which is found in Lake Victoria, reveals exceptional 'genomic potential'-hundreds of ancient haplotypes bear insertion or deletion polymorphisms, many of which are associated with specific ecologies and shared with ecologically similar species from other older radiations elsewhere in Africa. Network analysis reveals fundamentally non-treelike evolution through recombining old haplotypes, and the origins of ecological guilds are concentrated early in the radiation. Our results suggest that the combination of ecological opportunity, sexual selection and exceptional genomic potential is the key to understanding explosive adaptive radiation.