Organic matter: does size matter?

The Bentayga is an eddy that rotates clockwise, which affects how the organic matter produced and used by organisms that inhabit the ocean is distributed. Organic matter is made up of myriads of chemical compounds based on carbon atoms, forming carbon-carbon or carbon-hydrogen bonds. It can be divided into dissolved and particulate, depending on its size, and the limit is dictated by the pore size used to filter the water (between 0.2 and 0.7 micrometers).

José Luis filtering water to collect particulate organic matter (left) and an example of a filter with particles from the sediment traps (right)

Dissolved organic matter represents 97% of all organic matter in the ocean and represents one of the largest reservoirs of organic carbon on the planet. The highest concentration is found on the surface ocean, where the effect of the Bentayga is most noticeable. 95% of the dissolved organic matter comes directly or indirectly from phytoplankton, which are the organisms responsible for photosynthesis in the sea, and the remaining 5% from terrestrial or atmospheric sources. The functions of dissolved organic matter in the ocean are diverse, intervening in carbon dynamics in the ocean (Biological Carbon Pump), buffering and contributing to water alkalinity, complexing trace metals or damping the damaging effect of UV radiation on organisms inhabiting the ocean.

Particulate organic matter is made up of all living organisms that inhabit the ocean, and fundamentally, by particles of sizes ranging from micrometers to centimeters. These particles can be found in suspension or sedimentation more or less rapidly in the water column, being their main source waste materials from marine trophic webs. As they are larger, these particles sink, transporting carbon and other elements from the surface to the bottom of the sea, contributing to the Biological Carbon Pump.

In an eddy like the Bentayga we are observing that the main fractions of organic matter are dissolved and particulate matter in suspension and, therefore, the vertical transport of this matter dominates the flow of large particles, in sedimentation by gravity.