A project undertaken at the School of Biological Sciences, Victoria University of Wellington, New Zealand, and supervised by James Bell
Temperate mesophotic ecosystems (TMEs) generally occur between 30 and 150 metres, at the limit of photosynthesis, and represent a transition zone between deep and shallow ecosystems. Unlike shallow water rocky ecosystems, which are dominated by seaweed, mesophotic ecosystems are primarily dominated by animals. Although TMEs were first recognised as distinct ecosystems over 20 years ago, their role in supporting fisheries remains largely unknown.
In New Zealand, as part of this project, we found that sponges dominate rocky TMEs, covering over 90% of the substrate in some cases. Earlier research on coral reefs has demonstrated that sponges recycle dissolved carbon—such as amino acids, proteins, cellular material, and very small particulates—into detritus, which is consumed by detritivores. This process is known as the sponge-loop. It has been assumed that these detritivores are then consumed by organisms higher up the food chain. However, whether sponges perform the same role in colder temperate waters, where they are often much more abundant, remains unknown. The primary aim of this project was to understand the role sponges play in TMEs in moving carbon from the water column, through sponges, to detritivores on the seafloor, and then further up the food chain.
In the first part of this project, we measured how much dissolved and particulate organic matter (larger food particles between 2 and 20 micrometres) is consumed by sponges. Our focus was on TMEs in Fiordland, Wellington, and the Poor Knights Marine Reserve in New Zealand. We found that sponges consumed substantial amounts of particulate food, consistent with earlier studies, as well as dissolved carbon.
We then examined the detritus production rates of several abundant temperate sponge species to determine whether they produced detritus. We found that all the species examined produced detritus, although the amounts varied significantly between species. When scaled up in sponge-dominated ecosystems, sponges produce kilograms of detritus annually per square metre of the seafloor. To track sponge food sources through to detritus, we labelled dissolved and particulate matter using enriched, naturally occurring stable isotopes. We successfully detected labelled carbon in the detritus, confirming the presence of a sponge-loop in temperate species. Observational studies in the laboratory with several known detritivores provided evidence that small (<3 cm) triplefins feed on sponge-derived detritus. Triplefins are among the most abundant benthic coastal fish species and are highly likely to be consumed by larger fish species higher up the food web.
This work highlights the crucial role of sponges in TMEs, particularly in transferring food from the water column to the seafloor. The detritus produced by sponges provides a food source for a variety of detritivores. Future work will investigate whether other species feed on sponge detritus, including the small (millimetre-sized) organisms living within and between the sponges, and whether these are also consumed by fish. Ultimately, we aim to trace sponge-derived detritus to the highest levels of the food chain.