Monday, 25 April 2016

Coral Bleaching on Inshore Reefs near Cairns

The 2016 coral bleaching on the Great Barrier Reef has been received a lot of attention but I am not sure that anyone has documented what has happened on the inshore reefs and coastal fringing reefs.  In this post, I present my observations of Yule Reef before and after the summer of 2015-6. Yule Reef is part of the coastal reef complex just south of Port Douglas.  At low tide the 4.5 km long reef can be accessed by foot from the Captain Cook Highway which makes it the most easily accessible coral reef near Cairns.  The only equipment needed to get to the reef is a pair of shoes to protect from stonefish.

Yule Reef at low tide, before coral bleaching
Yule Reef, with Captain Cook Highway visible at Yule Point in lower right (Photo 1996, Beach Protection Authority)
Inshore reefs are not as pretty as off-shore reef but like any place with high biodiversity they are interesting.  Yule Reef supports a variety of large marine animals, particularly green and hawksbill turtles. Inshore reefs also function like a giant water filter that can make murky coastal waters clear. On the outgoing tide when clear water from the reef flat spills through channels in the edges of the reef, I used to snorkel between the coral platforms as the enclosed shallow waters were also safe from tiger sharks.

One of the channels between old coral platforms (July 2012) - click to zoom in
Live Corals at Pretty Beach
Closer to the reef edge there is respectable coral in the channels (June 2009)
Live rosette coral at Pretty Beach
Another coral view in a channel at Pretty Beach (about 50 m off the beach and 3 km further south)
I was slow to discover inshore reefs as even back in the 1980’s common people were saying that most of the inshore reefs were dead.  I now know that there has been approximately 1 m of tectonic uplift in the previous 6000 years and that this may have thrust the tops of reef up above the level of reef growth.  The channels in the reef and the outer edge are where most of the life is.  Corals on inshore reefs are also predominantly grays and brown rather than the riot of pastel colours seen on the outer barrier and this may have lead to the idea that the reefs were dead.
Soft corals at low tide on an inshore reef
Dense soft corals at the head of the shallow channel where people enter the reef from the sand flats (July 2009)
Many inshore reefs have parallel coral platforms tens of metres long and a few metres wide that point seaward like widely spaced piano keys (the streaks in the aerial photo). The tops of the platforms are brown and sometimes muddy but have their own life forms. Coral cover on the sides of the channels was high, although only a few coral species were present. At low tide, rays, turtles and large fish such as metre long Australian salmon are often present in the channels presenting an opportunity to observe these animals.

Coral platforms and filtered seawater, Yule Reef
Coral platforms that appear as a dark fringe near the seaward edge of the reef on the aerial photo.
Describing what the reef was like before the 1998 coral bleaching is very hard.  When I first visited these reefs there were no affordable underwater cameras and GPS did not exist so making a good record was out of reach.  Even aerial photography was hard to come by.  To remember where places were, I would try to line up coastal features with mountains, a method that was very approximate at best.  Added complications included persistent rough conditions  that would muddy the water and make navigating around reefs difficult and the presence a wide range of potentially lethal animals including tiger sharks, crocodiles and box jellyfish.  Seasonal growth of Sargassum weed can also cover much of the reef with floating towers of brown seaweed over 2 m tall.

Now I use a GPS (Garmin 64s) and an underwater camera and on my last day trip to the area, captured almost 700 photos.  Geotagging software is used to geotag the photos with the GPS trail, which results in a better than 3 m geographic precision.  In-camera GPS units are far inferior to hand held GPS units so I overwrite any coordinates recorded by the camera.  Now, I have a record that rivals the environmental monitoring undertaken by some scientific organisations.

Starting with my old memories from the time before the first coral bleaching in 1998, the reef edge near the mouth of the Mowbray River was a slightly threatening place.  Swells would swash between exposed dark brown living heads of coral that rose up approximately a metre from the bottom and stood more than half a metre exposed between waves.  These coral heads, which were shaped like tiny mountain peaks formed an irregular row with roughly 10 m spacings.  With care I could navigate around them with my 12 foot boat and motor.  The heads of living coral may in part be maintained by the splash of waves allowing the coral to grow taller than water levels usually permit.  I wonder if these tall raised coral colonies are still a feature of today’s reef.

