Hungry, hungry urchins: Can changes to grazing rates alter kelp forest ecosystems in a changing ocean?

A guest blog by Emily Donham

The great green unknown: A diver descends through a giant kelp forest. Photo credit: S Kram

The great green unknown: A diver descends through a giant kelp forest. Photo credit: S Kram

Diving in a kelp forest can be a bit overwhelming at first. The initial shock of cold as you plunge into the water quite literally takes your breath away. You then try to untangle yourself from the grips of the kelp canopy that only seems to tighten as you struggle with your arms and legs made heavy by your thick neoprene wetsuit. Finally, you descend through the forest, and if you’re lucky, you’ll be able to see 30’ ahead. At first this can be disorienting, but the more you do it, the more fun it becomes to use your compass and rely on landmarks to navigate around the reef. Once you reach the bottom, it’s usually much easier to take in all of the amazing critters that live in this underwater forest. There can be schools of rockfish in the water column ahead, brightly colored sea slugs crawling across bubblegum pink algae below, and curious harbor seals making brief appearances. When you are below the canopy, its clear that kelp forests are diverse and bustling ecosystems.

Despite the sense of wildness and life that you can feel when you SCUBA dive, kelp forests are still very much at risk from human activities. Anthropogenic carbon dioxide emissions are causing the ocean to warm and acidify. Although ocean temperatures and acidity can vary a little bit depending on the season or the location, the changes brought on by climate change and acidification are much more intense and are likely to impact algae and marine animals that live in all regions of the world's ocean, including the rockfish, sea slugs and calcareous algae found in kelp forests.

As a part of the Kroeker lab here at UCSC, my dissertation focuses on how kelp forest ecosystems found along the California coast will respond to climate change and ocean acidification. Kelp forests are dominated by large habitat forming brown seaweeds known as kelps. Kelps provide important habitat and food for the hundreds of species that live within them. These ecosystems are very much like our own bustling cities, made up of thousands of individuals interacting with each other and competing for the resources they need to survive. In this underwater “society,” one of the most important things for survival is making sure that you get enough food to eat.

Sea urchin grazing on “kelp rollup”, a mixture of ground kelp in agar. The number of squares of green kelp removed from the mesh grid are counted after 24 hours and used to measure a relative grazing rate. Photo credit: E Donham

Sea urchin grazing on “kelp rollup”, a mixture of ground kelp in agar. The number of squares of green kelp removed from the mesh grid are counted after 24 hours and used to measure a relative grazing rate. Photo credit: E Donham

Imagine for a moment that you’ve just gone on a long run and are looking forward to dinner that evening. You will likely plan to eat much more than you would if you had just spent the day working behind a computer. Ocean warming and acidification have the potential to affect marine life in ways that a long workout might affect you. For a sea urchin living on a reef, warming and ocean acidification could increase its energetic costs – such that the urchin will need more food than it does now to survive. If all the individuals in a kelp forest respond similarly to ocean warming and acidification, we could see big changes to the ecosystem as a whole. For instance, if sea urchins that feed on kelp start eating more, we might end up seeing less kelp within our kelp forests. Reducing the amount of kelp within the kelp forest might also impact other species that rely on kelp for food and habitat. Therefore, a change to one species' physiology (i.e., sea urchin) could impact other species within the ecosystem both directly (in this case, the kelp which is being consumed) or indirectly (by removing kelp that provides habitat for fish).

Undergraduate researcher Clare Peabody (Brown University) setting up large experimental tank. Photo credit: E Donham

Undergraduate researcher Clare Peabody (Brown University) setting up large experimental tank. Photo credit: E Donham

In order to better understand if different species will change their eating habits in response to climate change and ocean acidification, we’ve been conducting laboratory experiments, where we manipulate environmental conditions to mimic ocean warming and acidification. We do this by constructing small kelp forest communities in large experimental tanks and monitoring how the feeding rates of different species change in response to increased temperature and acidity. These mini communities provide us a window into the future of kelp forests that could ultimately help us to prepare for future environmental change. It’s my hope that by understanding future changes, we can help to protect these bustling ecosystems and the large diversity of marine life that they support off our coastline.

Emily Donham is a PhD student in Ecology and Evolutionary Biology at UCSC.