Distribution of heavy metals in Lake Erie sediments associated with cultural and climatic changes

As one of the five Great Lakes in the North America, Lake Erie is situated in a drainage basin of 99,600 km2 that has been densely populated and extensively developed over the past 200 years 1. Owing to a range of environmental changes such as land clearing, lake-level fluctuations, species invasions, and abiotic pollutants from various point and nonpoint sources, Lake Erie has changed considerably in terms of water quality and biological integrity. The recent resurgence of harmful algal blooms serves as a reminder that the future changes may exacerbate the vulnerability of this already stressed freshwater ecosystem. In order to better anticipate and mitigate the impact of the future changes, there is a need to address the response mechanisms of Lake Erie to cultural and climatic changes. Since the early work by Kemp and Thomas 2, the sediments of Lake Erie have been investigated extensively to understand the historical changes in cultural eutrophication and abiotic pollution over the past two centuries1,3,4. Despite early success from rigorous management efforts, harmful algal blooms have increased aggressively since the mid-1990s 5, highlighting a shortcoming of our understanding of how Lake Erie responds to the changing environmental conditions 6. Here we report a 250-year record of heavy metals (Cu, Pb, and Ni) from a short (39 cm) sediment core recovered from the Sandusky basin of Lake Erie, off the Vermilion coast in northern Ohio, USA. Our record shows some features similar to existing metal records from other two sites of this lake, with a unimodal rise and fall of Pb concentrations which peaked in 1960s. Lake Erie has been affected by increased nutrient loadings from land clearing and agricultural runoff since as early as 1800, at least a half century before the onset of metal pollution from various anthropogenic activities such as biomass burning, mining and smelting, coal burning, leaded-gasoline combustion, and municipal and industrial effluents. We find most of the metal pollution occurred between 1890 and 1950, an industrialization period with rapid population growth and economic develop in the region. Moreover, the ratio of Cu/Ni has increased steadily though concentrations of Cu and Ni have levelled off since 1950s. We find Cu/Ni ratios were relatively low in the Sandusky and central basins between 1960s and 1990s, corresponding to a period of decreasing occurrences of algal blooms with rising lake levels. Our results suggest a combined influence of cultural and climatic changes on the ecosystem health of the lake.