Course:EOSC270/2023/Group 6 - The Impacts of Salinity Change on Marine Ecosystems

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Background Information about Salinity? (Danny)


Salinity is a vital parameter for determining the state of the ocean and plays a critical role in understanding the ecological environment and ocean circulation[1]. Generally, ocean salinity has increased in some regions, particularly in the subtropics over several decades, and decreased in others, especially in the high-latitude oceans[2].

Salinity Measurement

Two typical methods used to measure salinity include a conductivity sensor and satellite measurements. The conductivity sensor is placed on the back of a fish, exposed directly to seawater, and measures temperature, pressure, light intensity, and electrical conductivity to calculate salinity using an equation[3].

Satellite measurements can also determine salinity levels in two ways. The first involves correlating satellite measurements of radiosity or reflectivity to salinity. The second is the detection of colored dissolved organic matter (CDOM) using Marine color satellite products, suitable for use close to the shore, which effectively calculates the actual salinity[4].

Salinity and The Marine Cycle

Ocean surface salinity is a critical factor in understanding how freshwater input and output impacts ocean dynamics, as the majority of global evaporation (86%) and precipitation (78%) occur over the ocean [5]. Notably, there is a vast expanse of highly saline water in the North Atlantic, similar to terrestrial deserts that experience minimal rainfall and significant evaporation[5].

Monitoring changes in ocean surface salinity allows us to keep track of fluctuations in the water cycle, as salinity variations very sensitively reflect the overall exchange of freshwater between the ocean and the atmosphere. The process of evaporation transfers freshwater from the ocean into the atmosphere, thereby increasing the ocean's salinity. Conversely, precipitation adds freshwater to the ocean, leading to a reduction in its salinity. Therefore, changes in salinity provide an excellent indication of changes in the water cycle, encompassing the impact of various factors over vast areas[6].

Salinity Problems

In the late 20th century, salinity levels decreased in the continental margin of Antarctica and the Pacific Ocean[7]. The El Nino Southern Oscillation (ENSO) is an oscillation of wind field and sea surface temperature in the eastern equatorial Pacific that can affect and alter atmospheric circulation, resulting in global environmental impacts, despite originating in the tropical Pacific Ocean[8]. La Nina events refer to a widespread and persistent abnormal cooling of sea surface temperatures in the eastern and central equatorial Pacific Ocean, also known as an anti-El Nino. Salinity plays a positive role in the emergence of El Niño by increasing the variation in density, which is more negative during El Nino and more positive during La Nina[9].

How does this problem impact marine ecosystems?

Changes in salt concentration will have a variety of effects on the world and marine life, including climate warming and a decrease in the area of the world's sea ice, which will alter the habitat of marine life. Changes in salt concentration will also cause changes in the organism traits of marine animals and plants.

The pacific ocean salinity decrase(Jeffrey)

Human activities impact the marine ecosystem, affecting temperature, acidification, and salinity[10]. There are three major types of salinity processes: pure warming, pure freshening, and pure heave[11]. . The north Pacific Ocean experiences pure freshening due to increased freshwater flux from nearby rivers[11]. The freshening of the Pacific Ocean can cause instability in the salinity level, which may lead to diseases in the ocean ecosystem[12].

Steady state surface temperature differences for 50 g/kg versus 20 g/kg salinity scenarios with each model configuration. Reds indicate higher temperatures with higher salinity while blues indicate lower temperatures with higher salinity. All high salinity scenarios are warmer on global average than the low salinity scenarios despite local cooling.
Steady state surface temperature differences for 50 g/kg versus 20 g/kg salinity scenarios with each model configuration.

Changes in habitants

Sea ice coverage for various model configurations and ocean salinities. Rows show the standard model with a fully coupled ocean-atmosphere, sea ice dynamics, and freezing point depression (“Full Physics”; top); a modified model configuration with ocean and sea ice dynamics but fixed freezing point for all salinities (“Fixed Freezing Point”; middle); and a slab ocean configuration lacking ocean and ice dynamics but including freezing point depression (“No dynamics”; bottom). Salinity differs between the columns, increasing from 20 (left) to 50 g/kg (right). Increasing salinity yields lower ice cover in all scenarios, but the effects are most pronounced in model scenarios that include both dynamical and thermodynamic effects.
Sea ice coverage for various model configurations and ocean salinities.

Increasing ocean salinity results in warming, particularly at high latitudes. With Fixed Freezing Point, increasing salinity from 20 to 50 g/kg results in warming of up to 11.3°C in the annual average at northern high latitudes (0.8°C warming on global average) and a 71% decrease in global sea ice coverage[13]. Global sea ice loss and melting will alter the habitats of marine animals and plants, especially those that depend on sea ice for mating, raising their young, and resting, such as seals, walruses, penguins, etc. Loss of habitat would result in a decline in the numbers of these marine mammals, as well as the marine species that depend on them.

Changes in organism traits

Salinity variations  can affect organism characteristics such as survival, fecundity, metabolic rate, and growth rate. For example, Garreta-Lara et al. [14] found a strong influence of salinity on the metabolomic profile of the invertebrate Daphnia magna, but no significant interaction with temperature. In estuarine and marine invertebrates, increased salinity generally protects against the negative effects of metals [15], which can be partly explained by competitive interactions with major cations for sensitive ion transport sites [16].

Example of changes in salinity affecting the Pacific

Salinity has an impact on the size of eggs in Pacific herring. By setting and controlling variables experimentally, it is observed that there was an inverse relation, between egg volume and salinity at all stages of egg development. Eggs transferred from 20‰ to 5 or 35‰ S, 87.4 hr after fertilization (90% blastodermal overgrowth of the yolk), showed only minor changes in total egg volume within the period of relative stability (100–240 hr)[17].

