From Lush Forest to Barren Wasteland: The Ecological Shift Transforming Kelp Forests
If you’ve ever been to the great sequoia forests, you have seen just how awe-inspiring it is to walk between trees so tall that the light just barely reaches the ground from the top of the canopy. Now imagine that sight, but underwater, with kelp stretching from the seafloor to the ocean surface, light dancing through leafy fronds as fish, sea otters, and sea lions dart in between. These are the California Coast kelp forests, the “sequoias of the sea”, a source of essential ecosystem services and the home of a multitude of species.
Kelp forests are a significant part of the California Current system, acting as important areas of primary production. They provide food and habitat to a variety of species, supporting biodiversity and local fisheries. Their presence allows for the existence complex food webs and distinct communities of marine life, including species like rockfish, sea lions, starfish, urchins, and sea otters.
Kelp forests are also important for coastal communities. They are able to slow down waves before they hit the shoreline by absorbing some of their energy. This lessens the wave impact along coasts and reduces erosion. In this way, kelp forests are partly responsible for the beautiful, calm beaches that can be found in California.
Currently, there is research into the viability of kelp growth as a method for carbon sequestration and for the mitigation of ocean acidification. Kelp is a fast growing algae, capable of growing up to two feet a day. As it grows, it photosynthesises and stores CO2 in organic material. This removes CO2 from the water, potentially reducing the effects of ocean acidification. The conservation and restoration of kelp forests could help offset some of the effects of CO2 emissions.
Now imagine these kelp forests reduced to barren fields, devoid of life save for spiny carpets of sea urchins. Imagine all of these benefits being lost as kelp forests disappear. This drastic shift is starting to become reality in many regions throughout the California Current system, largely due to climate change.
Climate change has caused an increase in global temperatures, affecting ecosystems across the globe. Kelp forests may be one habitat that is particularly sensitive to rising temperatures because kelp are naturally adapted to grow in colder waters. Elevated temperatures could place stress on kelp and reduce their growth rates.
The marine heat waves of 2014 and 2016, which introduced extended periods of high temperatures over a large portion of California coastal waters, provide an example of how kelp forests might respond to long term temperature shifts due to climate change in the future. These marine heat waves caused widespread kelp mortality and drastic shifts in the community structure of kelp forests. Some of the sites hit by these effects are still in recovery today. It is still unclear exactly how increased temperatures are affecting kelp, but researchers have developed some theories using data collected from these events.
One idea is that, beyond just the stress of adapting to a new temperature, kelp are also receiving less nutrients due to these changes. Kelp forests are usually located in areas of upwelling where cold, nutrient rich water is circulated to the ocean’s surface. Increased temperatures can cause increased stratification of the ocean, meaning that there is a larger layer of warm water sitting on the surface, making it more difficult for deeper waters to move up to where the kelp are. This reduced upwelling can cut off the supply of key nutrients that the kelp need to grow. Additionally, reduced upwelling means that rising temperatures are not offset by the colder waters that would have been added to the kelp forest system, perpetuating the cycle of stratification and temperature rise.
Another theory is that rising temperatures increase the frequency of sea star wasting syndrome (SSWS), a disease that cases sea stars to melt, dissolve, and eventually die. We still don’t know how exactly this disease spreads or what causes it. However, a spike in this disease was observed during the 2014 and 2016 marine heat waves, eliminating many sea star populations throughout California kelp forests. This led researchers to believe that there is some aspect of elevated temperatures that improves the ability of SSWS to infect sea stars.
In a complex community, like a kelp forest, the loss of any group of organisms can be devastating to food webs and to the ecosystem as a whole. Sea stars usually prey on sea urchins, keeping their populations in check. However, with sea stars gone, urchin populations can expand uncontrollably, grazing on kelp and placing them under increased stress. They can even consume entire forests until the habitat is dominated entirely by sea urchins. These regions are called urchin barrens and can replace kelp forests along with all the primary productivity, habitat, and ecosystem services they had once provided.
While it is widely understood that climate change will have a negative impact on kelp forests, we still don’t fully understand all the reasons for why increasing temperatures affect kelp forest growth and community structure. More research into the actual mechanisms causing kelp forest deterioration is required if we want to preserve these important ecosystems.
Rogers-Bennett, L., Catton, C.A. (2019) Marine heat wave and multiple stressors tip bull kelp forest to sea urchin barrens. Sci Rep 9, 15050. https://doi.org/10.1038/s41598-019-51114-y
