If it you can’t stand the heat, get out of the kelp forest!

Simonson, E. J., Scheibling, R. E., & Metaxas, A. (2015). Kelp in hot water: I. Warming seawater temperature induces weakening and loss of kelp tissue. Mar Ecol Prog Ser, 537, 89-104. doi:10.3354/meps11438

Kelp is a kind of large seaweed that grows in dense forests in shallow, coastal waters (fig. 1). Beach go-ers may be familiar with the wads of it that wash ashore after big storms. Aside from being fun for children to play with, kelp serves an important environmental function. Kelp beds and forests form vital habitat that support many organisms and ecosystems. These communities that live around kelp are characterized by high levels of productivity and species diversity.

Kelp is a finicky alga that is most productive and robust under particular environmental circumstances. It is especially sensitive to water temperature. Oceanographers are concerned that the global distribution of kelp will change dramatically as the temperature of the ocean rises in response to climate change. As the temperature of the water goes up, the amount and spatial extent of the kelp goes down. These changes could result in dramatic habitat loss leading to reduced ecosystem productivity and the extinction of many invertebrate species.

Such habitat degradation phenomena have already been observed in kelp forests in the Pacific and eastern Atlantic oceans. Fortunately, kelp loss has not yet been observed in the western Atlantic. But these waters are considered an active warming area. Water temperatures in the northwest Atlantic have increased about 1^o C (1.8^o F) in the past three decades. Climate scientists predict that the upward trend will accelerate, with waters warming an additional 4^o C by 2100.

Erika Simonson and her colleagues at Dalhousie University in Nova Scotia set out to determine how these changing temperatures might affect the local kelp species. The team designed an experiment to test the responses of the three dominant types of local kelp, Saccharina latissima, Laminaria digitata and Agarum clathratum, to warming water. They collected samples from the ocean, placed them in four identical tanks in the lab, and grew them under identical conditions for three weeks. They only varied the water temperature between the four tanks: 11, 14, 18, and 21^oC. These levels correspond to the growth optimum for two of the species, the average summer temperature, the current maximum summer temperature, and the predicted maximum temperature in 2100.

To assess how the kelp was doing, Simonson took weekly measurements the kelps’ growth, tissues loss, and tensile strength. To monitor the kelps’ growth rates the team punched a hole in a leaf near where it connected to the stem and noted the change in distance between the hole and stem. To record tissue loss, Simonson measured the length of the leaves before putting the kelp in the lab. She then kept track of the change in length of the leaves. Lastly, the tensile strength was measured by loading a small piece of kelp with weight. Using a camera and force meter, the group was able to measure how much the tissue stretched and precisely how much weight was required to break it (fig. 2). Simonson also monitored the mortality of the plants and examined tissue samples under a microscope.

All of the plants survived and thrived in the tanks with temperatures simulating contemporary waters. Much of the kelp in the 18^oC tank died before the end of the three-week experiment. None of the samples in the 21^oC aquarium survived to the two-week mark. Simonson and her colleagues suggest that this trend in mortality is linked to tissue loss. They observed that the plant tissue degraded more quickly as the temperature increased.

The tensile tests demonstrated that two of the three species were notably weaker after a week in the high temperature tank. The material properties of the third species, A. clathratum, appeared unaffected over the same time span. Indeed, examination of the tissues under a microscope revealed that the cells of A. clathratum remained mostly undamaged even at high temperatures (fig. 3).

Based on these results, the authors believe that A. clathratum might be more robust to temperature extremes than the other two local species of kelp. In a warmer ocean, A. clathratum might begin to creep into shallower water and to dominate the ecosystem. Simonson notes that while A. clathratum can support much of the existing kelp forest communities, certain kinds of invertebrates and fishes will inevitably be out competed for food and space.

Keep in mind; the 21^oC treatment is a predicted high temperature that the kelp might experience for a few days in 2100. If climate change continues at the current pace, then these waters would continue warming in concert. That might mean the kelp would experience long stretchs of warm waters. Simonsons experiment demonstrated that even A. clathratum could not survive at such elevated temperatures.

Scientists believe that in this environment, turf algae will replace most kelp. Even the A. clathratum samples could not survive two weeks at such elevated temperatures. Turf algae grow in mats over the sea floor and do not grow vertically in the water column. Without the habitat structure provided by kelp, whole ecosystem may shift beyond recognition. Such changes could have far reaching implications into the worlds of coastal ecosystem management and commercial fishing.

Eric Orenstein

Eric is a PhD student at the Scripps Institution of Oceanography. His research in the Jaffe Laboratory for Underwater Imaging focuses on developing methods to quantitatively label image data coming from the Scripps Plankton Camera System. When not science-ing, Eric can be found surfing, canoeing, or trying to learn how to cook.