Salinity matters – salt changes life in inland waters

Anthropogenic salinization of rivers as a result of various human activities is now a global phenomenon, observed on every inhabited continent. With climate change and increased demand for water, we can expect this problem to intensify. Numerous studies indicate that the increase in salinity of inland waters significantly adversely affects biodiversity and the functioning of these ecosystems. It promotes an increase in eutrophication, leads to the remodeling of biological communities and results in a decrease in their diversity. Let’s find out how the inhabitants of freshwater ecosystems are coping in the face of increasing salt pressure. And what factors are key to shaping the biotic characteristics of rivers in an era of widespread salinization.

Osmoregulation is the key

Salt concentration is the main factor determining a species’ tolerance to physiological salinity and its osmoregulatory capacity. A species’ tolerance to salinity results from its ability to regulate optimal internal osmotic concentration against external gradients, and varies widely among species and groups of organisms. Thus, which organisms live in waters with different salinity levels is a result of their adaptations and physiological mechanisms for maintaining water-ion balance in cells and tissues.

In most marine organisms, intracellular fluids contain so much salt that their intracellular osmotic pressure is equal to that of the external environment. If they end up in fresh water, the difference in concentration will cause their bodies – in order to balance the salt content – to absorb a large amount of water, leading to swelling and eventually death. Organisms living in fresh water have a small amount of salt in their bodies. Subjecting them to high-salinity water results in water escaping from their bodies, leading to an increase in the concentration of body fluids, resulting in reduced growth or death from the toxic effects of excess ions, water deficiency or both.

Tolerance is the key

Human activities usually cause moderate salinity in rivers (on the order of 1-10 mS ^cm-1 electrolytic conductivity), but sometimes this pressure can reach significant values, typical of brackish and even saline ecosystems (>100 mS ^cm-1). As a result, assemblages of organisms remodel, and taxa sensitive to higher salt concentrations are replaced by those with greater tolerance. A small increase in salinity during the initial phase can even slightly increase the species richness of some groups of organisms [1]. This is because a decrease in the difference in osmotic pressure between the internal and external environments reduces the energy requirements of osmoregulation. To some extent, this may explain why some freshwater species, such as some fish and mollusks [2, 3], achieve maximum growth at moderately elevated salinity.

A relatively small number of freshwater habitat taxa are salinity tolerant and capable of colonizing anthropogenically saline rivers. One of the best studied groups of animals in this regard are benthic macroinvertebrates, widely recognized as reliable indicators of water quality and ecosystem structure and functioning. It is known that particularly sensitive to salt are insects belonging to three orders, viz. mayflies (Ephemeroptera), pitchforks (Plecoptera) and caddisflies (Trichoptera) (together used as an indicator of EPT).

Their taxonomic richness and abundance decrease dramatically with increasing salinity, although there are known caddisflies that can inhabit saline environments (e.g. the exclusive saltwater marine caddisfly Philanisus plebeius). River flies (Diptera), beetles (Coleoptera), dragonflies (Odonata) and bugs (Hemiptera) (DCOH index) show considerable variation, ranging from sensitive to highly tolerant to salinity, nevertheless, an increase in water electrolytic conductivity above 1.5 mS ^cm-1 usually leads to a significant decrease in insect species richness and a decrease in the ratio of EPT species to DCOH [1].

Evolution is the key

Differences in the salinity tolerance of different organisms may explain the evolutionary distance between freshwater taxa and their marine or estuarine ancestors. Many freshwater crustaceans, mollusks and fish are closely related to marine or estuarine species and can tolerate certain levels of salinity. At least 33 modern snail lineages have colonized freshwater from marine environments, according to studies [4]. Freshwater fish also likely evolved from marine ancestors [5]. It has been documented that only 2 percent. amphibian species inhabit saline environments, so an increase in salinity pressure will lead to the elimination of this group of organisms from the ecosystem.

Insects are among the few invertebrate taxa with much higher species richness in freshwater than in marine environments. Freshwater insects, which dominate streams and rivers in terms of biomass, abundance and taxonomic richness, have evolved many times over from terrestrial insects, and salt-tolerant taxa derive in many lineages from freshwater ancestors [6]. The lack of evolutionary exposure to saline conditions may mean that most of them are poorly equipped with physiological mechanisms to adapt to saline conditions. Therefore, this group will show a significant decrease in diversity as a result of increasing salt concentrations. And it is worth mentioning that it constitutes a very important food base for animals of the higher levels of the trophic network.

Migration from other environments

Species already adapted to salt can spread and colonize saline rivers by migrating from nearby oceans, estuarine sections of rivers or naturally saline lakes. However, salinity aside, river environments differ from naturally saline environments in many ways, so colonizing species will have to cope with other aspects of river life, including unidirectional flow, greater variability in oxygen conditions, pH, water temperature, and habitat mosaicism. The fact that so few organisms from marine habitats live in saline rivers, even at similar salinity levels, suggests that the barriers to the spread and colonization of these ecosystems are formidable.

It is worth noting that the possibility of colonization of anthropogenically saline inland rivers by organisms of other saline habitats is limited geographically, since naturally such sites are common only in Mediterranean and (semi)desert climates. In addition, however, anthropogenically saline ecosystems tend to have lower salt concentrations than naturally saline ecosystems and may prove too low for typically saline organisms. Hence, such waters will be inhospitable to freshwater organisms and not hospitable enough to saltwater organisms.

Salinity matters

Salinity is one of the most important factors determining where aquatic organisms can live. An inevitable consequence of anthropogenic increase in salinity of inland waters will be impoverishment of communities, remodeling of their taxonomic composition, decline in biodiversity and disruption of ecological processes. This means that species that we are used to encountering in our waters may not survive there and be replaced by others that are better adapted to the new salinity conditions.

At the same time, the functioning of the entire ecosystem may change, as emerging species may perform different ecological functions or carry them out differently. And this will certainly affect the ecosystem services provided by water. Climate change will exacerbate this problem. So as we measure the electrolytic conductivity of our rivers, let’s remember that the salt in water matters not only for its taste, but most importantly for its inhabitants.

In the article, I used, among others. z :

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