A coral head on Little Reef, which is near Yule Reef (June 2009)
Wentworth Reef which is further offshore was a complex of metre high coral ridges covered with low brown seaweed and separated by sinuous sandy channels.  Where the edge of the reef dipped away to the sea floor which was too deep to see given the water clarity, there was a diverse continuous coral cover on the slope.  Large fish were virtually absent and I put this down the accessibility of the reef leading to overfishing.  Coral colonies were frequent and were attended by schools of small reef fish, mostly of the yellow and grey varieties.  Inshore reefs never were as exciting as the outer reef and to be fair, I did not want to swim in the low visibility waters on the reef edges where the best corals were.

Typical underwater view on Wentworth Reef (August 2013)
Heading back to the Mowbray River from Wentworth Reef, I found a slightly deeper reef just out from the main part of Alexandra Reef.  Snorkelling over this reef revealed an almost level surface of living massive coral that was so extensive that dragging a dinghy behind me, I did not have the energy to reach the other side.  In all that distance, probably more than 150 m, I did not observe a single gap in the coral cover.  It would not have been possible to slip a razor blade between the living coral colonies.  At the time, it was considered that a healthy reef would have 50% coral cover and 50% coralline algae cover and as I swam across the reef, I was thinking about how this reef absolutely defied that pattern.  Within the flat mid-brown surface at metre spacings were fist sized holes which were the last vestiges of gaps between colonies.  These gaps provide the only refuges for the small fish that were swimming around above the coral surface.  The reef was completely devoid of larger fish and had no brown macro algae at all.  I don’t know if this reef ever becomes exposed at low tide, but if it did it would have been a struggle for fish to survive on the reef during these periods.

A tiny patch of continuous coral cover showing how there is no space between colonies (July 2012)
I do not know if this reef survived the first coral bleaching in 1998.  It was boring by coral reef standards but from today’s perspective is was a living miracle.  It was a massive seamless monoculture of coral that was only limited by the depth of the available water.  A smaller example of the same type of reef was also present on a small south-facing bay at the far eastern end of Dunk Island (near mangroves).

I have looked for mono-culture brown reef a few times since the 1998 coral bleaching but finding even large reefs that are close to shore can be difficult.  On a day with a mirror-like surface I once failed to find a reef that was less than 1000 m from shore, even though I briefly saw the reef under my boat, the reflective surface defeated my attempts to relocate the reef and the next day, I bought my first GPS out of frustration.  When I searched Alexandra reef for the live coral surface, I found a thick forest of Sargassum which was over 2 m tall.  There was almost no live coral below the sargassum, just an irregular dead coral surface covered with a film of mud.  Very small massive corals were here and there and I put the survival of these corals down to being on vertical surfaces which resist the settlement of sediment.  Now with aerial photography, I have located a small, slightly deeper reef just the east of Alexandra Reef, I need to check that reef as it may be the reef with the living coral surface I once observed.

Sargassum at Yule Point, near Mowbray River
Sargassum dominates the shallows between the coral platforms near the reef edge and the green seagrass beds on the landward margin of the reef
Tall growths of Sargassum are a seasonal feature of inshore coral reefs and often form a single line close to the transition point from shallow to deeper water.  I cannot say whether the massive growths that now blanket inshore reefs during summer used to be present before the first coral bleaching to the extent they are now.  I also cannot say whether Sargassum competes with coral or protects it.  In deeper water with better water flow, Sargassum may shade the coral and help protect it from bleaching.  In other locations, I suspect that Sargassum alters the environment in ways that are not favourable for coral.  Some research is needed.

