Still dreaming of where I’d be right now if the pandemic hadn’t locked down the world. So I’ll take you along for a stroll on the beach and a dive underwater on a coral reef in the Caribbean Sea for a little exploration of reef sand.
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Naturally occurring sand on tropical beaches has a backstory filled with life. Unlike the quartz mineral sand we are familiar with on the temperate shorelines of both East and West coasts, the beaches lining coral reefs are largely composed of particles derived from the corals themselves (a rock source is limestone, also biological in origin), along with the shells and carbonate remains of countless other marine creatures. If you look closely at beach sand you’ll see a variety of hues and shapes in the sediment particles.
Reef-building corals extract ions from seawater to create their calcium carbonate structure, which anchors and protects fragile coral polyps from external threats. Over thousands of years living corals continue adding onto an existing reef, with the living layer on the outside near the ocean surface.
Coral reefs are under attack from many directions, both locally (pollution, overfishing, damage by anchors, sedimentation) and globally. The reefs of Little Cayman are fairly well protected from local problems, but are increasingly vulnerable to global threats, especially climate change. Warming oceans and increasing acidification are pushing many marine creatures to the upper edge of their survival window, and beyond. And it’s accelerating. Acidification stresses coral animals by requiring more energy to make their carbonate skeleton.
Meanwhile, reef structure is being eroded, in many places more rapidly than it’s being built. Sort of like the bones of people with osteoporosis. A certain amount of the breakdown of stony reefs to form sediment is mechanical, ie chunks breaking off, rolling around, washing up. But surprisingly a majority of the sand is derived biologically, ie bioerosion. About half is burrowed out from the inside, by sponges, worms, fungi, bacteria and algae.
The other half is scraped off from the outside primarily by urchins, fish and mollusks. Mostly these animals are herbivorous, going after algae on the coral surface but inevitably they chew away the coral skeleton mass itself.
Parrotfishes are primarily grazers but they bite stony corals heartily, swallowing chunks of coral. After extracting the nutrition, they poop out the ground up coral….as sand. The average parrotfish generates half a pound of sand per day, or about 200 pounds a year. Considering there are many thousands of parrotfish on a patch of coral reef — well you can do the math.
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That parrotfish poop is a lot of what you lounge upon when you’re at a tropical beach!
The beaches on Little Cayman Island, where we usually go each April, aren’t the long wide sparkling white Florida type. Those are actually more limestone derived, with sediment sorted and carried quite a distance in longshore currents. On small islands, beaches are shorter, with the sediment more local and less well sorted.
Topside, sand looks very different from where it’s generated and accumulates underwater. Consider this idyllic view of a coral island in Belize surrounded by aqua water. That bright blue color comes from shallow water over coral sand.
Below the surface, the sand is habitat for many creatures. These stingrays and garden eels are just two. There are many many other creatures: fish, nudibranchs, worms, crabs, conchs, etc. Coral reefs have the highest biodiversity of any ecosystem on earth. Some creatures are bright and obvious. Others are more cryptic and subtle. Sand flats are abundant in the latter.
Also in the sand flats are garden eels, plankton feeders who emerge vertically from burrows to feed, descending to hide. You can tell the direction of the current by which way the garden eels are facing. Watch what the garden eels do as I approach a field of them:
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Sand accumulates in flats between ridges of reef, which run roughly parallel to the shore. Sandy valleys and tunnels take you out onto the reefs from the flats. Nurse sharks tuck themselves into soft hiding spots to sleep, like this tunnel through the reef. (Divers call these tunnels “swim-throughs”).
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Out on the reef, the parrotfish are at work,...
grinding up coral that will eventually wash up onto shores.
It’s a balance between the generation of stony coral material and its breakdown by reef creatures large, small and microscopic. As ocean acidification accelerates, the millions of tiny coral animals are having a harder time keeping up, and on many reefs falling behind. Without the hard reef structure, all the multitude of reef inhabitants — on the reef, on the sand flats, in tunnels, in lagoons — will have no home or food or protection. The loss of coral has a cascading effect on the reef ecosystems that depend on them.
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What’s up in nature in your part of the world?