James Bell and Valerio Micaroni from Victoria University of Wellington and Rob McAllen from UCC dive deep into the world of Lough Hyne.
Deeper than most divers can safely work and above where most underwater robots are designed to descend are some of the world’s least studied ecosystems.
Between 30 and 150 meters deep is the ocean mesophotic zone, which means medium light. Communities of life exist here on the edge of where photosynthesis can occur. On rocky surfaces in cold water, seaweeds slowly give way to sponges, anemones and sea jets, small tube-shaped creatures that filter plankton from the water.
Sandwiched between shallow and deeper environments, these twilight ecosystems provide food and habitat to the fish and other species that we catch. Lower light levels mean they can feed with less risk of being seen and eaten by predators.
But the distance of these ecosystems to the surface does not exempt them from human influences. Sediments and nutrients from farms and mines obscure the light reaching the seabed, while pots and fishing nets can harm the fragile animals that live in mesophotic ecosystems. Increase in sea surface temperature they are likely to affect these areas in ways we don’t yet understand, as their remoteness makes it very difficult to study.
Surprisingly, one of our best guides to what happens there is found much closer to the surface, in a hidden saltwater lake off the southern coast of Ireland.
Lough Hyne Marine Reserve
Lough hyne It is the only marine reserve in the Republic of Ireland, an ocean protected area, and is home to more than 1,850 species in just half a square kilometer. The lake is more than 50 meters deep, but even in its shallow waters, animals and plants grow that would be more typically found in the mesophotic zone.
Sponges and anemones that are generally found 30 to 40 meters deep are found in the lake at a depth of up to five meters. In research published 20 years ago, we described the wide range of animals that live below the surface on rocky cliffs, including coral mug, wandering locusts and spider crabs. The most conspicuous are the sponges, which way dense gardens of more than 100 species.
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The lake is connected to the Atlantic Ocean by a narrow and shallow channel. The rocky threshold running through it restricts the flow of water in and out, with currents only detectable within the lake during the incoming tide. This relative calm allows sediments in the water to settle and reduces the amount of light that can penetrate.
These conditions, combined with the protected nature of the lake, create shallow ecosystems that would normally emerge in much deeper water. Lough Hyne allows scientists to study the mesophytic without the logistical challenges of working there.
A dramatic change
The lake’s mesophotic communities were thought to have changed very little over decades. That was until a 2016 visit, when we noticed a dramatic change.
On recently published research, we reported how the abundance of sponges in the lake was halved between 2000 and 2018. Slow-growing sponges, particularly the branch-forming species, were the most affected. In some places, the sponges had completely disappeared. Instead, faster-growing sea squirts and dense tufts of algae had proliferated.
These changes were most dramatic where the water currents were weakest in the lake to the west. There has been no constant monitoring of the lake’s underwater cliffs, so it is impossible to say exactly when the change occurred. But based on previous polls and conversations with regular visitors, we think it happened sometime between 2010 and 2015.
It is difficult to be sure what caused the change, whether it was a natural event or the result of human activities. There could have been a sudden increase in the amount of sediment reaching the lake from the surrounding land, or a unusual quirk in lough chemistry, or a sudden change in temperature.
Sponges that live on the outskirts of the lake in shallow water do not appear to have been affected. But we have no idea whether the mesophotic habitats around the coasts of Ireland and Britain, similar in species composition to those of Lough Hyne, have also changed.
Thanks to the support of Ireland National Park and Wildlife Service From the Department of Housing, Local Government and Heritage, we have established new long-term monitoring stations (areas of the seabed that we have marked to return to year after year) on the underwater cliffs, to assess any additional changes. Happily, we are already starting to see new sponges starting to settle and grow.
At this stage, it is unclear if all species of sponges will return, or how long it will take for larger sponges to grow back. As far as we know, the sudden disappearance of sponge gardens on this scale has never occurred on the lake before.
However, our new surveys will help reveal how nearly these unique communities rebound from shocks and allow us to track any future changes and their causes. This will not only help us better manage Lough Hyne, but also other mesophotic ecosystems around the world.
James Bell is professor of marine biology at the Victoria University of Wellington in New Zealand. Valerio Micaroni is a PhD candidate in Coastal and Marine Biology and Ecology, also at Victoria University of Wellington.
Rob McAllen is Professor of Marine Conservation at University College Cork. He is also the research coordinator for the university’s laboratories in Lough Hyne.