The research team unearthed a 380-meter-deep sediment core from the lakebed that provided evidence of significant changes in water levels in the lake over the last 1.3 million years due to changes in climate and moisture levels in the atmosphere.
According to King, the East African moisture history is much more complicated than anyone previously imagined. “These dramatic shifts in effective moisture resulted in large-scale changes in one of the most diverse freshwater ecosystems on Earth,” said King, a professor at the URI Graduate School of Oceanography. “This extreme variability had a profound influence on the evolution of the fish that live there.”
The research was published last week in the Proceedings of the National Academies of Science.
The sediment core collected from Lake Malawi showed evidence of 24 instances in its history that the lake level dropped by more than 200 meters, including 15 times when water levels were 400 meters lower than the present depth of about 700 meters. During these times, a significant portion of the lake became grassland or desert from the drier climate. At other times, the lake became overfilled, indicating a wetter climate.
“The changing water levels promoted the evolution of the cichlids as the habitat changed and groups of the fish became isolated from one another,” King said. “There were even times when there were two separate water bodies, which further enabled the fish to evolve differently.”
To determine the changing water levels in the lake, the researchers examined the sediment core for indications that the lake was deep enough to stratify into an upper, oxygenated layer and a lower, oxygen-deprived zone, which is typical of deep lakes. They then used radiocarbon dating of the top levels of the core to determine its age. For deeper layers they found layers of ash from known dates of eruptions of a local volcano—or in one case, shards of microscopic glass spewed from a volcanic eruption in Indonesia. The deepest layers of the core were assigned dates by comparing known changes in the Earth’s magnetic field to the observed magnetic field in samples of the core.
In addition to changes in the depth of the lake, the lake’s shoreline also had a varied history. King said that seismic data of the current lake bottom shows that different depths are more rocky, sandy or a mix of sand and mud. As the water level changed, these different areas became exposed. Because cichlids tend to live in the upper parts of the lake and often near its edges, changing shorelines led to repeatedly changing environments for Lake Malawi’s fish. This, in turn, provided additional pressure on the fish to adapt to local conditions—a key component in the development of new species.