Schuette's opinion released Monday found that dredging is upkeep and can't be paid for with Natural Resources Trust Fund money. The account approved by voters is funded with royalties from oil and gas drillers.
Gov. Rick Snyder recently approved emergency spending to deepen harbors that could become impassable as the Great Lakes hover near historically low levels. Schuette said the move was constitutional because the money is coming from a different account.
Some lawmakers say the recreation trust fund should be used for other purposes such as maintenance.
Schuette weighed in at the request of the Snyder administration. The Republican's opinion is considered binding unless reversed by the courts.