Sundried Tides: Timing Is Everything

While predictable, extreme high and low tide events are a naturally occurring disturbance in coastal marine ecosystems. In fact, tidal variation is the driving force behind most marine coastal zonation. Extreme high and low tides, or spring tides, on coral reefs have rarely been recorded as the cause of mass mortality. However, in years when extremely low tides coincide with high mid-day sun exposure--an event referred to as sundried tides--they have the potential to cause widespread damage.

A group of Australian researchers working on the Great Barrier Reef have studied and documented this effect that can silently wipe out coral reefs. Their analyses, published in the journal Marine Biology, have revealed for the first time that these highly predictable events can seriously impact the state of coral reefs at a time when they are preparing for the thermal stresses of summer as well as storing precious energy needed for spawning. During a sun-dried tide, 40-75% of the major coral taxa were either bleached or partially destroyed.

Ken Anthony and Ailsa Kerswell, of the ARC Centre of Excellence for Coral Reef Studies have revealed that extreme low tides on clear sunny days can lead to widespread damage of coastal coral colonies. Dr. Anthony explains,

Really low tides, where the local sea level gets to its extreme low for the year, can occur at different times of the day. In years where this occurs during the middle of the day when the sunlight is at its most intense and the reefs are almost fully exposed, there is a real risk of severe coral stress and death in the shallow reef zone. Just like cyclones and other natural disasters, these severe ‘sun-dry tides' rarely occurred since they relied on the alignment of numerous natural extremes. However, when these factors all aligned, by a combination of sun, moon and chance weather, an extreme event occurred which could leave coral colonies bleached and devastated.

So what can be done to protect already threatened reef areas that are vulnerable to inevitable sundried tides? Not a lot. Tour operators on Australia's Great Barrier Reef may be encouraged to help shade coral from the sun when the reef is exposed at extremely low tides, Dr. Anthony says. Temporary sunshades for small parts of the reef could help coral fight the effects of a sun-dried tide. It's not such a silly idea, Anthony says. There is no real way you can prevent corals coming out of the water and the Sun shining. But tour operators could provide shade; if you can shade out light you can save the coral.

Dr Anthony hopes that their predictable nature will also lead to improved warning systems and better models for predicting stress and mortality in corals. Dr Anthony elaborated, If we better understand the timing and severity of natural stressors on reefs, we will be able to better predict the risks of human-induced stressors, and hopefully better manage for healthy reefs.

Source: ARC Centre of Excellence for Coral Reef Studies