Analysis of a comprehensive database has revealed strong links between biological productivity in the surface oceans and patterns of biomass and abundance at the seafloor, helping to explain large regional differences.
The vast majority of the biological production in the world’s oceans occurs within sunlit surface waters – the “photic zone“. Through the process of photosynthesis, tiny marine plants called phytoplankton use the energy of sunlight to build the carbon-rich molecules needed for growth. When they die, a proportion of this organic matter sinks to the ocean depths, where it is used as food by all manner of deep-sea life.
“With the exception of chemosynthetic communities living around hydrothermal vents and other chemical energy sources, deep-sea ecosystems are entirely dependent on the supply of organic material from miles above,” said Dr Brian Bett one of the researches from the National Oceanography Centre (NOC).
“An important question is whether variation in food supply explains patterns of regional seafloor biomass, or whether other oceanic factors such as seafloor relief and bottom water characteristics are responsible,” he added.
Seafloor relief is the terrain or topography of the seafloor.
To help address the issue, the researchers assembled data on the biomass and abundances of species living at the seafloor in different regions.
According to their analysis, seafloor biomass is highest around the poles, and on continental margins, where upwelling waters supply nutrients needed for phytoplankton growth in the photic zone. Values are also high near the equator where the interaction of wind-driven currents causes upwelling. In contrast, they found consistently low seafloor biomass on the central abyssal plains of the major ocean basins.
The dominance of different groups of organisms in terms of their abundance or biomass depends on seafloor depth. Smaller-bodied organisms tend to dominate the food limited depths of the ocean.
Understanding patterns of seafloor biomass, National Oceanography Centre