Marine Atlas: Maximizing Benefits for Tonga

There is also seasonal variation in photosyntheti- cally available radiation in Tonga. The greatest variation occurs around the islands and in the very northern part of Tonga’s waters, where photosynthetically available radiation varies by up to 15 per cent throughout the year. This is in part due to changes in atmospheric conditions, such as cloud cover. In Tonga, the average percent- age of the sky covered by clouds experiences significant seasonal variation over the course of the year, with the cloudiest days occurring from December to March and the least cloudy days from July to September. The chlorophyll-a concentration in Tonga’s waters is generally very low, with concentrations in the offshore waters less than 0.1 gram per m3 of seawater. Most of the tropical regions of the open oceans have similarly low chlorophyll-a concen- trations. In contrast, within temperate and arctic regions, these concentrations can approach 1 gram per m3 of seawater. The shallow coastal regions have increased chlorophyll-a concentra- tions, with up to 2.2 grams per m3 of seawater. Again, this is low compared to many coastal regions around the world, where chlorophyll-a concentrations can reach over 10 grams per m3 of seawater. The low concentrations of chloro- phyll-a in Tonga’s waters reflect the low availabili- ty of key nutrients. Compared to large continental landmasses, with large river discharges that can carry nutrients into the sea, Tonga is a small island nation with comparatively small nutrient inputs into the marine environment. However, at the local or bay scale, nutrient inputs may still be significant. In the south-western tropical Pacific Ocean, strong seasonal and inter-annual variabilities in the chlorophyll-a concentration have been observed (Dupouy et al., 2004). Strong chlorophyll-a enrich- ments have been documented around the Solo- mon Islands, and between New Caledonia and Vanuatu, with weaker enrichments found around Fiji or Tonga. The annual variation in chlorophyll-a around Tonga is up to 2 grams per m3 of seawater in some coastal areas.

14°S

176°W

172°W

CHLOROPHYLL A CONCENTRATION (milligram/meter 3 )

0.2 mg/m 3

0.04 mg/m 3

75 150 km No Data

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

18°S

22°S

Euphausia superba, phytoplankton from the Antarctic, is an example of the basis of the marine food chain.

26°S

MAXIMIZING BENEFITS FOR TONGA

SUPPORTING VALUES

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