Oysters and other Pacific invertebrates rely heavily on their habitat for growth. In high growth sites, oysters had greatest increases in shell height and weights of whole oyster, shell and dry meat which were attributed to high phytoplankton biomass (chlorophyll a) and suitable temperature and salinity regimes[18]. Oyster quality and production fall when the salinity of the Pacific Ocean varies due to the decline in phytoplankton population and the shift in salt concentration.

The extent of the problem and future trend

Current situation of the Pacific Ocean

In summary, the present ocean salinity situation is very complex and influenced by a combination of IPO, global warming and salinity advection.


Two phases of IPO

IPO is a large-scale, long-term oscillation affecting the climate of the Pacific basin. During the positive phase of the IPO, precipitation in the northeastern South Pacific Convergence Zone (SPCZ) is generally higher, while precipitation in the southwestern SPCZ is generally lower[19]. For the Pacific Ocean, decadal salinity changes are closely related to the decadal Pacific Oscillation (IPO)[20]. A salinity pattern is followed: salinity is low where precipitation is high and high where precipitation is low[20].

Global warming

Notably, previously mentioned salinity pattern is now reinforced by the effects of anthropogenic global warming, implying that the ocean becomes fresher in precipitation-dominated areas and more saline in evaporation-dominated areas[21]. Argo data shows that the contrast between low and high salinity is increasing in all regions except the subpolar North Atlantic[22].

Salinity advection

The salinity advection is mainly driven by winds, where seawater with high salinity in the subtropics is carried to the tropics, while fresher water in the tropics is transported to higher latitudes[21]. In summary, salinity advection contributes negatively to salinity variability in the ITCZ and SPCZ regions, positively to the equatorial central Pacific, and smooths the salinity gradient[23].

different level of salinity and affect to shell species

Salinity drop and costal ecosystem species

The salinity decrease is now influencing the coastal species. Scientists found some species with hard shell in the costal low salinity environment has higher chance to loose their shell than open ocean high salinity environment[10]. Due to the increasing amount of freshwater being added to the ocean, the salinity levels near the coast are decreasing. This is causing living environment of shell species closer to the coast is fresher than before[10].

What may happen if Salinity increasing in future

On the other aspact, The increasing ocean salinity level may cause some diseases in the ocean ecosystem in the future. A study of a disease shows that a kind of sea grass called eelgrass has a lower chance to be infected by disease in the high temperature and low salinity environment[24]. This disease was appear in 1903s in Altlantic ocean, that was cause a large area infected of the sea grass[24]. If human not control the waste water gose into the ocean, in the future the salinity may increase, and it will lead to the disease outbreaks like the seagrass in the Altlantic ocean.

disease possibility in different salinity level

The future if human do not act

A recent study suggests that the global ocean is a rapidly changing system that is impacted by variations in temperature and salinity[25]. .Salinity changes can occur not only on the top layer of the ocean but also in deeper parts of the ocean, and the rate of change may increase in the future[25].The study suggests that the majority of observed changes in ocean water patterns are the result of human activity and that these changes will worsen if humans continue to emit high levels of carbon dioxide(CO2)[25]. In the future decades, if humans still do not control greenhouse gas emissions, and slow down global warming, the influence of ocean salinity change will become more visible to humans. . The study predicts that human-related influence on deep ocean basins in the Pacific Ocean will increase from 40-65% in 2050 to 55-80% in 2080[25]. This means that in the future, more and more water areas will be impacted by human activity if we continue to ignore these issues.

Given the impact, what are the solutions?

Climate Change

Principles of three main CO2 capture options.

Changing precipitation patterns and increased evaporation due to global warming could also play a role in exacerbating ocean salinity.[26] To address this issue, several mitigation strategies can be employed, such as reducing greenhouse gas emissions through the adoption of renewable energy sources, improving energy efficiency, and implementing more efficient carbon capture and storage technologies (Carbon capture and storage is the technological process of collecting carbon dioxide at sources that produce it, such as industrial facilities, and then storing them in locations that are isolated from the air).[27]

Increase River Input

Map of South-to-North Water Transfer Project

As population and industrial facilities continue to grow, the demand for freshwater resources increases, leading to a decrease in the flow of rivers entering the sea and a concomitant increase in ocean salinity.[28]Inter-basin water transfer (IBWT) is a method of reallocating water resources across geopolitical boundaries, meaning the construction of diversion projects between different basins to regulate the water balance in each basin.[29] For example, China has implemented the South-North Water Transfer Project. The South-to-North Water Transfer Project brings water from the precipitation-rich southern regions of China to the northern regions through man-made canals.[29] This reduces the northern demand for freshwater resources from rivers such as the Yellow River, increases the amount of freshwater flowing into the Bohai Sea, and regulates the salinity of Bohai Sea water.

Proper Treatment of Brine

The brine discharged from desalination is closely related to the uneven distribution of seawater salinity.[30] Measures such as locating desalination facilities in areas with ocean currents and building longer outlets can mitigate these adverse effects.[31] Studies of salinity dispersion models in Bohai Bay (an area with limited seawater exchange capacity) suggest that desalination plants should discharge their concentrated seawater at high tide and maintain a capacity of less than 10x10^4 t/d.[32] Therefore, for the ocean salinity problem caused by desalination, a rational planning of the siting and discharge pattern is essential to reduce the salinity variability.

Monitoring Salinity Change

Ocean salinity is monitored by means such as conductivity sensors and satellite technology. In order to effectively study changes in ocean salinity and develop countermeasures, ocean observing systems that can detect and monitor continuous changes must be relentlessly maintained and enhanced.[33]


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