Stirring stuff
It might be tempting to think of the ocean as a huge garden pond: a vast, still expanse of water with surface ruffled by wind into waves. However, the waters of the ocean are constantly moving, from the surface right down to the abyssal depths, kilometres below. The perpetual movement of these currents is a life support system for marine life, directly supporting the microscopic phytoplankton upon which the marine ecosystem depends.
Phytoplankton have few requirements to grow and proliferate. Adequate sunlight and nutrients would allow them to double their population as fast as once every day if they weren’t in turn being consumed by other organisms. However, sunlight is strongly absorbed by water and only the top 100m of the ocean receives enough sunlight for phytoplankton to grow. Furthermore, their ability to grow quickly means that much of the world’s oceans have been stripped of nutrients in these surface waters. Unless the waters can be ‘fertilised’ the phytoplankton would die out. The ocean’s currents perform the vital task of carrying nutrients up to the surface from depth, where sinking dead material has gradually decomposed. However, there is no single nutrient conveyor pipeline to the surface. Ocean circulation systems vary widely, from those spanning entire ocean basins to those spinning over just a few metres. A fundamental and hotly debated question of oceanography is which type of circulation is most important in fertilising the surface waters.
Circulations spanning oceans
It should not be thought that nutrients fuelling phytoplankton in the waters off the UK were formed by organic matter decomposing in the waters deep below them. There are currents that span whole oceans (see our pages on ocean circulation), such that nutrients used by phytoplankton in the North Atlantic may have been formed in the deep waters off Antarctica.
Winter convection
The dropping air temperature in winter cools the ocean surface to the extent that it contracts, becoming denser than the water below it. This drives vigorous vertical mixing until the balance of light water over dense water is re-established. This convective mixing, as it is termed, can penetrate hundreds of metres into the ocean. It can consequently mine the deeper waters for nutrients, by mixing them up into the surface. As the impact of cooling varies with latitude, so the depth of winter convection varies with latitude. Hence, this process is potentially a much greater source of nutrients further from the equator. As the air temperature plays such a critical role in this, this path for nutrients is potentially sensitive to predicted global warming.
Ocean weather
The ocean shares many phenomena with that other Earth-spanning fluid, the atmosphere. We are all too familiar with fronts and storms from weather forecasts. The ocean has similar circulation patterns, but the higher density means that the "storms" in the ocean are smaller. They are typically 100km in diameter in contrast to those of 1000km in the atmosphere. The manner in which the eddies (mini-storms) and fronts in the ocean supply nutrients to the surface is one of the most contentious aspects of the problem. Some scientists argue that nutrients are primarily ‘pumped’ to the surface in the centre of eddies. Others maintain that perturbations in the otherwise circular currents comprising eddies are chiefly responsible. Another view is that the tight ‘jets’ and strong currents formed by eddies squeezing together are dominant. This is an area where ocean modelling provides us with the most information as it is exceptionally difficult to measure these phenomena.
