Scuba researchers from the University of Alaska Fairbanks are studying the links between killer whale feeding habits and decreasing density of kelp forests around Alaska’s Aleutian Islands.
Dense kelp forests play an important role in coastal areas as a nursery for fish and invertebrates, and they protect shorelines from coastal erosion. One of the main grazers of kelp is the sea urchin, which can decimate kelp forests and turn them into rocky urchin barrens.
Researchers spent two weeks in June 2016 onboard the R/V Oceanus mapping out underwater community diversity around the Aleutian Islands to better understand the recent decline in kelp beds.
“It’s a trickle-down effect,” said Brenda Konar, a professor at the UAF School of Fisheries and Ocean Sciences. Sea otters eat urchins, and killer whales eat marine mammals. Konar explains that if killer whales don’t have the large marine mammals they normally eat, they will eat more sea otters. With fewer predators, sea urchin populations explode, and they eat all the kelp.
‘We wanted to look at how this trickle-down effect, from killer whales to the kelp, has impacted productivity and diversity across the Aleutian Islands,” Konar said.
SFOS graduate student Jacob Metzger is working with Konar to understand how biodiversity varies between kelp forests, urchin barrens and transition beds, the intermediate habitat between a kelp forest and an urchin barren. The research is funded by the National Science Foundation.
Some of this biological variability stems from how resistant certain kelp species are to urchin predation. Metzger explains that certain canopy kelps are more resistant to urchins because of their body structure.
“Eualaria, one species of canopy kelp, have large fleshy sporophylls or reproductive structures at the bottom of the plant,” said Metzger. “If urchins start crawling up the Eualaria, the movement of the sporophylls in the water can eject the urchins.” These plants tend to be found in transition beds because of their increased ability to resist urchins.
Similarly, coralline algae called Clathromorphum are abundant in urchin barrens. Their sturdy calcium carbonate exterior helps them withstand urchin grazing, and serves as a habitat to many invertebrate species.
In June 2016, the researchers sampled six islands between Tanaga and Dutch Harbor. To study these varying habitats, the researchers collected species samples in half-meter by half-meter quadrats, and measured species diversity over 10-meter transect lines. When possible, they measured two kelp bed habitats, two urchin barrens and two transition zones at each island.
Some of the selected study sites have been studied since the 1970s. By contributing to this long-term record, the researchers can begin to understand how regions are changing, and which areas are transitioning most quickly from kelp beds to urchin barrens. Konar explains that in some places, up to 80 or 90 percent of the ecosystem has become urchin barrens.
Other regions, like islands east of Samalga Pass, still pose biological mysteries. The researchers explain that dive sites east of Samalga Pass were filled with kelp forests and no urchin barrens, while west of Samalga Pass they found urchin barrens, but no kelp forests. More research is necessary to better understand why this is the case, although this region is known to be a biogeographic break for some species.
In summer 2017, the researchers will conduct scuba surveys on the western side of the Aleutian chain, between Adak and Attu.