Denise D’Ambrosia and her colleagues could barely contain their excitement as they crowded around Northwell Health chief executive Michael Dowling on Tuesday to get pictures with their cellphones.

“We consider ourselves innovators and we know Northwell Health system are innovators and we can’t wait,” said D’Ambrosia, a palliative care nurse practitioner at Peconic Bay Medical Center. “We are very happy and very proud.”

D’Ambrosia was one of about 100 donors, doctors, nurses and local leaders who gathered Tuesday at the 200-bed Riverhead hospital to witness the formal signing of an agreement between Peconic Bay and Northwell, making it the 21st hospital in the health system and anchoring Northwell’s presence on the East End.

Dowling spoke of the agreement as a marriage “but with no divorce.”

“We will go up or down together,” he said.

Andrew J. Mitchell, president and CEO of Peconic Bay Medical Center in Riverhead, left, and Michael J. Dowling, president and CEO of Northwell Health, officially sign the documents as Peconic Medical joins with Northwell Health on Tuesday, Jan. 19, 2016 at Peconic Bay Medical Center in Riverhead. Photo Credit: Randee Daddona

Peconic Bay chief executive Andrew Mitchell said the agreement signals the beginning of a “new era for health care in central and eastern Suffolk” that will make Peconic Bay, which serves about 200,000 patients and employs 1,300, a regional health care provider.

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It also seals the end of Peconic Bay’s 10-year relationship with Stony Brook University Hospital and the demise of the East End Health Alliance formed in 2008, which included Peconic Bay, Southampton Hospital and Eastern Long Island Hospital in Greenport.

The alliance began to unravel when Southampton announced in October 2012 it was allying with Stony Brook. Peconic Bay’s board of directors voted last March to join Northwell, and last July, Eastern Long Island Hospital announced it was joining with Stony Brook. The Southampton and Eastern Long Island agreements have not been finalized but are “moving forward,” a Stony Brook spokeswoman said.

The new alliance will mean changes for Peconic Bay.

Mitchell said Tuesday that Northwell filed a certificate of need on Dec. 30 with the state for construction of a 15,000 square foot cardiac catheterization lab to be built on top of the emergency department — the first on the East End, he said. They also are planning to expand the emergency department, become a state-designated trauma center, construct a 16-bed cardiac intensive care unit and build a helipad on top of the building, Mitchell said. If all goes well, the $50 million project could be completed next year, he said. And the health system is planning to expand ambulatory services at several sites on the East End.

Northwell also is providing personnel and expertise. Dr. Stanley Katz, Northwell’s senior vice president of cardiovascular services, said the health system would be adding cardiologists to Peconic Bay along with support from Southside Hospital in Bay Shore and North Shore University Hospital in Manhasset.

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Dr. John D’Angelo, executive director of Northwell’s emergency medicine service line, said three to four new emergency medicine doctors would be joining Peconic Bay by the summer. And Dr. Jason Naidich, senior vice president of imaging services for Northwell, said radiologists — especially those in specialties like pediatric radiology — would visit on site and be available via computer remotely.