As disturbing as the recent numbers on declining print circulation and plunging advertising revenue at newspapers have been, less attention has been paid to ominous signs of a slow-down on the online side as well:

– Most newspaper chains reported online revenue growth in single or low double digits this quarter, compared with growth rates of 15-20% or more a year ago.

– The amount of time the average visitor spent at most newspaper web sites declined in February compared with a year ago, according to an Editor & Publisher report on Nielsen Online data. E&P reported similar data for January.

The stagnant economy and the tailspin in the housing market no doubt contributed to the deflated online revenue numbers. And there’s the statistical reality that sustaining high percentage growth rates becomes more difficult as the base number gets larger.

But that doesn’t account for all the sluggishness in online revenue, and newspapers appear to be losing ground to other web sites that advertisers prefer. And if growth in online revenue is slowing, at best that means stretching out any projections about when online advertising at newspapers will finally make up for the decline in print ads.

The drop in average time spent at newspaper web sites may be the result of success on another front: newspapers are expanding their overall online audience, bringing in new visitors who aren’t staying at a newspaper’s website as long as regular readers.

But can newspapers effectively sell advertising if many readers just dive bomb in and skim a story? Will advertisers, especially local ones, be interested in such readers, many of whom may be arriving from afar via a search engine or news aggregator?

Which leaves me with several questions:

– Are there national advertising networks that are proving effective in selling ads for only casual readers of newspaper web sites?

– What newspapers are effectively selling ads to local businesses and still showing robust growth in online revenue?

– Or is there some other explanation for the dip in time spent online at newspapers and the tapering off of online revenue growth that yields some light somewhere in this tunnel?