If we hope kelp is going to mitigate climate change, then we might need to think again about over-fishing and kelp harvesting.
When it comes to global climate change and the potential of photosynthesis to remove excesses of CO2 accumulating in the atmosphere, the focus to date has been largely on higher plants, in terrestrial habitats. And, whilst the role of angiosperms in near-coastal marine habitats, e.g. mangrove forests and seagrass meadows, has also been considered, any potential that marine macroalgae might have in this regard has been largely ignored.
Well, not any more, if work by Dorte Krause-Jensen and Carlos Duarte gets the attention it deserves. These workers conclude that macroalga, such as kelps, could represent an important source of carbon that is locked-up in marine sediments and the deep ocean, i.e. far-removed from the atmosphere thereby removing that carbon and its potential to increase global warming (at least temporarily). They propose that this is achieved by two main routes; the drifting of macroalgal material through submarine canyons, and the sinking of negatively buoyant macroalgal detritus. Importantly, Krause-Jensen and Duarte calculate that this carbon sequestration mechanism exceeds that of angiosperm-based coastal communities. That’s one in the eye for ‘higher’ plants! And, there’s more macroalgal good news.
Added to concerns of increased global temperature from elevated atmospheric CO2 is the issue of enhanced ocean acidity (lowered pH) as that gas dissolves in sea water. Studying kelp forests during the prolonged days of the Arctic region during summer, Dorte Krause-Jensen et al. conclude that the enhanced photosynthesis under those conditions, which can mean 15 hours of daylight in a 24-hour period, can support sustained up-regulation of pH in kelp forests, i.e. such photosynthesis helps to increase pH (reduce acidification). This, in turn, is inferred to benefit organisms that use calcium carbonate in their structure – so-called calcifiers such as bivalve molluscs, brittle stars, and sea urchins – and which are at risk from an increasingly acidic ocean. Furthermore, and somewhat ironically, that acid reduction mechanism may actually increase as Arctic vegetation is expected to expand its territory in response to loss of sea ice from global warming from elevated atmospheric CO2 levels.
But it’s not all good news as far as help from kelp is concerned. A study by Adriana Vergés et al. in Australian waters concludes that warmer oceans lead to an increase in warm-water species (a phenomenon known as tropicalization) in those regions. A consequence of which is an increase in herbivory of macroalgae, such as kelps, by fish, which contributes to the seaweed’s decline.
So, different outcomes in the Arctic compared to the Antipodes? Or a warning that any photosynthesis-enhanced pH increase will be short lived as those cold northern waters are tropicalised by warm-water herbivores migrating north poleward? The study highlights the complicated interactions and inter-relations between macroalgae and other biota, which is further underlined by an analysis of 50 years of global patterns of kelp forest change by Kira Krumhansl et al., that adds other human activities such as over-fishing and direct harvesting of kelp into the mix.
Krause-Jensen, D., & Duarte, C. M. (2016). Substantial role of macroalgae in marine carbon sequestration. Nature Geoscience, 9(10), 737–742. https://doi.org/10.1038/NGEO2790
Krause-Jensen, D., Marba, N., Sanz-Martin, M., Hendriks, I. E., Thyrring, J., Carstensen, J., … Duarte, C. M. (2016). Long photoperiods sustain high pH in Arctic kelp forests. Science Advances, 2(12), e1501938–e1501938. https://doi.org/10.1126/sciadv.1501938
Krause-Jensen, D., & Duarte, C. M. (2014). Expansion of vegetated coastal ecosystems in the future Arctic. Frontiers in Marine Science, 1. https://doi.org/10.3389/fmars.2014.00077
Verges, A., Steinberg, P. D., Hay, M. E., Poore, A. G. B., Campbell, A. H., Ballesteros, E., … Wilson, S. K. (2014). The tropicalization of temperate marine ecosystems: climate-mediated changes in herbivory and community phase shifts. Proceedings of the Royal Society B: Biological Sciences, 281(1789), 20140846–20140846. https://doi.org/10.1098/rspb.2014.0846
Vergés, A., Doropoulos, C., Malcolm, H. A., Skye, M., Garcia-Pizá, M., Marzinelli, E. M., … Steinberg, P. D. (2016). Long-term empirical evidence of ocean warming leading to tropicalization of fish communities, increased herbivory, and loss of kelp. Proceedings of the National Academy of Sciences, 113(48), 13791–13796. https://doi.org/10.1073/pnas.1610725113
Krumhansl, K. A., Okamoto, D. K., Rassweiler, A., Novak, M., Bolton, J. J., Cavanaugh, K. C., … Byrnes, J. E. K. (2016). Global patterns of kelp forest change over the past half-century. Proceedings of the National Academy of Sciences, 113(48), 13785–13790. https://doi.org/10.1073/pnas.1606102113