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Why is the deep ocean water so mysterious and rich in nutrients?
As we study the oceans, we are increasingly aware of the mysteries that lie beneath deep waters, particularly how these layers of water play a critical role in marine ecosystems, supplying vast amounts of nutrients and biodiversity. This is largely due to an ocean phenomenon called upwelling, which is driven by wind and pushes deep, cool, nutrient-rich water to the ocean surface.
During upwelling, cold water replaces warm surface water, and the nutrient-rich cold water stimulates the growth and reproduction of primary producers such as phytoplankton.
Upwelling areas typically show cooler sea surface temperatures and have higher concentrations of chlorophyll, nutrients that are absorbed by phytoplankton, which in turn fuels the food chain. The rise of these deep waters not only increases the productivity of the entire ecosystem, but also means that a quarter of the global fish catch comes from five major upwelling areas that cover only 5% of the ocean. It is noteworthy that these upwellings are mainly driven by coastal currents or diversions in the open ocean, and their impact on global fish production is quite significant.
Mechanisms of upwelling
The formation of upwelling is mainly driven by three driving forces: wind, Coriolis effect and Ekman transport. When the wind blows in a particular direction over the ocean, the movement of the water causes the water layer to deviate at a 90-degree angle to the wind direction due to a combination of the Coriolis force and the Ekman transport.
The water movement brought about by this process forms a spiral structure, which eventually causes deep water to rise to the surface for replenishment.
In the Northern Hemisphere, water currents are deflected to the right of the wind, while in the Southern Hemisphere they are deflected to the left. Such movement creates a diversion of the surface water, which further triggers the rise of deep water. The rising waters are rich in nitrates, phosphates, and silicates, which are formed by the decomposition of organic matter in the surface waters.
Types of upwelling
The main upwellings in the ocean are related to the diversion of water currents and can be divided into several types, including coastal upwellings, large-scale wind-driven inland sea upwellings, and upwellings associated with eddies.
Coastal upwelling
This is the best-known type of upwelling and the one most closely associated with human activities because it supports some of the world's most productive fisheries. Coastal upwelling occurs when winds blow parallel to the coastline, driving currents that move surface water outward and replenish it with deeper, colder water.
Upwelling areas can account for 50% of global ocean productivity, and their interconnected life cycles make the prosperity of the entire ecosystem particularly evident.
Equatorial and Antarctic upwelling
Upwelling in the equatorial region is closely related to activity in the Tropical Convergence Zone, while the Antarctic is a large-scale upwelling, both of which can effectively bring deep water to the ocean surface.
Upwelling Productivity
Given the productive mechanisms of upwelling, these areas often become the richest ecosystems in the ocean. Upwelling not only enhances productivity, but also attracts aggregations of hundreds of species, making biodiversity a focus of marine research.
These upwelling areas often display a pattern called "waist-honeycomb richness," where species diversity is quite high at the high and low ends, but very little in the trophic layers in between.
This middle layer consists of small pelagic fish, usually anchovies and sardines, which provide a food source for upper-level predators and contribute to the flow of energy throughout the ecosystem.
Threats to upwelling ecosystems
However, these rich upwelling areas face many threats, most notably commercial fishing. Overfishing can have a significant impact on entire ecosystems, especially small fish that serve as middle tiers of the food chain.
ConclusionIf the number of these key intermediate species is reduced, the overall ecological balance will not be maintained, which may eventually lead to the collapse of the entire ecosystem.
The deep waters of the ocean are not only mysterious but also vital, bringing rich nutrients to the ecosystem through the process of upwelling. However, the future of these ecosystems is challenged by increasing commercial fishing and environmental changes. How can we protect these important upwelling areas and ensure their long-term survival?
