Ecological regime shifts reduced burial ability of aromatic hydrocarbons by both inland and coastal waters but driven by various nutrients

Huge synthetic chemicals and hydrocarbons have been released to inland waters and oceans, composing anthropogenic dissolved organic carbon (ADOC). They complement a large budget for CO2. Burial by inland and coastal marine sediments is crucial to reduce this budget. How ecological regime shifts influence the burial ability of ADOC by inland waters and coastal oceans, and what are the differences between them remain largely unknown. We collected sediment cores from an inland lake (Lake Qianhu) and the largest coastal lagoon (Lagoon Pinqing) in China, and chose 16 polycyclic aromatic hydrocarbons (PAHs) to address these issues. Burial ability of PAHs by sediments decreased by 55.1% -98.5% in Lagoon Pinqing in the period from 1963 to 2018, and by 91.5% -99.5% in Lake Qianhu in the period from 1970 to 2018. Burial ability and its decrease rate for most PAHs in Lagoon Pinqing were larger than those in Lake Qianhu in the same period. PAHs with higher hydrophobicity were more ready to be buried by both lake sediments and lagoon sediments. Burial ability of most PAHs in Lagoon Pinqing was negative correlated with total phosphorus concentration. In contrast, burial ability of most PAHs in Lake Qianhu was negative correlated with total nitrogen concentration. Regime shifted from phytoplankton to submerged macrophytes dominance in the year of 1976, and from submerged macrophytes to phytoplankton dominance in the year of 1999 in Lagoon Pinqing, driven by nitrogen. Regime shifted from vascular plants to phytoplankton dominance in Lake Qianhu in the year of 1991, driven by phosphorus. Different aromaticity and sources of organic matter related to regime shifts were responsible for the discrepancy of burial ability for PAHs by sediments of these two waters. Our study suggests that burial ability of ADOC by inland and coastal marine sediments will be reduced if eutrophication results in ecological regime shifts, which may undermine the efforts to mitigate global warming.

Automated summary

It has been found that huge amounts of synthetic chemicals and hydrocarbons released into inland waters and oceans have a significant impact on the carbon budget, especially the anthropogenic dissolved organic carbon (ADOC). The burial ability of ADOC by sediments in inland waters and coastal oceans is affected by ecological regime shifts, with differences between these two water systems. The burial ability of polycyclic aromatic hydrocarbons (PAHs), a type of hydrocarbon, decreased significantly over the years in Lake Qianhu and Lagoon Pinqing in China. The burial ability of PAHs was found to be correlated with different factors in each water system, such as total phosphorus concentration in Lagoon Pinqing and total nitrogen concentration in Lake Qianhu. Ecological regime shifts driven by nitrogen and phosphorus were responsible for the difference in burial ability between the two water systems. The study suggests that the burial ability of ADOC by sediments may be reduced if eutrophication leads to ecological regime shifts, which could undermine efforts to mitigate global warming.