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Solving ‘Darwin’s Paradox’

The productive waters surrounding a remote coral reef atoll (Palmyra Atoll) teeming with marine life The productive waters surrounding a remote coral reef atoll (Palmyra Atoll) teeming with marine life Zafer Kizilkaya
09 Apr
2016
An historic phenomenon explaining why fertile coral reefs form as oases in the oceans has finally been proved correct

During Charles Darwin’s many years studying wildlife species around the world, he found himself pondering an observation he couldn’t explain. While most of the open ocean is very low in nutrients, the isolated hotspots where coral reefs form are instead teeming with the energetic resources that stimulate marine ecosystems. ‘He travelled for weeks on end and saw nothing, then came across what he believed to be one of the most diverse ecosystems on the planet,’ explains Gareth Williams, lecturer in Marine Biology at the School of Ocean Sciences, Bangor University. ‘That became known as Darwin’s Paradox.’

Sixty years ago, a hypothesis entitled the ‘Island Mass Effect’ (IME) claimed to have helped solve the famous naturalist’s quandary. The IME proposes that a variety of differing factors work together in the near vicinity of oceanic islands, reinforcing each other and creating a positive feedback system that accumulates and retains the nutrients required to stimulate phytoplankton production and coral reef development. A new paper in the journal Nature Communications (doi:10.1038/ncomms10581) has found the IME to be ‘a near-ubiquitous feature among a majority (91 per cent) of coral reef ecosystems surveyed’, proving the hypothesis correct.

The island creates its own environment that enhances and maintains phytoplankton production within its vicinity

‘The Island Mass Effect,’ continues Williams, senior author of the paper, ‘is the idea that as a reef system starts to form, there are changes in the circulation of flow around that physical structure. The island creates its own environment that enhances and maintains phytoplankton production within its vicinity.’

Some of the most influential factors of the IME include local geomorphology, such as island type and reef slope gradient, both of which create variations in how ‘upwelling’ sweeps nutrients from the deep ocean up into the shallows, providing basal energetic resources for higher trophic levels such as corals and reef fish. ‘Areas that have steep slopes have a less pronounced IME than those that have gradual slopes,’ explains Williams. ‘If you have a very gradual slope, it facilitates the movement of water up into the shallows and with it cooler, nutrient-rich waters from the deep.’ He also highlights the impacts of lagoons in low-lying atolls, which often support large bird populations who bring further nutrients. ‘The flushing of the lagoon system will carry all those biologically-derived nutrients out onto the reef, and cause local phytoplankton enhancement.’

The report also studies the impacts of human-induced nutrient addition, such as agricultural and urban runoff. Williams stresses the need for more research into how these affect coral reef ecology, pointing out that while increases in nutrients initially support reef development, after a certain point it appears to do the opposite. ‘When and how humans become a negative effect is yet to be fully quantified,’ he adds.

This was published in the April 2016 edition of Geographical magazine.

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