When you're Google, subtle shifts have a big impact. That was the case last month, when the search giant announced it would severely limit the information publishers – or anyone else – would receive on the keywords driving traffic to their websites.

Google still provides keyword data to search advertisers, but the move changed the game for organic search, leaving some publishers and advertisers in the dark.

"It's one of the most significant losses of data marketers have seen in half a decade," said Conductor CEO Seth Besmertnik, who claimed that on average half of the traffic to the search-optimization vendor's clients' sites comes through organic search.

Google didn't cut off publishers completely. They can still get information on the top-2,000 most-popular keywords, just not in real-time and only through Google's webmaster tools, which limits the way publishers can adjust content on the fly to attract more organic search traffic. The shift is having an impact on the earned-media strategies of marketers.

Soon after Google turned out the lights, Rosetta partner Jason Tabeling received a panicked call from one marketer who is a huge Google advertiser that also depends heavily on organic searches.
"His concern was, why is Google doing this?" Mr. Tabeling said.

The answer to that varies, depending on whom you ask: Google is protecting users' privacy; it's pushing sites to run ads to recoup the lost keyword data; it's preventing sites from gaming its search-ranking algorithms. All a Google spokesperson would say is that the company has been increasingly privatizing people's searches and this most recent change extends that to users who are not signed in to a Google account.

Over time, Google had been showing advertisers and publishers less search-query data. Wister Walcott, exec VP-products and platform at Marin Software, said that a few months ago, "half or 40% [of queries] were dark." Now, he said, "they're all dark."

The shift is a bit like climate change -- everyone anticipates it will have an effect, but it's not felt immediately. Further, because the shift affects everyone equally, the competitive pecking order seems to be staying intact. "Clients care, but they care less because it's happening to everybody and it's not changing their business results immediately," said Rosetta's Mr. Tabeling.

The biggest impact could be on sites like The Huffington Post and Demand Media, which create keyword-laden content to drive traffic from search. Dean Praetorius, director of trends and social media at The Huffington Post, said he's "not freaking out as much as you think" because HuffPost can look to its seven-year trove of articles and "look for similar situations and language patterns" to determine search-friendly headlines.

Demand, for its part, said it uses many different data sets, and this one hasn't been significant.
But it will make it harder to decipher newly popular terms driving people their way. Mr. Tabeling said marketers will likely examine how every aspect of a web page beyond keywords affects its search ranking.
The process of crafting content to rank highly on search results pages, or search engine optimization, "is always a little bit of a guessing game," he said.

Correction: An earlier version of this article said that Google had stopped passing organic search keyword data to site publishers. After the article was published, a Google spokesperson clarified that some keyword data is still relayed, but that site publishers have to use Google products to access that data. Ad Age regrets the error.