Refugee arrivals to the Bay Area and communities across the nation are plunging under the Trump administration, even as courts block the president’s effort to halt refugee resettlement, researchers said Thursday. Nationwide, 3,316 refugees were admitted in April, down two-thirds from October, when 9,945 people were admitted, according to a Pew Research Center analysis of government data. Only four states saw an uptick in resettlements. In California, 338 refugees arrived in April, down 58 percent from October’s total of 814. The dip — which has alarmed advocates for refugees and prompted questions from some lawmakers — has been particularly striking in Alameda County, data show. One of a handful of places in California that hosts a significant number of refugees, Alameda County typically welcomes people from Iran, Iraq, Eritrea and other countries plagued by war or human rights abuses. But while 70 refugees arrived between October and January, only nine came in February and March, according to the state Department of Social Services. “It’s a shame, because people who are eligible for refugee status are the most highly vetted (for security threats) to come into the U.S. compared to other ways of entry,” said Amy Weiss, director of refugee and…