When the river meets the ocean: an amazing estuary adventure

All rivers flow into the sea, which not only refers to the fact that all rivers flow into the sea, but also has the implication of being the destination of public expectations, because what converges with the rivers are not only fresh water and mud, but also merchant ships and merchants floating downstream.

The estuaries of large rivers are often important ports, where inland products gather and are taken on ships to go out to sea, and where marine fish are landed and distributed inland. San Francisco in the United States, London in the United Kingdom, Tokyo in Japan, and Shanghai in China are all centers of estuary-based ocean-going trade.

Shanghai Port, the largest hub port on my country's coast. Image source: China Port Network

However, in the Earth system, estuaries are not only the hub where rivers and seas meet, but also the hotspots where land and sea interact. So, how big is this area, and what are the characteristics of the organisms and environment in the area?

Part 1

The rare yellow-blue dividing line

At the mouth of the Yellow River, you can sometimes see a yellow/blue dividing line, which simply means that there is river water on one side and sea water on the other. Regardless of whether this is the boundary between the river and the sea, if there is no such dividing line, how can the estuary determine where the river and the sea are divided?

Image source: Wetland China

In fact, not only does fresh water flow into the ocean, but salt water also flows back into the land. The area between these two lines is what we call an estuary. In theory, the land boundary of an estuary is at the edge of the supratidal wetland, with the tidal line or saltwater intrusion limit as the boundary, and the sea boundary is the farthest end affected by the fresh water. In short, the land boundary is the farthest point that seawater reaches, and the sea boundary is where fresh water can reach.

Obviously, this boundary is not fixed, but changes with multiple factors such as runoff, tides, waves, etc. Moreover, both boundaries are difficult to detect. Considering the characteristics of the estuary where the river and the sea meet, using the distribution area of ​​brackish water organisms as the scope of the estuary is a relatively clear standard. Through this indicator, we can intuitively and clearly distinguish the "edge" of the river and the sea.

In this case, the yellow-blue line mentioned above is not necessarily the boundary between the river and the sea in the scientific sense. If you look closely at the salinity of the water, you will find that the yellow water has become salty and the blue water has been diluted. The number of organisms in the water on both sides may be different, but the species composition will never be so clear-cut.

Therefore, the yellow/blue dividing line at the mouth of the Yellow River is more like the outer edge of the maximum turbidity zone. The world's rivers transport 3.5´1013m3 of fresh water to the ocean every year, carrying 20~22´109 tons of silt and sediment. Therefore, tidal estuaries generally have a maximum turbidity zone, which is also one of the most important environmental characteristics of estuaries.

The Yellow River pouring down with a large amount of silt. Image source: Sogou Encyclopedia

Essentially, this is a body of water with a sand content several or even dozens of times higher than that of the upstream and downstream. Its position is not fixed and it migrates regularly within a certain range. In other words, this yellow/blue line will only appear at the estuary if certain conditions (water volume, sand volume, tidal intensity) are met.

The environmental significance of this line is that many heavy metal elements, organic matter and nutrients are retained here. Ecologically, primary production and photosynthesis within the maximum turbidity zone are limited by light, while outside the maximum turbidity zone, they are more likely to be limited by nutrient supply.

Part 2

Sea sediment: I am the nutritional supplement of the ocean

A trip to the estuary is bound to bring you delicious seafood and colorful birds, but the source of these delicious seafood and colorful birds is the silt mentioned above.

We can distinguish China's major estuaries based on runoff and sediment transport: the Yellow River has relatively large runoff and extremely high sediment content, the Yangtze River has abundant runoff and low sediment content, and the Pearl River has very abundant runoff and extremely low sediment content.

The sediment brought by rivers is vital to the ocean because the inorganic and organic substances in it are exogenous nutrients for biological production. The shelf marginal sea under the influence of large rivers is a key interface area where rivers, land, ocean and atmosphere interact. It produces 30% of primary productivity and 90% of fish catch with 7% of the global ocean area, and buries about 80% of the organic matter in the global ocean.

Biogeochemical Cycles of Biogenic Elements in Large River Estuaries and Shelf-Margin Seas

Image source: Marine Biogeochemistry Laboratory, Ocean University of China

The estuaries of large river deltas are the areas with the highest level of biological productivity in the continental shelf sea. They receive the continental marginal materials transported by rivers and determine the transportation of terrestrial materials to the open sea through controlling effects such as sedimentation, transformation and remineralization.

