Marine Atlas: Maximizing Benefits for Kiribati

STIR IT UP: MIXED LAYER DEPTH

Kiribati’s waters are stirred by winds and heat exchange. How deep this disturbance goes influences both the climate and the marine food chain.

The waters surrounding Kiribati are often choppy and turbulent, creating a ‘mixed layer’ in the upper portion of sea surface where active air–sea exchanges cause the water to mix and become vertically uniform in temperature and salinity, and thus density. The mixed layer plays an important role in the physical climate, acting as a heat store and helping regulate global temperatures (see also chapter “Hotter and higher”). This is because water has a greater capacity to store heat compared to air: the top 2.5

metres of the ocean holds as much heat as the entire atmosphere above it. This helps the ocean buffer global temperatures, as the heat required to change a mixed layer of 25 metres by 1°C would be sufficient to raise the temperature of the atmosphere by 10°C. The depth of the mixed layer is thus very important for determining the temperature range in Kiribati’s waters and coastal regions.

drives global variability, including El Niño (see also chapter “Go with the flow”).

tiny marine plants known as phytoplankton are unable to get enough light to maintain their metabolism. This affects primary productivity in Kiribati’s waters which, in turn, impacts the food chain. Mixed layer depth can vary sea- sonally, with consequential impacts on primary productivity. This is especially prominent in high latitudes, where changes in the mixed layer depth result in spring blooms. The depth of the mixed layer in Kiriba- ti’s waters ranges from 37 metres to 79 metres, and there is a considerable differ- Getting to the lower layers Local fishermen in the southern islands are well known for the type of vertical tuna longlining known as drop-stone fishing, or Te kabwara, in Kiribati. With their small canoes, they travel to the outer reef where they employ this tech- nique to catch tuna in the deep ocean. The technique involves a long, flattish stone weighing 1–2 kilograms, around which a wire trace with baited hook is wrapped several times and tied with a quick-release knot. This allows the fishermen to get the baited hook down

ence between the three island groups. The shallowest mixed layer depths are found in the Gilbert Islands and the northern part of the Line Islands. The deepest mixed layer depths are found through the centre of the Line Islands and Phoenix Islands. This area corresponds to the strongest sea surface currents from the South Equatorial Current. Globally, mixed layer depths range from 4 metres to nearly 200 metres depth. The deepest mixed layer depths are generally found in the sub-Antarctic regions and the high latitudes of the North Atlantic. to the required depth and then release the stone, so that the hook hangs free. This method often uses chum, or finely chopped bait, to attract tuna at specific depths. The ingenuity of this technique is not only in the manufacturing of the gear itself, but in the very sophisticat- ed understanding of the depths of the ocean as well as the mixed layer depth on any given day. Only in this way can fishermen find the right depth for spe- cific species of tuna, be it yellowfin, skipjack or albacore.

The mixed layer also has a strong influence on marine life, as it determines the average level of light available to marine organisms. In Kiribati and elsewhere in the tropics, the shallow mixed layer tends to be nutrient-poor, with nanoplankton and picoplankton sup- ported by the rapid recycling of nutrients (e.g. Jeffrey and Hallegraeff, 1990; see also chapters “Soak up the sun” and “Travellers or homebodies”). In very deep mixed layers, the

In addition, the heat stored within the oce- anic mixed layer provides a heat source that

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Kiribati Provisional EEZ Boundary

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Copyright © MACBIO Map produced by GRID-Arendal Sources : Becker et al, 2009; Claus et al, 2016; Scott and Dunn, 2006; Smith and Sandwell, 1997.

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MAXIMIZING BENEFITS FOR KIRIBATI

SUPPORTING VALUES

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