Marine Atlas: Maximizing Benefits for Fiji

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 Fiji’s coastal ecosystems.

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 Fiji’s waters, there is certainly no shortage of sun, and thus photosynthetically available radia- tion, but there is a general limit of phosphate and nitrate. Once these nutrients are added from the land-based activities such as farming and waste- water treatment, primary productivity increases dramatically. The impact of too many nutrients (eutrophication) is especially significant in coastal waters, where increased 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 biodiver- sity (Fabricius, 2005).

On a global scale, Fiji’s waters have a moderately low phosphate concentration, ranging from 0.15 to 0.28 umol/L. The highest concentrations are observed in the northern waters and gradually decrease to the south. At the global level, nitrate concentrations in seawater are generally low, with the highest concentrations found in high latitudes and some areas of coastal upwelling. Within Fiji’s waters, the nitrate concentration ranges from 0.7 to 1.3 mmol m3. The highest concentrations of nitrate in Fiji occur in the south-west, but the South-West Tropical Pacific (SWTP) is generally considered a nitrogen-limited area. Phosphate and nitrate concentrations are slightly higher in the waters close to the main islands due to land and coastal inputs, which can include inorgan- ic fertilizers, wastewater treatment from municipal

Seafood “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. Humans need to eat food to survive, though only in moderation, since too much food can undoubtedly do more harm than good. Similarly marine organisms need food and nutrients. Phosphate (see map) is one such important nutrient that supports biological activity and is needed for the growth of tiny plants known as phytoplankton, which are the basis of many marine food chains (see also chapter “Soak up the sun”). Another food source is nitrogen (see map), which exists in various forms in the marine environment, with nitrate the principal form used by organisms. Phytoplankton produc- tivity at the surface of the ocean is often limited by the amount of available fixed inorganic nitrogen (Falkowski et al., 2009).

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PHOSPHATE CONCENTRATION (umol/L)

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Fiji Provisional EEZ Boundary Archipelagic Baseline

As the chapter “Plastic oceans” and “From ridge to reef” as well as the graphic show, excess nu- trients are only one type of pollution and threat to Fiji’s marine values. To keep Fiji’s coastal habitats

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

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MAXIMIZING BENEFITS FOR FIJI

THREATS

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