Marine Atlas: Maximizing Benefits for Kiribati

SOAK UP THE SUN: PHOTOSYNTHETICALLY AVAILABLE RADIATION The amount of light available in Kiribati’s waters determines the growth of plants, including tiny phytoplankton—the basis of the marine food chain—and thus the rate of carbon capture.

However, in Kiribati’s coastal waters, in- creased nutrients from land-based activities, such as farming and wastewater treatment, can result in harmful algal blooms. These blooms can affect coastal habitats, for example the growth of macroalgae can smother coral reefs and limit light availability, both of which can lead to rapid declines in reef biodiversity (Fabricius, 2005). Blooms can therefore have a detrimental impact on living creatures and ecosystems, resulting in fish die-offs, water being unsafe for human consumption, or the closure of fisheries.

Marine phytoplankton, however, play a key role in the global climate system and in supporting Kiribati’s complex marine food webs. Understanding their spa- tio-temporal variability by analysing chlo- rophyll-a concentrations is therefore an important goal of present-day oceanogra- phy. Consequently, chlorophyll-a concen- tration is routinely measured in the ocean and is also considered to be an important parameter of global physical-biological oceanic models.

Globally, photosynthetically available radia- tion is highest in the tropics and decreases at high latitudes, with some variation due to cloud cover and other atmospheric condi- tions. As a result, photosynthetically availa- ble radiation is moderately high in Kiribati’s waters and mirrors the global pattern, with higher amounts in parts of Kiribati’s waters along the equator (0 degrees), and decreases to the north and south. Within this overall trend, there are other variations: for example, photosynthetically available radiation is higher in the easterly islands of

Ocean gardens For plants to thrive, they need three things: water, sunlight and nutrients. In Kiribati’s sea, the first is obviously not an issue. The second is also not a problem, with the sun shining on Kiribati’s tropical waters year-round. Thus, there is always radiation available for photosynthesis— the process used by a plant to convert light energy into chemical energy that can later be released to fuel its activities. However, the third requirement, nutri- ents, is often the limiting factor in the seas of Kiribati. The energy from sunlight is absorbed by green chlorophyll pigments that trans- form sunlight into energy. Only sunlight of a specific wavelength range (400 to 700 nanometres) can be converted into energy. This wavelength range is referred to as photosynthetically available radi- ation, also known as photosynthetically active radiation. Growing in Kiribati’s sunlit surface waters is a myriad of tiny plants called phytoplankton, which literally means drifter plants (see also chapter “Trav- ellers or homebodies”). They are full of chlorophyll, which gives them their

greenish colour. Chlorophyll absorbs most visible light, but reflects some green and near-infrared light. There are six different types of chlorophyll molecules, with chlorophyll-a the most common type in phytoplankton. Meas- uring chlorophyll-a concentration gives a good indication of primary productivity in the oceans. Nevertheless, marine plants cannot live off water and light alone. They also require nutrients, including iron, nitrate and phosphate (see also chapter “The dose makes the poison”). Since these nutrients are generally low in Kiribati’s waters, phytoplankton quickly consume nutrients whenever they do become available. There is a school of thought that fertilizing areas of ocean may stim- ulate phytoplankton growth, capturing carbon which may sink to the ocean floor (see also chapter “Pump it”). Could this be the solution to climate change (see also chapter “Hotter and higher”)? However, the many ocean fertilization experiments worldwide using iron, phos- phate or nitrate have yet to show feasi- bility on a scale large enough to reduce global emissions (Matear, 2004).

5°N

CHLOROPHYLL A CONCENTRATION (milligram/meter 3 )

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0.2 mg/m 3

0.05 mg/m 3

Kiribati Provisional EEZ Boundary

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400 km

Sources : Becker et al, 2009; Claus et al, 2016; NASA Goddard Space Flight Center, 2014; Smith and Sandwell, 1997. Copyright © MACBIO Map produced by GRID-Arendal

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5°S

170°E

175°E

180°

175°W

170°W

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10°S

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160°W

155°W

150°W

MAXIMIZING BENEFITS FOR KIRIBATI

SUPPORTING VALUES

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