Reposted from Jennifer Marohasy’s blog
January 20, 2021 by Jennifer
At the end of last year I went on an expedition looking for a type of very old coral known as massive porite. Stuart, Shaun and I dived five very different coral reefs between Cairns and Townsville.
The first was a coastal coral reef on the leeward side of High Island. High Island is near mainland Australia and not far from the Russell and Mulgrave Rivers, which drain a drainage basin of sugar cane farms. More than almost any other coral reef along the Queensland coast, this coastal reef would likely have an impact on agriculture – if any.
High Island with its coral reef.
This coral reef has a very wide and long ridge. On the afternoon of September 28th when we visited, this habitat was covered with maybe a meter of water. On the seaward edge of this coral reef, the ridge gives way to the reef slope where we found so many massive pores – so many of the ancient corals we were looking for.
The water was quite cloudy, which is common on these coastal reefs depending on the prevailing wind.
On the reef slope, many of the porites (those massive bolder corals) were more than two meters high and wide and were golden in color. There were lots of small fish. I found a pen that might mark a porit that was gutted more than 17 years ago. The Australian Institute for Marine Science (AIMS) once gutted corals on this reef.
Once upon a time, scientists drilled into the corals and extracted a core from which they created a hatch. Then they counted the annual bands under X-ray, the different lengths and thicknesses of each band being a measure of the annual growth rates. Yes, these massive bolder corals have annual growth rings – like tree rings.
Then we found another pen that might mark a different one of the corals that were previously gutted. However, it was not our job to count the pegs, but to measure the height and width of the porite – the ancient corals.
The massive pores at the beginning of the reef slope had a uniform height. This is because their vertical growth is limited by the sea level. Coral colony growth rates slow down when they are within 2 to 3 meters of the surface. As a result, the annual growth streaks of these porites on this reef may not be a good indicator of climate change as they are so constrained by sea level.
Our expedition was looking for porites, but not porites: porites suitable for coring to determine the effects of climate change on coral growth rates.
The porites grow to a uniform height just below sea level.
In every place, the sea level changes with the tides. The sea level, the tides, can vary by 3 meters on this day on this reef. And the lowest tide in the 18.6-year lunar declination cycle can cause a tide 30 cm lower than the lowest tide in an average year. (More on this in our new book ‘Climate Change: The Facts 2020’, see page 267, and there is also a factsheet about tides that fall on corresponding El Nino events on the book’s special website, see Figure 4 under the second Link.)
The tides and so much other thing that happens on coral reefs have a lunar / lunar influence – both the good and the bad. Coral spawning occurs exactly 5 days after a full moon, and the worst coral bleaching events, which occurred in 1998 and then again in 2016, correspond exactly to the 18.6 year lunar declination cycle. If the massive pores on the coral reef on the edge of High Island weren’t constrained by sea level, they would be more domed. But as they get closer to the surface, their tips are bleached on the lowest tides and then die off, making them shallow.
The crumbling tip of a massive porite that grows on the slopes of this coral reef.
As the reef crest grows seaward and the coral colonies that are now growing on the slopes are built into it, these massive porites can eventually grow into a donut-like shape known as a micro-pit. A microatoll has dead coral in the middle and is surrounded by a ring of live coral.
There are so many “donuts” – porites microatolls maybe two meters in diameter – in the aerial photos over the top of the reef on High Island. These corals may have been incorporated into the reef crest when the coral reef was expanding seaward, and therefore may be six feet deep, or the coral colonies / Porites microatolls may have fairly shallow bottoms after they started their lives on the existing reef have platform and only ever had the option to grow sideways because they were always constrained by the sea level.
The High Island reef crest from 10 meters high. The High Island reef crest from a height of 120 meters.
From a height of 120 meters above the reef crest on High Island, and indeed above most reef crests, everything – apart from the donuts – mostly looks pretty monotonous and bleak. At about this height, the chief scientist at the ARC Center for Coral Reef Studies at James Cook University has determined that more than 50% of the Great Barrier Reef is bleached. His reports make newspaper tops all over the world, and so most people today believe the Great Barrier Reef is half dead.
To be clear, Professor Terry Hughes flies over hundreds of reef crests, looks out the airplane window, and rates them. If more than 60% of the corals look dead to him, the reef is given a score of 4.
Apart from the fact that at this altitude at which the plane flies, there are actually no individual corals to be seen – beyond the large donuts, which are Porites-Mikroatolls.
