Marine Atlas: Maximizing Benefits for Tonga

TURNING SOUR: OCEAN ACIDITY Climate change is not only causing sea temperatures and levels to rise but also its acidity, which causes serious problems for many marine organisms.

Seawater acidity can be measured using the pH, a numeric scale to specify the acidity or basicity of a solution; a pH of 7 is neutral—neither acid- ic nor basic. A decrease in pH by one means a solution is twice as acidic, whereas an increase by one means a solution twice as basic (see graphic). The pH of the global oceans ranges from around 7.5 to 8.4. Tonga’s waters are at the higher end of this range, with pH between 8.25 and 8.29. Increasing CO 2 in the surface water leads to increased acidification (lower pH). Already, CO 2 emissions have resulted in a 26 per cent increase in the acid content in the ocean (see small map). In this context, it is important to look at calcite, which is another vital element found in seawater (see map on the right), as calcium carbonate is a building block of the skeletons of most marine organisms, including corals. Globally, calcite con-

centrations are highest in the high latitudes and in coastal areas. The calcite concentrations in Tonga’s oceanic waters are low, with the coastal areas around the islands having a higher concen- tration (see calcite map). How does acidification affect calcite levels? Firstly, CO 2 in the water transforms into carbonic acid and the carbonate saturation decreases. This is problematic for all animals that use marine carbonate to make their shells, such as mussels, snails, corals and sea urchins, among many oth- ers (see also chapter “Travellers or homebodies”). The less carbonate there is in the water, the more difficult it is for them to make suitable shells. The effects can already be seen among foraminifera: tiny calcifying creatures that make up an impor- tant part of the plankton. The shell-thickness of animals in the Southern Ocean has noticeably decreased compared with specimens from the pre-industrial period. The effect on oysters is slightly different: it has been observed that the thickness of their shells does not decrease, but only because they invest so much energy into shell production that it stunts their overall growth. This makes them easier prey for predators, such as murex snails. Ocean acidification Tonga is suffering the effects of global warming, with greenhouse gas emissions not only heating the nation’s sea, but also ending up in it. In fact, worldwide the oceans have absorbed about one third of the carbon dioxide (CO 2 ) produced by human activities since 1800 and about half of the CO 2 produced by burning fossil fuels (Sabine et al., 2004). As CO 2 in the ocean increases, ocean pH decreases, resulting in the water becoming more acidic. This is called ocean acidification, the “evil twin” of sea temperature and sea level rise, described in the previous maps.

14°S

176°W

172°W

pH

8.3

8.2

75 150 km

Sources : Becker et al, 2009; Claus et al, 2016; Smith and Sandwell 1997; Tyberghein et al, 2011. Copyright © MACBIO Map produced by GRID-Arendal

18°S

22°S

26°S

CLIMATE CHANGE THREATS MAXIMIZING BENEFITS FOR TONGA

60