Sargassum laying down in a shallow lagoon at low tide (April 2016)
Did this coral survive due to its wig of Sargassum?
Turtles were sometimes abundant and one day I counted 30 turtles on a 4 km section of reef front.  When turtles are surprised by an object shaped like a shark, they present their back to the shark’s mouth as a turtle's back is its shield.  To angle themselves in this way, they actually have to swim up into the path of the moving object.  In this case, the object was my competition surf ski and I had the unusual experience of turtles sometimes rising from the bottom and deliberately colliding with the front of my craft.  With one turtle, I paddled backwards a few metres, changed direction and set off again to miss the turtle by a few metres only to have the turtle swim into my craft a second time.  Other large wildlife includes dugong, which are almost impossible to sneak up on and you only know they are close when you see metre wide patches of smooth water forming on the surface that indicate a dugong has swum below.  Large predatory fish such as Australian salmon and sharks swim through the shallow waters and stingrays of many species were usually present in large numbers.  I actually stopped swimming in the shallower sandy channels due to the numbers of stingrays in them and this was long before the Irwin tragedy.

After Cyclone Yasi destroy feeding grounds, many exhausted Green Turtles would beach when the tide retreated (July 2012)
By 2012 most of the reef appeared to have recovered from the 1998 bleaching.  The only element that I have not seen again is the massive coral surface and that is possibly due to navigation issues. Some off-shore reefs that were stunning were laid waste by the 1998 bleaching to the point of being unrecognisable so to see that a reef had recovered was really encouraging.

Well before the event, the 2016 coral bleaching was predicted.  Satellites had identified a vast pool of hot water in the eastern pacific ocean and from past experience, it was known that this pool of water would stream across the pacific to affect the Great Barrier Reef.  The 2015-6 wet season was poor and the skies often crystal clear.  On kayaking my wet hands would be cooled by the breeze but when they dipped into the water, the water was as hot as bath and beyond anything I had experienced.  I heard from others that Yule Reef had gone white so I missed the onset of the event and perhaps have only captured the tail end.

On walking out onto the reef flat, the most obvious change were large patches of white coral.  White coral is dying coral which will either die or if conditions suddenly change for the better, recover.

One of the first corals I saw was completely bleached with more exposed parts already dead
In April 2016, Yule Reef was dominated by patches of stark white in a brown Sargassum matrix
The stark white corals are eye-catching however a second view shows than some corals were not bleached
The area shown above was in or near a shallow lagoon behind the coral platforms of the main reef.  Just to the north is a strip of fringing reef where I discovered that fewer people walk and the coral was in great condition (seen on left of aerial photo).  The entire reef flat was covered with a gray turf of living coral.

Living coral grew like a lawn up to the upper limit of growth (July 2012)
Pre-bleaching coral cover, Yule Reef
A close up of the coral lawn showing two species of branching coral
On revisiting the reef in April 2016, at first sight the reef looked the same. However most of the larger tracts of coral on the reef flat are completely dead.  Some smaller patches of coral in this area still struggling to survive.

The coral is still standing but is dead
A close-up view shows brown slime rather than star shaped coral polyps
Corals in areas with better water flow survived at a least in part
Massive corals (species that grow like boulders) are also present in lower densities and some appear to be fine but others have taken on strange colours.  For massive corals, it seems preferable to be exposed to air, rather than be in the hot waters of the reef shallows.  In the shallows, may corals are not pure white but a florescent light green or yellow.  There were no simple patterns to the bleaching and unaffected corals were scattered through devastated areas.

Stressed massive corals were an abnormal pink colour
Same species of coral bleached to a florescent light green shade
It is clear that huge damage has been done to Yule Reef with coral mortality above 50%.  Inshore reefs regularly take a beating from cyclones and floods so have great powers of recovery, however if the frequency of events increases, there will not be enough time to recover.  There may also be a breakdown of essential processes such as the processes that provide clean hard surfaces for coral larvae to settle on.  To some extent, the process that create good conditions for coral are provided by the coral reef itself.  So Yule Reef survives for now, but it is a diminished place and is on the edge of survival.

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