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Biology

Corals better learn to keep up or they may drown!

Article: van Woesik R, Golbuu Y, Roff G. 2015 Keep up or drown: adjustment of western Pacific coral reefs to sea-level rise in the 21st century. R. Soc. Open sci. 2:150181. http://dx.doi.org/10.1098/rsos.150181

 

Introduction

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Fig 1. Porite microatolls.

Coral reefs are an ever adapting environmental system, whether it be from thermal stress due to increasing atmospheric and oceanic temperatures, increases in nutrient levels due to runoff, and vertical space constraints due to sea level, they are ever dynamic. In the western Pacific Ocean, Porites coral species (a stony coral that grows in finger like patterns (Fig 1)) comprise the primary reef framework. These corals grow rapidly, 1m every 100years (that’s fast for corals!), and despite the increasing ocean temperature and sea level, the massive Porites have continued to grow with fever, while other coral species have slowed their growth and some have even died. Because of the resilience of these corals, reef flats are dependent on these corals for their survival. Massive Porites grow in a very characteristic “microatoll” formation, expanding laterally rather than growing upward, resulting in a flattened ball-shaped colony. But in environments where the sea level is higher, providing more vertical space, Porites have been seen to form more dome shapes, expanding both laterally and vertically (Fig. 1).

Van Woesik et al. set out to take a closer look at the growth patterns of these Porites species in the Western Pacific, primarily looking at how the increasing sea level has affected intertidal reef flats. Can these predominately low, laterally expanding Porite corals keep up with the rising sea level by growing more vertical to meet their light and nutrient requirements?

 

Methods

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Fig 2. The island of Palau

Sampling reef flats in the islands of Palau (Fig. 2) in the western Pacific, the researchers obtained sea level data from a tide gauge over a 45-year time frame. Time series analyses were set up to examine any trends in the water levels over monthly averages as well as yearly high and lows.

Ten sites around Palau were selected for analyses of microatoll growth. Sixty Porites microatolls were measured for vertical extension at each site as well as yearly growth marks. Just like a tree, corals have growth rings that can be examined by slicing microatoll rims and examining them under a microscope. These growth rings were then lined up with the sea level measurements, as well as the ocean temperature estimates.

 

Results and Implications

The growth rates of the Porites microatolls were consistent with previous studies on the Great Barrier Reef, and while large thermal-stress events occurred on Palau in 1998 and 2010, the Porites survived, some bleached slightly and growth slowed, but no Porites species failed to continue growing. This again reflects how resilient these corals are! These corals have proven to fight the battle and have kept up with the rising temperatures and rising sea levels the ocean has thrown at them, but it doesn’t mean that they can continue to do so.

The rate of sea-level rise is projected to continue, estimating a rise between 36 and 81cm by 2100. Because of this rise, anywhere from 1-3°C is expected, which will cause substantial declines in the growth rates of corals. The small, high island of Palau is likely to be impacted by this projected sea level rise, and the Porites microatolls will again be put to the test if they can keep up. Without a substantial coral reef, the island of Palau will be unprotected from storms. Further research is needed to ensure Porites won’t “drown” in the rising sea levels and reef management strategies will need to be put in place immediately if there is any threat of them not keeping up. Let’s hope, for the sake of the island of Palau and for the reef community, that the microatolls adapt and continue to be resilient!

If these corals are unable to keep up, what management strategies would you put in place to help protect them from drowning?

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