Parts of the warming Gulf of Maine have become inhospitable for kelp forests, according to new research from the Bigelow Laboratory for Ocean Sciences in East Boothbay.
Between 2001 and 2018, a team led by senior research scientist Doug Rasher combined dive surveys of kelp population and data on ocean temperature to compile the first detailed census of Maine's kelp forests in nearly 20 years.
The results were startling, Rasher said. Maine's kelp forests were devoured by a green urchin overpopulation in the 1980s and 1990s, but rebounded around the turn of the century.
"We anticipated that with the rise and fall of the sea urchin fishery and the absence of sea urchins in the ecosystem, that kelp forests should have been widespread and pretty healthy across the coast of Maine," Rasher said.
But that's not what his team found, according to the results of their research published in the journal Ecology. Kelp forests persisted off Maine's northern coast but south of Casco Bay they had almost disappeared.
"And so we were quite surprised to see that kelp forest health had declined in the northern part of our state and that kelp forests had collapsed writ large in the southern part of our coastline," Rasher added. "That was really surprising to us, that we saw such dramatic change in a relatively short period of time."
The sudden and unexpected decline in kelp forests coincided with the rapid warming in the Gulf of Maine driven by climate change caused by fossil fuel greenhouse gas pollution.
South of Casco Bay, the kelp that remains does not grow in dense forests that form critical ecosystems for young fish. Water temperatures in the spring and summer are just too hot for kelp, which depend on cold, nutrient rich conditions.
"So when water temperatures reach a particular tipping point, it just becomes too hot for adult kelps to survive," Rasher said.
Marine heat waves are particularly harmful to kelp and warmer water can also impact their reproductive ability.
In the meantime, as kelp declined it has been replaced by new species such as filamentous red algae that don't provide the same kind of beneficial habitat to other species, Rasher said.
North of Penobscot Bay, however, lush kelp forests are still thriving, according to the study. And those areas could continue seeing health kelp populations for decades, Rasher said.
In order to sustain those ecosystems will require exercising sound management of fisheries, aquaculture and conservation measures, Rasher added.
Kelp forests cover about 25% of the world's temperate coastline, and the paper could serve as something of a bellwether for what's likely to unfold elsewhere in the coming decades, Rasher said.
In Maine, the research project served as a springboard to examine the ecologic and economic consequences of kelp decline, as well as map out changes occurring in the warming Gulf of Maine.
"You know, some of the good news from this work is that we still do have kelp forests across a large portion of our state, and that there's opportunity to manage for their health and their persistence in the face of rapid warming," Rasher said.
