Marine Atlas: Maximizing Benefits for Vanuatu

THE DOSE MAKES THE POISON: PHOSPHATE AND NITRATE CONCENTRATION While nutrients including phosphate and nitrate provide much-needed nutrients for the marine food chain, too much from agricultural run-off and other sources negatively affect Vanuatu’s coastal ecosystems.

wastewater treatment from municipal sources, and soaps and detergents. This is where the dose makes the poison: while phosphate and nitrate are important nutrients, too much of them can be bad for marine and coastal ecosystems. In Vanuatu’s waters, there is certainly no shortage of sun, and thus photosynthetically available radiation, but there is a general limit of phosphate and nitrate. Once these nutrients are added from the land-based activities such as farming and wastewater treat- ment, primary productivity increases dramatically. The impact of too many nutrients (eutrophication) is especially significant in coastal waters, where in- creased nutrients can result in algal blooms. These blooms can affect coastal habitats such as coral reefs by smothering, in the case of macro-algae, or limiting light availability, which can lead to rapid declines in reef biodiversity (Fabricius, 2005).

On a global scale, Vanuatu’s waters have a mod- erately low phosphate concentration, ranging from 0.17 to 0.20 umol/L. The highest concentrations are observed in the north-eastern waters and gradually decrease to the south and west. At the global level, nitrate concentrations in seawater are generally low, with the highest concentrations found in high lat- itudes and some areas of coastal upwelling. Within Vanuatu’s waters, the nitrate concentration ranges from 0.3 to 0.7 mmol m3, which is very low com- pared with global levels. The highest concentrations of nitrate in Vanuatu occur in the south-west, but the South-West Tropical Pacific (SWTP) is generally considered a nitrogen-limited area. Phosphate and nitrate concentrations can be higher in the waters close to the main islands due to land and coastal inputs, which can include inorganic fertilizers,

Sea food “All things are poison and nothing is without poison; only the dose makes a thing not a poison”, stated the Swiss physician Para- celsus 500 years ago. And indeed, the dose makes the poison. Marine organisms need food and nutrients, with tiny plants known as phytoplankton forming the basis of many marine food chains (see also chapter “Soak up the sun”). These phytoplankton rely on the nu- trients phosphate and nitrogen, principally in the form of nitrate (see map). Phytoplankton productivity at the surface of the ocean is often limited by the amount of available fixed inorganic nitrogen (Falkowski et al., 2009). However, where there is too much of these nutrients, algal blooms can occur, which can have negative impacts on the environment. As the chapter “Plastic oceans” as well as the graphic show, excess nutrients are only one type of pollution and threat to Vanuatu’s marine values. To keep Vanuatu’s coastal habitats healthy (see also chapter “Home, sweet home”), it is important to

PHOSPHATE CONCENTRATION (umol/L)

0.27

0.16

Vanuatu Provisional EEZ Boundary Boundary as deposited at UN Archipelagic Baseline

15°S

100 50

200 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

20°S

25°S

165°E

170°E

THREATS MAXIMIZING BENEFITS FOR VANUATU

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