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What do you foresee?

Public awareness of many climate change-related issues is rising globally, as well as in New Zealand. (1) However, ocean acidification (OA) seems to be one of the least perceived and understood threats to the marine environment. (2)

Increasing levels of atmospheric carbon dioxide are taken up by the global oceans, causing changes in the water chemistry, a process known as ocean acidification. An estimated 2.6 billion tons of carbon dioxide are absorbed by the ocean every year. (3) While the water is not quite literally turning into acid, the water pH shifts to a less alkaline state (or 'more acidic').

It has been estimated that the global average surface seawater pH has dropped by 0.1 units since the beginning of the Industrial Revolution; taking into consideration the fact that pH is measured on a logarithmic scale, this shift translates into a 30 percent increase in acidity. New Zealand marine ecosystems are already experiencing OA-related stress and it is expected that they will continue to do so in the near future. (4)

The chemical changes in the water come at a cost for many marine organisms and may have severe impacts on key physiological processes such as growth, shell formation, reproductive capabilities, competitive fitness and photosynthesis. Furthermore, lowered pH may lead to shell dissolution in shell-building organisms. (5)

One organism that has been identified as most vulnerable to OA is a group of calcifying algae called coralline algae. (6) Coralline algae belong to an evolutionary old group of algae, the red algae or Rhodophyta ([phrase omitted], 'rose,' and [phrase omitted], 'plant'). As the name suggests, they come in all shades of red, from dark red or purple to bright pink (Figure 1). The colour arises from the photosynthetic pigment Phycoerythrin. (7) Unlike other algae within the Rhodophytes, they form a solid carbonate structure. Some coralline algae grow as tufts and have thin jointed branches that sway with the currents (geniculate coralline algae), while others grow as thick pink crusts (crustose coralline algae (8)).

Coralline algae thrive globally in almost all coastal habitats where there is sufficient light and hard substrate to attach. Their distribution ranges from the cold polar waters to the tropics, and they can be found from the harsh intertidal down to the deep subtidal zones. The deepest ever discovered macroscopic plant life was a coralline alga found at a staggering 210m deep (9).

While mostly overlooked or even considered a nuisance among aquarium keepers--as they grow rather fast on the walls of the tanks--they nevertheless fulfil a multitude of ecological functions. New Zealand has a particularly rich diversity of coralline algae and they can extensively cover hard substratum (Figure 2). Coralline algae are ecosystem engineers and consolidate the reef structure, provide habitat and protection and constitute a food source; they have an intricate relationship with invertebrate larval settlement (10).

The New Zealand abalone is among those invertebrate larvae that depend directly on the presence of crustose coralline algae. Commonly known as paua, it is a key coastal species, given its huge economical, ecological and cultural importance.

Paua reproduce through an indirect cycle, which is characterised by the presence of free-swimming larvae that metamorphose into juveniles after settling on their preferred substrate. Paua larvae use a signal given by the crustose coralline algae as a cue to settle and metamorphose into juveniles. Once metamorphosed, the juveniles will use the crustose coralline algae as a source of food. Besides paua, many other organisms rely on crustose coralline algae as a preferred settlement substrate for their larvae and as a source of food for juveniles.

OA-induced changes in settlement substrates such as species composition or distribution could cause changes in settlement rates of paua--as well as other invertebrate--larvae. Altered settlement rates could alter the abundances, distributions and ecology of future marine communities. Aquaculture services that rely on a continuous supply of (paua) adults might be negatively influenced as well.

Anna Kluibenschedl and Nadjejda Espinel Velasco are PhD candidates in Marine Science, University of Otago. Nadjejda studies the effects of ocean acidification on settlement of NZ invertebrates. Anna works on crustose coralline algae--calcifying algae that facilitate settlement of larvae and connect their planktonic and benthic life cycles.

(1.) T L Milfont, M S Wilson and C G Sibley, "The Public's Belief in Climate Change and its Human Cause are Increasing over Time," PLoS ONE, 12:3 (2017): e0174246. doi:10.1371/journal.pone.0174246.

(2.) S Gelcich, P Buckley, J K Pinnegar, J Chilvers, I Lorenzoni, G Terry, M Guerrero, J Carlos Castilla, A Valdebenito and C M Duarte, "Public Awareness, Concerns, and Priorities about Anthropogenic Impacts on Marine Environments," Proceedings of the National Academy of Sciences, 111:42 (2014), 15042-47. doi:10.1073/ pnas.1417344111.

(3.) C Le Quere, R M Andrew, J G Canadell, S Sitch, J Ivar Korsbakken, G P Peters, A C Manning, et al., "Global Carbon Budget 2016," Earth System Science Data, 8:2 (2016), 605-49. doi:10.5194/essd-8-605-2016.

(4.) C S Law, J J Bell, H C Bostock, C E Cornwall, V J Cummings, K Currie, S K Davy, et al., "Ocean Acidification in New Zealand Waters: Trends and Impacts," New Zealand Journal of Marine and Freshwater Research, 52:2 (2018), 155-95. doi:10.1080/00288330.2017.1374983.

(5.) K J Kroeker, R L Kordas, R Crim, I E Hendriks, L Ramajo,

G S Singh, C M Duarte and J-P Gattuso, "Impacts of Ocean Acidification on Marine Organisms: Quantifying Sensitivities and Interaction with Warming," Global Change Biology, 19:6 (2013), 1884-96. doi:10.1111/ gcb.12179.

(6.) W J Woelkerling, "An Introduction," in Biology of the Red Algae, eds K M Cole and R G Sheath (Cambridge: Cambridge University Press, 1990), 517

(7.) W A Nelson, "Calcified Macroalgae--Critical to Coastal Ecosystems and Vulnerable to Change: A Review," Marine and Freshwater Research, 60:8 (2009), 787-801. doi:10.1071/MF08335.

(8.) M M Littler, D S Littler, S M Blair and J N Norris, "Deepest Known Plant Life Discovered on an Uncharted Seamount," Science, 227:4682 (1985), 57-59. doi:10.1126/science.227.4682.57.

(9.) Ibid.

(10.) Nelson, "Calcified Macroalgae".

Caption: Figures 1-2. A paua larvae settling on a coralline algae. Image: Anna Kluibenschedl. Subtidal coralline algae assemblage. Image: Peri Subitzky.

Caption: Figure 3. Kaikoura coastline one year after the 2016 earthquake. The newly exposed coastline is extensively covered in bleached coralline algae. Image: Anna Kluibenschedl.

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Title Annotation:ocean acidification
Author:Kluibenschedl, Anna; Velasco, Nadjejda Espinel
Publication:Junctures: The Journal for Thematic Dialogue
Geographic Code:8NEWZ
Date:Dec 1, 2018
Words:1073
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