NOAA Fisheries researchers studying the shallowing of the seafloor around Chignik, some 250 miles southwest of Kodiak, say combined forces of volcanoes and eelgrass are the likely culprit.
According to their research, published in early March in the online Elsevier journal Science Direct, the sediment shallowing Chignik waters is coming from volcanic ash which is entrapped and stabilized by eelgrass beds
Mark Zimmermann, a biologist with the Alaska Fisheries Science Center, and led author of the study, found that the rate of shallowing in Chignik was consistent with rates of sediment accretion in eelgrass beds reported in previous studies.
Chignik lies on the Pacific Ring of Fire, surrounded by active volcanoes, and large runs of wild sockeye salmon spawn in Chignik lakes. Eelgrass beds in surrounding bays serve as nurseries where young salmon come to feed and acclimate to saltwater.
The loss of inshore habitat could have a significant, long-term impact on the local salmon run, along with populations of other fish, birds and other wildlife that come there to feed and shelter, researchers said. Their hope is that their research will help managers and communities to respond proactively to the situation at hand.
Zimmermann’s interest was piqued by another project to create fish habitat maps based on old hydrographic maps called smooth sheets. As he analyzed smooth sheets from the 1920s and 1990s, he quantified shallowing of the seafloor in five of six bays in the Chignik region.
Then Zimmermann learned from a colleague who had studied salmon runs in Chignik for decades that the upper Chignik lake, which is important to salmon reproduction, was disappearing under shifting deposits of volcanic sand and gravel.
NOAA Fisheries’ report on the research notes that Zimmermann then recruited a multidisciplinary team who ruled out other causes of shallowing, including the 1964 earthquake that shook much of the state. Then they checked for changes in the tidal range and found that it had increased between the 1920s and 1990s, which would put more water into the bays. They concluded neither land movement or tides were the cause of shallowing.
With the only notable source of sediment coming from nearby volcanoes, and other common causes of depth change eliminated, they concluded that volcanic ash deposition was the most likely cause of shallowing, Zimmermann said.
They also confirmed that eelgrass beds had thicker sediment than other areas without eelgrass beds.
This phenomenon of volcanoes and eelgrass working together to turn a large portion of the Chigik sites into land is probably widespread especially throughout the North Pacific, where volcanoes, eelgrass and salmon are common components of the ecosystem, Zimmermann said.
And he noted that the world’s biggest eelgrass bed is at Izembek Lagoon, west of Chignik on the Bering Sea side of the Alaska Peninsula. Understanding that a similar dynamic could operate there and in other areas could provide insight for marine coastal management of commercially valuable species like salmon, he said.
Eelgrass beds play a role as buffers against storm surge and also as deposition areas that prevent land-based contaminants from entering the ocean.
There is concern that in Alaska some of these eelgrass beds, which provide high quality habitat for commercially important fish during the larval and juvenile stage, and seabirds may become too shallow and cease to provide food and shelter.
Zimmermann said their study underscores the importance of long term monitoring programs and correctly using historical hydrographic surveys to understand inshore habitat change and vulnerability.