BAA figures show Stansted Airport handled 1.55M passengers last month, down 6.3% on the same month last year.
For the year to date, passenger numbers at Stansted show an increase of 1% over last year. Growth at the airport had been at 15% to 20% in recent years.
SSE campaign director Carol Barbone said: "We take no comfort from the present uncertainty at Stansted. We have always acknowledged that the airport is an important local employer but there are risks in over-expansion, not least the risk of the local economy becoming over-dependent on a single employer.
"This is a vibrant and diverse local economy and we should keep it that way. Nobody knows what the future might hold and so it makes sense to avoid having too many eggs in one basket."
SEE say that the slowdown is due to Ryanair withdrawing ten aircraft from service at Stansted, while Air Berlin, Stansted's third biggest customer, has also cut back on its Stansted operations.
A spokesperson from BAA said that the drop was part of its forward planning, and that number would be "very high" in the summer.
BAA also confirmed that some job losses would occur at Stansted, but new posts would also be created.