Climate change impacts kelp composition

But first, marine ecosystems are built around plant and algal species that capture solar energy and feed it up the food chain. Kelps are major players, especially around the Arctic, and, as well as being a source of food, kelp forests also provide shelter and breeding grounds for marine life. But a new study suggests that some species are very vulnerable to climate change, which can alter water temperatures and water turbidity, which can affect the amount of light that makes it through. These altered conditions exert a selection pressure, shifting the balance of kelp species away from beneficial forms towards those that afford animals a less hospitable environment, which can have economic costs for us too, as Chris Smith hears from Sarina Niedzwiedz…

Sarina - My name is Sarina Niedzwiedz and I'm a postdoc at the University of Bremen and I'm a marine biologist. Kelps can actually be compared to trees on land because they can grow several metres in height and thereby build complex three-dimensional structures above the seafloor. They provide a lot of habitat and nursery ground for a variety of species and also food as their primary producers. So, they are very, very important for those ecosystems and act as foundation species.

Chris - And why do we feel that they might be threatened or impacted at least by climate change?

Sarina - Because the Arctic is one of the regions that is actually most affected by climate change and we see temperature rises at a rate about four times the global average and that leads to a variety of cascading other changes such as an increase of marine heat waves - but also all of this temperature rises cause glacier snow and permafrost to melt, and the huge amount of melt water that results from this melt is washing a lot of sediments into the fjords which is then changing the light availability underwater.

Chris - So, it's effectively a one-two punch that might hit the kelp then. Not only is the water temperature departing away from where they've evolved to be most happy but the water could become very, very cloudy and cut off the light supply.

Sarina - Exactly. Both temperature and light are very, very important drivers for those kelps. We need to know how the kelp forest dynamics is changing in the future.

Chris - And how can you get at this? Is there an easy way, because you can't speed up time and ask, well, this is what it was like before climate change, here it is afterwards. So, how can you get at this to get a reasonable answer to what the impact might be?

Sarina - We designed a multifactorial experiment where we exposed those two locally abundant kelp species to, first of all, different light conditions. Those light conditions mimicked either a clear water column or a turbid water column with all the melt water. And then on top of that, when they were acclimated to those light conditions, we exposed them to a marine heat wave.

Chris - And how do you gauge what impact that's having on the kelp?

Sarina - Yes. So, we measured their physiology, for example, their growth, their photosynthetic activity, and also their photosynthetic rates and respiration rates. And, on top of that, we also measured their biochemical response. So, their pigment composition, for example.

Chris - And when you do this, I presume you're looking at two different kelps, because then you can ask, well, what's likely to happen to a range of populations? But when you do this, how do those two variables: the temperature and how cloudy the water is, affect the outcomes for the kelps?

Sarina - When both of the kelp species were exposed to high light conditions, so basically a clear water column, they experienced physiological stress levels, which was actually destructive for them. This stress level got even higher when the high light conditions interacted with cold temperatures. And stress level was then mitigated by the marine heat wave. So, they actually benefited from warmer temperatures because both of the kelp species we were investigating are not Arctic endemic. So, their optimum growth temperature is warmer than that, what we currently see in the Arctic.

Chris - And of course, the warm temperature would also bring with it more meltwater, which would cloud the water, which would reduce the light, which would reduce the light stress.

Sarina - Yes. We also saw that with lower light intensities, the light stress got reduced, but the productivity and growth of both kelp species also got reduced.

Chris - Well, what do we take away from this then? It sounds like it's not necessarily all bad news for kelp, because there's going to be some that may do quite well from climate change then. They will be less stressed and they will retain their productivity. Is that good news then, that we can relax on that front?

Sarina - Physiologically, kelps can cope with those conditions that we will expect in Arctic near future. However, what we also saw is that depending on the conditions, the competition balance between both kelp species also changed. So, we saw that one kelp species, the sieve kelp, actually did better in darker and warmer conditions.
So, basically near future compared to the other species, the sugar kelp. The sieve kelp is also characterised by high contents of herbivore deterrents. So, if that kelp species, the sieve kelp, would become dominant in a future kelp forest, the nutritional value of the kelps might be reduced for higher trophic levels.

Chris - And hence, there would still potentially be knock-on effects further up the food chain.

Sarina - Yes, exactly. Up to the humans, because then commercially used fish or invertebrate species also are reduced in their abundance, then that might also have socio-economic impacts.