How new technology reveals the secrets of a little-researched underwater world

James Bell, Alice Rogers, Francesca Strano and Valerio Micaroni, The Conversation

Credit: Unsplash / CC0 Public Domain

Despite their close ties to the New Zealand oceans, very few will hear of the “temperate mesophotic ecosystem” (TME). Even fewer will appreciate their importance for coastal fishing, and perhaps for mitigating climate change.

TMEs typically occur at depths of 30 to 150 meters, at sunset in our oceans, where there is little sunlight left. But science has begun to shed light on these significant ecosystems and the need to protect them.

Although much research has been done on the deep oceans (greater than 200 m) and shallow seas (less than 30 m), TMEs have received little attention. They have only been recognized as a separate ecosystem for the last 15 years.

TMEs are available to most scientific divers, but relatively small and inexpensive long-distance vehicles (ROVs) now allow greater access to these special submarine regions.

ROVs, such as Boxfish made in New Zealand, can be spread from small boats and equipped with high-resolution cameras and robotic arms to identify organisms and collect grains. We are now able to observe TMEs regularly and our understanding of them is growing rapidly.

What are the rocky TMEs like?

Unlike shallow seas, which are generally dominated by juicy algae that make up the habitat, animals predominate in TMEs.

At best, they allow a mixture of algae and animals, but as they deepen in low light conditions, algae and special animal species begin to predominate.

Animals adapted to low light conditions include sponges, sea breezes, and sea faucets. In fact, recent research in New Zealand has found that sponges can occupy more than 70% of the space available in rocky TMEs.

Given that these ecosystems will be spread across temperate seas, it is possible that sponges may be even more abundant in coastal ocean regions than algae.

Ecological and economic importance

Although little is known about the ecology of TMEs, they are important in many ways for wider coastal ecosystems.

The three-dimensional nature of sponges and other animals that predominate in TME habitats creates structural complexity on the seabed. This provides a habitat for a variety of organisms, from small and young fish to crabs, which are likely to be used by this habitat to prevent predators.

In addition, many fish species migrate between these shallow waters and deeper ecosystems at dusk, probably in search of food and shelter.

TME-dominated sponges filter large volumes of water and are able to trap dissolved carbon and turn it into detritus. For example, small crustaceans and worms can eat sponge debris. These small creatures then eat large organisms (such as fish) above the food chain.

Therefore, TMEs may be very important for coastal fishing.

An assessment of temperature-related changes in depth suggests that TMEs may also be important in mitigating the impacts of climate change, especially sea heat waves that cause extreme sea water temperatures.

At the depth at which TMEs occur, we found that the water temperature is several degrees lower than at the surface, which may be a refuge for mobile fish species in shallow water.

In addition, if the shallower populations are harmed by human activities, the deeper water TME populations may regenerate by giving larvae.

Human impact on TMEs

It is likely that TMEs are affected by the same anthropogenic factors as surface water, which may have a greater impact on certain stressors.

Standing (often slow-growing) TMEs are dominated by many tree-like forms, including sponges and sea winds, making these ecosystems particularly vulnerable to physical disturbances.

Rocky TMEs often overlap with fishmongers that use pots and traps, such as lobsters and crabs. These fishing activities can break and damage sponges and sea winds, and can take many years to recover.

Rocky TME organisms that feed on filters and their proximity to the surface can be affected by the increase in sediment in the water column, which increases turbidity and the amount of sediment that settles in the organisms.

Increased sediment can result from changes in land use in coastal areas, such as changes in buildings or farms, or from drag, dredging, or seabed mining.

Our recent study has shown that few rocky TMEs in the world’s oceans have been explored and characterized. They are even less protected within existing management and conservation frameworks.

In most protected areas, it is often a side effect of protecting shallow-water ecosystems bordering on TMEs.

The diverse and ecologically important communities found in TMEs need greater recognition and protection for the unique biodiversity we now understand.

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Dedicated to The Conversation

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