While it’s impossible to see the tiny corals and colorful fish at this altitude – from this distance – there were as many delicate and pretty corals and even pretty blue fish on this reef crest as you can see in the transect photos. (There is a new webpage with the photographic transects and a bit more. It will hopefully become the data page for the High Island Reef, which will hopefully contain species listings and analysis of coral cover and health.)
The chief scientist claims to have brought his results from the aerial overflights to the ground by laying a ten-meter-long belt transect over part of the reef crest. We laid such transects on November 28, 2020 on the High Island reef crest. We also placed transects at a depth of 5 meters. We plan to go back and relocate transects at 10 meters.
Photo of the first transect 2 meters from the top of the reef on High Island.
If I only look at the first 10 meter section from the reef crest on High Island: At 2 meters I can see such a pretty beige Lobophytum pauciforum coral. The tentacles extend along several branches. From this I can conclude that it is a healthy coral.
Photo of the 5 meter transect on November 28, 2020 on the High Island reef crest.
But what about the coral 5 meters away along this photographic transect? It looks bleached! But maybe this is actually a soft coral, maybe Sinularia polydactyla, a type of coral that never has zooxanthellae and therefore cannot bleach.
Photo from the 10 meter high transect, taken on November 28, 2020 at the reef crest of High Island.
But the last photo in this transect at 10 meters: the corals are almost all dead and covered with algae. However, there are two young noble porites that emerge from the destruction and could eventually become microtolls. After a hundred years, these two pink noble things could reach a meter in diameter and have a donut shape.
While taking the transect photos at this reef crest, Shaun found and photographed a blue lionfish, Pterois sp ..
A lionfish on the High Island reef crest.
While Shaun was taking the photos, Stuart videotaped the corals along the tape measure, along the cuts. This video transect offers a more continuous and wider view of the corals along each of the ten meter long sections of the reef. How would you rate this reef crest if you were only looking at the last three of the ten meter long sections of the video transect that came from the reef crest?
Given the great variability in coral cover and coral health, as well as the diversity of coral species and only a 10 meter transect, it is quite difficult for me to make such a judgment.
Science is not a truth. It is a way to get to the truth.
Using the method used by the Chief Scientist, we only need to choose one of five categories for this reef. According to his method, the results of which are regularly reported around the world, you must choose one of the following categories:
<1% dead, 1-10% dead, 10-30% dead, 30-60% dead, >60%.
So what is your decision! Is this reef crest 10-30% dead?
Perhaps it would be easier if you just looked at the reef crest from 120 meters up and made a decision based on your impression of the corals from that height, as the chief scientist does.
The High Island reef crest from a height of 120 meters.
So what’s your decision Can it be concluded from this height that this reef crest is more than> 60% dead? Would this be a fair assessment and would it then be fair to conclude that the entire ecosystem on this marginal coral reef is more than 60% dead?
Indeed, I think there are major problems with the methodology that Professor Hughes uses to determine the condition of our coral reefs. I feel it is inappropriate to categorize the condition of corals from such a great height from more than 100 meters away. I think the professor should go in and under the water.
If there is one thing that high altitude aerial photography is useful for, besides being able to find Porites microattoles on reef crests, it may differentiate the different habitat types on coral reefs. From the air over High Island it is easy to see that the reef crest is very different from the reef slope.
The reef crest ends at the reef slope, which is along the sea side of the high island coral reef. This antenna was captured on November 28, 2020. Our 40 foot boat offers a certain size, you can see roughly half of it in this antenna.
The reef slope is the section that always falls to the sea floor. The most productive growth and biodiversity on a marginal coral reef is typically down the reef slope. I wonder how far down the porits and other corals grow on this reef. I wonder how far from the reef crest is the reef slope from the ocean floor?
Massive porite towards the top of the reef slope on High Island, photographed November 28, 2020.
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Stuart Ireland took all aerial photos and filmed the transect video. Shaun Frichette took all the transect photos. These provide some records of the health of this coral reef on the edge of the high island for this moment, on Saturday, November 28, 2020.
There is a High Island data page with the transect photos and I hope to add species lists and more to this page: https://jennifermarohasy.com/coralreefs/high-island/
Part 1 of this series concerns the Pixie Reef north of Cairns and can be found here: https://jennifermarohasy.com/2020/11/measuring-old-corals-coral-reefs-part-1/. Like High Island, Pixie is so close to Cairns, but such a different reef, perhaps partly because it doesn’t border a continental island.
This website is archived each year by the Australian National Library:
https://webarchive.nla.gov.au/tep/66941
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