EXETER, N.H. (AP) — Reducing the amount of nitrogen in the Great Bay estuary is either essential to preserving a treasured natural resource or an economic crisis in the making, according to testimony before a congressional committee Monday.

U.S. Rep. Frank Guinta, R-N.H., requested the field hearing, which was also attended by U.S. Rep. Darrell Issa, R-Calif., chairman of the House Oversight and Government Reform Committee. At issue was whether the Environmental Protection Agency has acted appropriately in setting strict limits on how much nitrogen can be discharged into the watershed from five wastewater treatment plants.

After a loss off eelgrass and a decline in the oyster population in Great Bay, the state declared the estuary ‘‘impaired’’ in 2009 and later developed a nutrient plan that called for a reduction in nitrogen.

The EPA followed by issuing draft permits to several communities calling for a limit of 3 milligrams per liter, but a coalition of five communities — Exeter, Portsmouth, Newmarket, Dover and Rochester — have argued for phasing in an 8 milligram per liter limit. They say science doesn’t support the lower limit and accused the EPA of basing its decisions on fears of being sued by environmental groups.

Rochester Mayor T.J. Jean said complying with the strict limit would cripple his city, which is still paying for a facility it built to comply with EPA improvements ordered in 1997 and now likely would abandon. At a time when the city already faces significant economic hardships, residents can’t afford to see their sewer bills double, he said.

‘‘EPA’s indifference to mandating major local expenditures every time a new permit is issued based on the flimsiest of information simply must cease,’’ he said. ‘‘We cannot afford such multimillion-dollar guesswork.’’

Dean Peschel, an environmental consultant for the Great Bay Municipal Coalition, said it would cost the five communities about $588 million to upgrade their plants to meet the proposed nitrogen limits.

Curt Spaulding, the EPA regional administrator, said the agency is aware of the costs and will work with communities on extended compliance schedules. But he also defended his agency’s handling of the permitting process and emphasized that the bay’s environmental quality is at stake.

‘‘There is still time, though probably not much time, to arrest the decline of Great Bay before it bottoms out,’’ he said.

While the coalition communities have opposed the nitrogen limits and have sued the state over water quality standards, other communities have accepted the proposed limits, including Durham and Newington.