Atolls and coral reefs sustain life far beyond their locations

A recent study published in Nature Communications uses satellite imaging to show that coral reefs and atolls may provide enough nutrients to sustain biological hot spots far beyond their geographical location.

Using data acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite, researchers found that over 90 percent of coral reef islands and atolls raise the biological diversity in the ocean around them. The data shows that on average, there is 86 percent more phytoplankton on the outer fringes of reefs than would otherwise be expected.

For years, scientists have been aware of the “island mass effect,” (IME) – which states that the productivity of islands can extend far into nutrient-depleted waters. However, it hasn’t been clear how prevalent the IME was or what role phytoplankton play in increasing productivity and diversity in the surrounding areas. Research oceanographer and lead author of the paper Jamison Gove from NOAA’s Ecosystems and Oceanography Division said “There’s been sort of a longstanding paradox in how island reef ecosystems are so productive when their surrounding environment isn’t. But this is the first research to show that IME is a ubiquitous phenomenon.”

In the map above, the green areas correlate to higher chlorophyll, which indicates blooming phytoplankton. The blue areas correspond to lower concentrations of chlorophyll and thus less phytoplankton. The large stretch of green around the equator is a well-known area of ocean upwelling called the equatorial cold tongue. It is where colder, nutrient-rich water is brought from the depths to the surface.

The IME actually resembles large ocean upwelling events in many ways. When ocean currents and waves encounter an island, they flow around it, which brings the deep, nutrient-rich water upwards. This reversal in the water column is a precursor to biological activity, which carries increased productivity and diversity as a result.

These green “hot spots” occur around the majority of the islands in the Pacific, indicating high levels of chlorophyll – and phytoplankton – in those areas. The amount of activity around the Gilbert Islands is particularly intriguing, as the islands themselves are far smaller in size than the phytoplankton-filled areas around them.

These findings are important for oceanographers, as coral reefs and atolls can be incredibly important patches of life in the many dead zones of the Pacific. Understanding what sustains the productivity and diversity of these hot spots will be vastly important moving forward as we attempt to combat the negative effects of climate change on our oceans.

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Credit: Earth.com author Connor Ertz