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WASHINGTON — The Interior Department’s internal watchdog has opened an investigation into ethics complaints against the agency’s newly installed secretary, David Bernhardt.

Mr. Bernhardt, a former lobbyist for the oil and agribusiness industries, was confirmed by the Senate last week to head the agency, which oversees the nation’s 500 million acres of public land and vast coastal waters. He has played a central role in writing policies designed to advance President Trump’s policy of “energy dominance” and expanding fossil fuel exploration. He has been dogged by allegations of ethics violations since joining the Trump administration as the Interior Department’s deputy secretary in 2017.

Eight senators, all Democrats, and four government ethics watchdog groups have requested that the Interior Department’s inspector general open formal investigations into various aspects of Mr. Bernhardt’s conduct. Among the chief complaints have been allegations, revealed by three separate New York Times investigations, that Mr. Bernhardt used his position to advance a policy pushed by his former lobbying client; that he continued working as a lobbyist after filing legal paperwork declaring that he had ceased lobbying; and that he intervened to block the release of a scientific report showing the harmful effects of a chemical pesticide on certain endangered species.

In a letter sent Monday to the senators who filed the ethics complaints, Mary L. Kendall, the deputy inspector general of the Interior Department, wrote that she had received seven complaints from “a wide assortment of complainants alleging various conflicts of interest and other violations” by Mr. Bernhardt, adding that she had “opened an investigation to address them.”