Water salinity

Water salinity is a core factor regulating the structure and function of aquatic ecosystems. Its changes directly affect the survival and reproduction of organisms, community structure, nutrient cycling, and can even trigger regional ecological imbalances. The specific impacts are as follows:

I. Direct Impacts on Aquatic Organisms

Impact on Cell Osmolarity and Survival

Aquatic organisms have specific tolerance ranges for salinity; exceeding these thresholds disrupts cellular osmolarity balance:

Freshwater organisms: such as carp and grass carp, have a higher internal osmolarity than the surrounding freshwater and rely on ion regulation systems in their gills and skin to maintain balance. A sudden increase in salinity can lead to cell dehydration and shrinkage, resulting in respiratory distress and death.

Marine organisms: such as corals and tuna, have a lower internal osmolarity than seawater and need to excrete excess salt through their salt-excreting organs. A sudden drop in salinity can cause cells to absorb water, swell, and rupture, leading to mass mortality.

Euryliferous organisms: such as fiddler crabs and bass in estuaries, can adapt to certain salinity fluctuations, but exceeding these limits will still cause physiological disorders.

Constraining Reproduction and Development

Salinity is a key trigger for the reproduction of aquatic organisms, with varying sensitivities to salinity at different developmental stages:

Fish spawning and fertilized egg hatching require specific salinity environments. For example, salmon migrate to freshwater rivers to spawn; excessive salinity prevents egg hatching.

The larval stage of marine plankton is extremely sensitive to salinity changes; abnormal salinity reduces larval survival rates, thus affecting the supply of the bottom of the food chain.

II. Impacts on Biological Community Structure

Altering Species Distribution and Dominant Species

Salinity gradients determine the spatial distribution pattern of aquatic communities:

In freshwater lakes, increased salinity leads to the death of low-salinity-loving submerged plants and the proliferation of halophilic algae, causing algal blooms.

In estuaries, salinity fluctuates with tides, forming a "freshwater-brackish-seawater" species gradient. Abnormal salinity disrupts this balance, leading to the replacement of dominant species.

Disruption of Food Chains and Biodiversity

Sudden changes in salinity can lead to the death of basic producers, resulting in food chain disruptions:

For example, a sharp drop in nearshore salinity can reduce phytoplankton populations, leading to a decrease in the number of zooplankton that feed on them, thus affecting the food supply for fish and birds;

Long-term high salinity stress can lead to reduced aquatic biodiversity, the proliferation of salt-tolerant monocultures, and decreased ecosystem stability.

III. Impacts on Aquatic Material Cycling and Physicochemical Properties

Affecting Nutrient Migration and Transformation

Salinity changes alter the form and solubility of nutrients such as nitrogen and phosphorus in water:

When salinity increases, phosphates in water readily combine with calcium ions to form precipitates, reducing bioavailability;

High salinity environments inhibit microbial nitrification and denitrification, leading to nitrogen accumulation and exacerbating the risk of eutrophication.

Altering Water pH and Dissolved Oxygen: Increased salinity enhances the buffering capacity of water bodies, stabilizing pH levels. However, high-salinity wastewater, under the action of microorganisms, produces hydrogen sulfide, leading to water acidification, decreased dissolved oxygen, and the formation of "dead water zones." High-salinity water bodies also have higher density, resulting in more pronounced vertical stratification. The bottom water is prone to oxygen deficiency, triggering the release of harmful substances from the sediment.

IV. Indirect Impacts on Terrestrial Ecosystems: Soil Salinization: High-salinity farmland runoff and river water infiltration into the soil lead to salt accumulation, damaging soil structure, reducing soil fertility, affecting crop growth, and even causing farmland abandonment.

Impact on Wetland Ecosystems: Wetlands are transitional zones between land and water. Increased salinity causes the death of freshwater wetland plants, which are replaced by salt-tolerant plants, altering wetland ecological functions, reducing their ability to purify pollutants, and diminishing their habitat value for migratory birds.