Cold Water Diving: How Sea Surface Temperature Influences Coral Reefs

Cold Water Diving

Shortly after sunrise the main dive boat, Calcutta, set out from Golden Shadow to survey a gently sloping spur and groove reef system on the south eastern side of Ill des Pins. The dive team and crew hunkered down out of the cold sea spray and wind for the forty minute transit out to the first dive site. It is noticeable how much more bundled up the scientific divers are on the New Caledonia trip compared to recent expeditions. Water temperatures recorded on dives frequently drop below 23°C (73°F). While this may sound warm, the water is actually relatively cold by tropical standards, and rapidly saps body warmth during the long days of survey. Hoods and layers of thick wet-suits are the order of the day, while one diver has resorted to a dry-suit to ensure comfort while he works – gear typically reserved for temperate waters. After cold water diving, researchers rapidly remove their wetsuits, towel off, and don dry clothes or wind breakers to elevate their core temperature. Positions in the sun are at a premium on surface intervals during cold water diving as divers do all they can to get warm before the second dive.

Cold water diving: Members of the dive team clad in layers of wetsuits, hoods, and windbreakers – cold but happy (left to right: Andrew Bruckner, Alex Dempsey, Kate Fraser, Dawn Bailey, and Ken Marks)

How Sea Surface Temperature Influences Coral Reefs

As we move from Ill des Pins to the atolls and reefs north of the main island (Grand Terre) in the coming weeks we expect to see temperatures rise by several degrees. This thermal trend provides a natural laboratory for studying how sea surface temperature influences coral reefs. It will be interesting to compare the biological communities in the northern reefs to those around Ill des Pins. Understanding the relationship of coral species to temperature is important. The trend of rising sea surface temperatures is predicted to continue under most climate change scenarios, and species with narrow thermal tolerance may change their distribution to cope with the changing thermal gradient.

Many algal species such as the green algae Caulerpa macrodisca identified during the surveys of Dr. Claude Payri are more commonly associated with temperate reefs to the south, such as around New Zealand, yet are found in Ill des Pins. We expect this species will be rare or absent in the reefs we will visit north of Grand Terre.

One example of how sea surface temperature influences coal reefs: Green Caulerpa algae are often found in reef environments. The fern-like Caulerapa taxifolia may be found in many regions of New Caladonia, the disk like algae Caulerpa macrodisca (just left of center) is only associated with the cool waters in the south of Ill des Pins.

We also see differences in the coral community during our cold water diving. Coral species such as the Leptoria phrygia pictured below have been common on our surveys. On the Global Reef Expedition we have tended to see this coral at deeper sites where the temperature dips below ~26 C.

Leptoria phrygia is common in colder locations such as New Caladonia.

These are just a few examples we’ve seen during our cold water diving of how sea surface temperature influences coral reefs.

Photos by: 1 Ken Marks; 2 Andrew Bruckner; 3 Alex Dempsey

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Coral Bleaching Awareness Month: Giving Coral Reefs a Chance to Recover

For hundreds of millions of years, coral reefs have supported life in our oceans. Although they cover less than 1% of the ocean floor, they provide habitat for at least 25% of all marine species. They also support people around the world by sustaining fisheries and tourism, providing food and livelihoods, and helping protect coastlines from waves and storms.

But coral reefs are under increasing pressure. Pollution, overfishing, coastal development, physical damage, and increasingly warm ocean temperatures can all affect reef health. One of the most visible signs that a coral reef is under stress is coral bleaching.

This September, in recognition of Coral Bleaching Awareness Month, we’re taking a closer look at what happens when corals bleach, why bleaching matters, and what scientists around the world are doing to better understand, protect, and restore coral reefs.

Everything You Need to Know About the 2027 Science Without Borders® Challenge

Are you ready to combine your creativity with ocean science?

The 15th Annual Science Without Borders® Challenge is now open! Whether you’re participating for the first time or returning for another year, this guide will help you get started.

This year’s theme, Navigate With Care, invites students to explore how responsible navigation helps protect both people and life in the ocean. Through your artwork, you can compete for scholarships and international recognition while inspiring others to protect both people and life in the ocean.