The estuarine ecosystem that has flourished here has become an important habitat for marine shellfish, and a place for spawning, raising young and feeding for major marine economic organisms such as fish, shrimp and crab. For example, the Yangtze River estuary in my country has traditionally produced economic estuarine fish and shrimp such as anchovies, anchovies, silverfish, loach and white shrimp, and has also become a spawning ground for Chinese mitten crabs, with a considerable output.

Image source: Veer Gallery

Part 3

Estuarine delta: Welcome to a biological paradise

What about the silt that did not enter the sea? As the cross-section of the estuary expanded, the water flow rate dropped sharply, and a large amount of silt was deposited, forming a triangular sandbar with a pointed inside and a wide outside, which is what we commonly call a "delta".

The sediments are rich in nutrients, and the freshwater and nutrient supply is guaranteed, forming a rich estuarine wetland community. Moreover, due to the confluence of salt and fresh water, there are both common freshwater wetland plants and salt-tolerant plants, and the biodiversity is extremely high.

This is a paradise for birds, as the lush vegetation provides a hiding place. In addition to plant roots, stems, leaves and fruits, insects in the air and wetland animals on the mudflats also become high-quality food for birds. Many migratory birds, such as the Oriental White Stork, the Red-crowned Crane, the Whooper Swan, the White Spoonbill, etc., use the estuary wetland as a transit station or habitat.

Wild ducks at the Dagu River estuary in Qingdao (photo taken by the author in 2009)

Part 4

Behind the prosperity of nutrients, there are hidden environmental risks

However, rivers carry not only nutrients, but also environmental risks. In addition to facing similar human activity pressures as other sea areas (overfishing, urbanization, etc.), industrial and agricultural pollutants in the basin flow into rivers and concentrate at the estuary before entering the sea, causing more serious eutrophication and pollution problems here than in other sea areas, such as red tides.

Red tide phenomenon in seawater Image source: China News Service

The Yangtze River estuary in my country is a high-incidence area of ​​red tides. According to statistics, the annual flux of pollutants entering the sea has no obvious relationship with runoff, but is highly consistent with the process of social development.

Why did the estuary prosper due to abundant nutrient supplementation, but fail due to eutrophication? Everything has its limits, and more nutrients are not necessarily better. The silt in the estuary blocks the light of the water body, and the inorganic nutrients that can be absorbed by photosynthesis are limited. Once the excess nutrients gather in the sea area with suitable temperature and light, it will cause a large outbreak of phytoplankton.

Lake water polluted by green tide Image source: Veer Gallery

The imbalance in the supply of nutrient types can also cause environmental problems. For terrestrial plants, the most important nutrients are nitrogen, phosphorus, and potassium, while for marine phytoplankton, the most important are nitrogen, phosphorus, and silicon. The redundant fertilizers in terrestrial agriculture contain a lot of nitrogen and phosphorus, but it is impossible to provide silicon to match them.

At the same time, the construction of water conservancy projects has increased the sedimentation in the reservoir area, and the diatoms in the reservoir water have also used and deposited more silicates. The final result is an unbalanced increase in nutrients in the estuary, with a large increase in nitrogen, a small increase in phosphorus, and no increase in silicon.

It can be seen that the fate of the estuary ecosystem is determined by the entire river basin. To study the estuary, we must "go downstream". The topography, landforms and vegetation coverage of the upper reaches determine the material flux into the sea; to manage the estuary, we must "go upstream" and determine the management strategy of the upper reaches based on the main threats to the estuary ecosystem. Only by taking a two-pronged approach can we truly understand and maximize the value of the estuary.

References:

Land-Ocean Interactions in the Coastal Zone,Science Plan and Implementation Strategy, IGBP Report 51/IHDP Report 18, IGBPSecretariat. http://www.loicz.org, 2005.

River inputs to the ocean systems, Final Report ofSCOR Work Group 46, Editor JD Burton, UNESCO technical papers in marinescience 55, 1988.

Produced by: Science Popularization China

Author: Zhang Guangtao, Institute of Oceanology, Chinese Academy of Sciences

Producer: Computer Network Information Center, Chinese Academy of Sciences

The article only represents the author's views and does not represent the position of China Science Expo

This article was first published in China Science Expo (kepubolan)

Please indicate the source of the public account when reprinting

Please indicate the source of the reprint. Reprinting without authorization is prohibited.

For reprint authorization, cooperation, and submission matters, please contact [email protected]