Benthic boundary layer

The BBL serves as a transitional zone between the water column and the sediment layer by regulating biogeochemical processes and the flux of nutrients and organic materials. This zone also serves as the main layer of resistance for the shift of mass, heat, and nutrients from the sediment to the water, or vice versa. It is this area of interaction between the two environments that is important in many species' reproductive strategies, particularly larvae dispersal. The benthic boundary layer also contains nutrients important in fisheries, a wide array of microscopic life, a variety of suspended materials, and sharp energy gradients. It is also the sink for many anthropogenic substances released into the environment as the substances commonly sink to the bottom of the water column.

Life in the Deep Sea Benthic Boundary Layer

The benthic boundary layer (BBL) represents a few tens of meters of the water column directly above the sea floor and constitutes an important zone of biological activity in the ocean. It plays a vital role in the cycling of matter, and has been called the “endpoint” for sedimenting material, which fuels high metabolic rates for microbial populations.

After passing through the BBL, this degraded material is either returned to the water column or mobilized into the sediment, where it may eventually become immobilized. While the supply of POM (particulate organic matter), or marine snow, is relatively limited and inhibits species abundance, it sustains a complex yet understudied microbial loop that can maintain both meiofaunal and macrofaunal populations. In the microbial loop, non-moving benthic organism living in the benthic boundary layer supply nutrients to the loop by releasing unused particles for use by microbial communities. In a study by Will Ritzrau (1996), it was determined that microbial activities were up to a factor of 7.5 higher in the BBL than in adjacent waters. While this study was completed between 100-400m depth, it could have implications for the deep-BBL.

Organisms that live in the benthic boundary layer are known as being benthopelagic. All organisms living predominantly in the benthic boundary layer must acquire their food from falling particles in the water column. Bacterial growth and consumption of falling organic detritus is hindered by the hydrostatic pressure of water and increase in depth. This allows for changeable and consumable matter to reach the ocean flood and be consumed by benthic organisms. The quality and quantity of nutrients reaching the sea floor play a major role in the development of benthic communities. These organisms ultimately play a vital role in the remineralization of matter and aid in breaking down POM that may eventually become permanent sediment. Excluding hydrothermal vents, much of the deep sea benthos is allochthonous, and the importance of bacteria for substrate conversion is paramount.

Presently, it is known that deep-BBL bacterial populations are able to support protozoan bacterivores like foraminifera and some metazoan zooplankton, which in turn can support larger organisms. Meiofauna and macrofauna found in the deep-BBL include: copepods, annelids, nematodes, bivalves, ostracods, isopods, amphipods, arthropods and gastropods, to name a few. The current number of species living in the benthic boundary layer is widely unknown. However, it is theorized that up to 10,000,000 species are living in the BBL. These organisms ultimately play a vital role in the remineralization of matter and aid in breaking down POM that may eventually become permanent sediment.