Density currents are critically important to life in the deep sea primarily because they are a major mechanism for transporting essential resources and dispersing organisms in a largely dark and food-limited environment.
Density currents, also known as thermohaline currents when driven by differences in temperature and salinity, are deep-ocean currents that form when surface waters become dense enough to sink. This sinking is often triggered by cooling and/or increased salinity (like during ice formation in polar regions). These dense waters then flow along the ocean floor, influencing vast areas of the deep sea.
Drawing from the understanding that "Currents are important in marine ecosystems because they redistribute water, heat, nutrients, and oxygen about the ocean. At the same time, currents inevitably sweep over and carry off living organisms," we can see several key roles density currents play in the deep sea:
Key Roles of Density Currents in the Deep Sea
1. Redistribution of Essential Resources
- Nutrients: The deep sea is far from the sunlit surface layer where photosynthesis occurs. Life relies on organic matter and nutrients that sink from above (marine snow) or are brought in by currents. Density currents are major conveyors of nutrients and dissolved organic matter from productive areas or the surface to the deep-sea floor, fueling chemosynthetic communities near vents or providing food for filter feeders and detritivores.
- Oxygen: Unlike surface waters that are oxygenated by interaction with the atmosphere and photosynthesis, the deep sea relies on the transport of oxygen from the surface. Density currents form from surface waters that are rich in dissolved oxygen. As these dense, oxygenated waters sink and flow across the abyssal plains and through deep trenches, they replenish oxygen levels, preventing anoxia (lack of oxygen) which would be lethal to most deep-sea organisms.
2. Transport and Dispersal of Organisms
- Larval Transport: Many deep-sea invertebrates have larval stages that live higher in the water column or can be swept along by currents. Density currents can play a role in the dispersal of these larvae, helping populations spread and preventing genetic isolation.
- Scavenger Attraction: Carcasses of large marine animals that sink to the deep-sea floor (whale falls) provide significant but localized food sources. Deep currents can carry the scent plumes from these falls, attracting highly mobile deep-sea scavengers over vast distances.
- Carrying "Marine Snow": As the reference notes, currents can "sweep over and carry off living organisms." While often referring to plankton, this also applies to marine snow (dead organic matter, fecal pellets, etc.). Density currents contribute to the distribution of this crucial food source across the deep-sea floor, supporting benthic communities.
3. Habitat Shaping and Sediment Transport
While not directly from the provided reference, density currents are known to influence the physical environment of the deep sea. Their flow can sculpt sediments, creating ripples and other bedforms, and transport fine particles, affecting the types of habitats available for benthic organisms.
In summary, density currents act as vital arteries of the deep ocean, connecting different regions and transporting the fundamental requirements for life – oxygen, nutrients, and food – as well as facilitating the movement and dispersal of deep-sea inhabitants in an otherwise isolated and challenging environment.
