Each year, oceans absorb about 25 percent of the carbon dioxide released into the atmosphere, and this absorbed CO2 fundamentally changes seawater chemistry by creating a more acidic marine environment. The consequences of this acidification are felt—often significantly—by many marine organisms, particularly those whose shells and skeletons are weakened by dissolved carbonic acid.
In an effort to quantify the impacts of ocean acidification (OA), University of Maine professor Robert Steneck and Postdoctoral Research Associate Douglas Rasher are looking to a species of calcareous algae in Alaska’s Aleutian archipelago and the local food web it supports. Due to ocean acidification, this alga is threatened by extinction, which may have devastating effects on the local ecosystem.
“These microhabitats are a critical component of the ecosystem, so understanding how climate change is altering the strength of the algal bioherms, which form the basis of the Aleutian food web, will shed light on the fragility and resilience of this ecosystem, as well as what we might expect for other marine ecosystems,” says Rasher.
Current studies on ocean acidification tend to forecast how increased carbon dioxide in the atmosphere will change future ecosystems and climate. Steneck and Rasher believe that increase is already affecting the oceans, especially at high latitudes, and this research aims to provide a baseline of the impact of acidification.
“Cold water absorbs carbon dioxide more readily, so cold-water ecosystems are more susceptible to ocean acidification,” says Steneck. “We are looking at the alga both in and out of kelp forests to piece together growth rates and density over time. This will help provide a measure of ocean temperature and acidity changes.”
Food Web Foundation
The slow-growing red algae can live up to 2,000 years. They incorporate calcareous material into their cell structure, which makes them hard and rock-like. As they grow, they create massive bioherms – encrusting, coral-like substrate. Dense kelp forests find foothold on these bioherms and many varieties of mollusks, fish, and sea urchins use them as food and habitat.
“The algae forms the basis of the food web in the Aleutians,” explains Steneck. “Each algal cell is surrounded by a microscopic shoe box made of calcareous material. Together, these shoe boxes impart structural integrity, which allows them to exist and persist in a world of grazing herbivores like sea urchins. With ocean acidification, the density of their calcareous shells breaks down and makes them more susceptible. They may be excavated by urchins into extinction and then the ecosystem will fall apart.”
Aleutian Island Fieldwork
During a three-week cruise aboard the RV Point Sur in summer 2014, Steneck’s team of ten, including Rasher and Estes, sailed in Alaska’s western Aleutian Islands. The group worked across seven islands between Adak, in the middle of the island chain, and Attu, at the chain’s western end almost to Russia – a distance of about 700 km. At 20 sites they did nine dives, breaking into teams to study both forested and deforested areas of algal bioherms.
The dive sites were all about 10 meters deep and had roughly the same amount of light reaching the bioherms, says Rasher. Then they surveyed the ecosystem and collected algae both in and out of the kelp forest.
“In a healthy forested area, there is less light,” says Rasher. “We want to know if kelp forests decrease acidity through photosynthesis. We expect to see a record of lower skeletal densities and higher growth rates among algae collected outside the forest.”
At each dive site a reconnaissance team of two or three divers first scouted the area to ensure the site contained both forested and deforested areas. Next, another team performed a habitat survey by taking a number of photographs for shipboard image analysis, then quantifying seaweed and fish species to measure site biodiversity. They also counted and collected urchins at each site, up to 200 specimens per location, which were brought back to the ship for biological analyses. Autonomous pH sensors inside and outside forested areas measured water acidity for several days and water samples were taken.
Into the Deep
Steneck and Rasher mapped the extent of each algal bioherm, and then collected specimens for ab analysis.
“I dove with two [air] tanks, one for breathing and one for the pneumatic chisel. Most of the time we used a hammer for hand-sized samples. We used the pneumatic drill more in the western sites where bioherms were larger. Once we dislodged the larger corallites, we rolled them into a cargo net and fitted them with air lift sacks to float them to the surface. Then, a zodiac towed them back to the ship where they were hoisted on deck by crane,” Steneck says.
Triage of Specimens
Once on deck, Rasher says the scene was a “triage of specimens.” First, the best five samples were chosen from each site. Next, using tile saws, the team sliced samples into thin slabs and polished. This enhances growth bands, allowing the team to accurately determine the age of each specimen by counting growth bands.
“We work on them the way geologists would; we mark important visual horizons, so if it falls apart then we can still tell where we are in time,” says Steneck. “Like tree rings, the algae create a layer of skeleton each year, roughly 300 micrometers thick. These growth bands give us the algae’s age and growth rate. Typically those that grow outside of kelp forests grow faster because there is more light. They have thicker crusts. Hand-sized samples usually represent decades to centuries. There are not a lot of giants – they are a real rarity, but they give us a few longer term records – up to a couple thousand years.”
Slabs were archived, catalogued and dried in an oven. Polished slabs were scanned so that growth bands could be counted more precisely using special software. About 300 lbs of samples were crated and shipped back to the lab for further analyses.
Understanding the Data
Analysis of Uranium and Thorium, which occurs naturally in the skeletons, will provide detailed dating. A scanning electron microscope will yield a detailed digital record of each specimen and help assess growth rates more precisely. A method called micro CT, basically a CAT scan, will be used to determine how the skeletal density of the coralline alga has changed through time. Raman spectroscopy provides another test of skeletal weakening on the molecular level. Lastly, a method called microindentation, wherein pressure is applied to growth bands until the material breaks, provides another measure of skeletal strength.
“Ultimately, high-tech lab methods will help us track changes in the algae’s calcification since the industrial revolution and, hopefully, for a significant amount of time before that,” says Rasher.
The scientists will correlate their findings with climate change records to determine causes of change, a process that should allow them to gauge “whether the alga has recently become susceptible to urchin predation as a result of climate change, the loss of sea otters, or both,” says Rasher. —Marcy Davis