APPLEDORE ISLAND, Maine — Attentive Seacoast beachgoers may have noticed much of the seaweed washing ashore these summer days has a marked pink tinge to it — a signal of a changing seaweed population in the southern Gulf of Maine that could have long-term impacts on fish and shellfish.

A team of University of New Hampshire researchers working on Appledore Island at the Isles of Shoals and at off-shore sites in southern York County and Seacoast New Hampshire recently published a study that reaches some unsettling conclusions. Essentially, the ocean floor in the Seacoast is seeing a marked decline in the often tall, leafy native kelp populations and an inundation of short, shrub-like invasive seaweed.

Key among those invasives is the short, red fiber-like seaweed Dasysiphnia japonica, a transplant from Japan that is taking over the ocean floor in this region — covering as much as 90 percent of some areas. It is this invasive that people are noticing on the beach.

“We were very surprised by what we saw,” said Jennifer Dijkstra, research assistant professor in the Center for Coastal and Ocean Mapping at the University of New Hampshire and the lead author of the study. “In some areas what was once a forest of tall blades of kelp with a high canopy height was now composed of bushy invasive seaweed species, which had a much shorter canopy and a very different physical form.

“In the 1970s and 1980s, there was more of a three-dimensional habitat, with kelp as the canopy, and now in some sites, it has gone to a one-story habitat.”

Dijkstra’s team compared photos of sections of the sea floor collected over 30 years in this region. They also collected individual seaweed species to determine their complexity.

The often tall, stringy-looking kelp, she said, offers protection from predators and a source of food for many juvenile species of fish, including Pollock and cod and juvenile and adult shellfish like crab and lobster. It’s also a food source for seabirds like terns and gulls. Dijkstra said studies have found kelp forests are one of the most productive ecosystems in the ocean, with high biodiversity and ecological functions.

While the shrubby invasives offer little protection from predators, the study found these introduced species actually support two to three times more meso invertebrates, the so-called insects of the ocean, than the kelp does. So the ocean appears to taketh away and to giveth, essentially.

The study focused on the shifting seaweed population only and did not concentrate on the cause. But Dijkstra said warming water temperatures do play a part in the decline of kelp.

“Kelp tissues tend to decline at warmer water temperatures,” she said, as determined in a 2015 Canadian study.

She referenced a 2013 Gulf of Maine Research Institute study reporting that between 2004 and 2012, sea surface temperatures rose as much as 0.5 degrees Fahrenheit a year.

“This is a concern in terms of the kelp population,” she said. “If water temperatures get too warm, it is possible certain populations may be stressed and decline.”

By the same token, “invasive species seem to do quite well here.” She said that could be attributed to a number of factors, warming waters being potentially one. “It’s a complex ecosystem, so I wouldn’t necessarily attribute it to one particular cause,” she said.

The study is the first of what she anticipates will be an ongoing series of studies as the team works to discern long-term trends to fish and shellfish populations.

“One of the things we’re hoping we can show is how invasive species support or don’t support fish populations, and if shellfish like lobsters and crabs are found in the invasive habitat in the same numbers as they are in kelp beds,” she said.

For instance, what are the implications of the increase in those insects of the ocean?

“We don’t know how these changes in the ecosystem will propagate through the entire chain,” Dijkstra said. “Even though there may be more creatures at the base, it’s not clear what their effects will be.”

University of New Hampshire research professor James Coyer, assistant director of academic programs at Shoals Marine Laboratory on Appledore Island, has been studying seaweed at the Isles of Shoals for more than 30 years. He said in that time, he has seen invasive species come and go. For instance, a type of seaweed called codium was introduced in 1983 and quickly became an invasive. But in the past five years, codium has declined significantly.

“Will this boom-bust happen with Dasysiphnia [the Japanese red invasive]? Difficult to predict,” Coyer said. “Now it is very common off Appledore and probably can be classified as an invasive species. A similar introduction scenario of Dasysiphnia occurred in Norway several years ago, but my Norwegian colleagues say it is now uncommon.

“I think the only certainty is that other algae species will continue to be introduced to the Gulf of Maine and that some of them will be invasive.”