Taking a stroll through the underwater woods

Sprawling underwater forests of macroalgae offer a habitat for a variety of fishes, crabs and worms. But climate change is now impacting these algae. Through field observations and laboratory tests, researchers at the Alfred Wegener Institute are investigating these seemingly simple organisms – and comparing what they see in the North Sea with their findings in the polar regions.

The narrow, yellow-brown seaweed blade that Dr. Inka Bartsch has pinned up on the wall of her laboratory in Bremerhaven is roughly two metres long. In the past, the ocean washed up masses of sugar kelp from the nearby rocky shores on the beaches of Helgoland, where Inka Bartsch often conducts research. But those days are gone. As the AWI biologist relates, “The populations of these and other brown algae have changed across Europe. Off the coast of Helgoland, the sugar kelp has shifted to deeper waters, while researchers in southern Norway found much less kelp after two consecutive warm summers. Thankfully, the populations have since bounced back to some extent.”

The long and leathery blades of brown algal seaweeds, also referred to as Laminaria, form entire underwater forests. These areas, fondly known as “kelp forests”, offer crabs, worms and hosts of fish a living room, safe haven, nursery and feeding ground. In addition, smaller algae attach themselves to the large plants. Brown algae and other macroalgae grow on sunny rocky coasts around the globe; the largest can be found on North America’s Pacific Coast and many coasts in the Southern Hemisphere.

Impressions of a kelp forest

But as our climate continues to change, the living conditions in many places are changing with it. Inka Bartsch has been researching the “giants” among the algae for years and is still fascinated: “Algae look so simple. Yet their reactions are highly complex, and in some cases very sensitive, when their habitat begins to change,” she explains. For example, if the water temperature remains above 20 degrees for an extended length of time, it’s a death sentence for many types of brown algae.

The depths at which macroalgae can be found depends on how clear the water is – in other words, on how much sunlight makes its way to the seafloor. Since the coastal waters off Helgoland have to date been comparatively turbid, around 1970 brown macroalgae only grew there at depths of up to eight metres. But changing water currents are now making Helgoland’s waters clearer. As a result, some algae are now expanding into several-metre-deeper waters, effectively enlarging their habitat.

However, the North Sea has also grown warmer: by an average of 1.7 degrees Celsius from 1962 to the present. This change has likely been hard on oarweed. Inka Bartsch has determined that it reproduces much more poorly at summertime temperatures above 16 degrees; at 20 degrees it can’t reproduce at all, and begins dying out when exposed to prolonged water temperatures above 20 degrees.

Hurricane Kyrill likely swept everything away

Higher temperatures also often translate into more frequent extreme weather events. When the entire covering of the brown algae Fucus in the intertidal zone was “swept away” in January 2007, most likely by the cyclone Kyrill, it took three years to recover. According to Bartsch, “If we start seeing storms over the North Sea more often due to climate change, some species might not have enough time to bounce back.”

This became clear to Bartsch through her years of conducting research, supported by a collaboration with the state of Schleswig-Holstein’s Ministry of Agriculture, the Environment and Rural Areas. Every summer and winter they work together to precisely record which types of algae – and how many of each – grow in small 100-square-meter patch of the intertidal zone, which provides them with information on the seasonal, annual and longer-term fluctuations.

How much change in their environment can macroalgae tolerate? That’s a question Inka Bartsch is currently working to answer. To do so, she subjects the algae to a broad range of conditions. “My goal is to determine how well algae can withstand rising water temperatures, and to what extent sunlight and drifting suspended particles influence their reproduction and growth.”

In this regard, lab work is just as important as field observations. And the research that Bartsch is conducting in Helgoland has also led her to the Arctic – an opportunity that only few researchers share. “We’re currently monitoring how the ecosystems in these very different regions are responding to climate change, and comparing those responses.”

So, are the macroalgae in Kongsfjord on the coast of Spitsbergen reacting the same as those in Helgoland? “Even though the kelp forests at both sites have similar ecosystems – at least on the surface – they are responding to warmer waters in very different ways,” claims Bartsch. In Helgoland it is already so warm at the southern border of kelp coverage that a summertime temperature increase could threaten their survival. Yet, as Bartsch relates, “In the Arctic, the summertime warming of the otherwise extremely cold water actually improves algae growth. In addition, thanks to the warming, lighting conditions are developing very differently in Kongsfjord than in the North Sea.”

More light in the spring, less in the summer

Because the air temperature is rising, Arctic glaciers are melting more quickly. “Because of this meltwater, more sediments are finding their way into the Arctic Ocean, where they cloud the water,” says Bartsch. However, at the same time a continuous sheet of ice no longer covers the water’s surface every year. “That means more sunlight penetrates the topmost water layer in the spring, but most likely less makes it through in the summer.” As a result, since the first monitoring period (between 1996 and 1998) the kelp’s growth rate has increased many times over, but its depth range has declined.

Coming back to the North Sea, rising summertime temperatures are endangering the algae. At the same time, the algae’s habitat has now grown, because they are now expanding into deeper waters where more light gets through. According to Bartsch: “The comparison with the Arctic clearly shows how, in the wake of global climate change, the factors water temperature and light are changing and interacting differently in different places, producing a diverse range of effects on local ecosystems.”