When fresh water is salty, or the answer to a not stupid question

woda słodka

Readers whose childhood and early adolescence occurred in the late 1980s. In the 1970s, they may remember the booklets written by Jan Ruranski, bravuraly illustrated with drawings by Edward Lutczyn, in the series Why…, or Answers to Stupid Questions. I particularly remember two of them: Why is salt salty? and Why is water wet? Today I’m going to take the liberty of making a submission that has taken on special meaning in recent months: why is water salty? And maybe some more… When is water (too) salty?

How much salt in salt water?

The salinity of water is a measure of the salts dissolved in it, mainly chlorides, sulfates and carbonates. We speak of salt water when the salt content is higher than 30 g per liter (or 30‰), and of fresh water when it does not exceed 1 g (0.5 g in some systems) per liter. The salt content between these values designates brackish water.

Interestingly, when we talk about water salinity, it almost always refers to the seas and oceans. Textbooks of hydrology and ecology in the context of saltwater present characteristics of these very ecosystems. But after all, inland waters can also be salty, so calling them freshwater in general is wrong. The Dead Sea is widely considered to be the world’s most saline inland body of water (about 231‰, or 6.6 times the average ocean salinity of 35‰). However, there are reports of lakes that are much saltier, such as Lake Gusgen, located at the foot of Mount Ararat in Turkey, Lake Assal in Djibouti, Lake Patience in Saskatchewan, Canada, and Don Juan Pond in Antarctica – all of which show water salinities of 350 to even >400‰.

A less spectacular example are the coastal lakes, located along the coasts, which, depending on the hydrological connection to the sea, can be brackish to varying degrees. In Poland, the most saline coastal lakes are Resko Przymorskie, Bukowo and Łebsko, while the least are Wicko, Kopań and Sarbsko, with waters that are essentially fresh. All of these ecosystems have their own specific ecological characteristics, and the chemical composition of their waters, which determines ecological functioning, arouses scientific curiosity at best, but rather not concern. The problem arises when waters that should be sweet become salty.

How much salt in fresh water?

Well, just when does fresh water become salty? The sources of salinity in inland waters are many, although the main cause is one – man. Surface runoff and wastewater discharges from various economic sectors quite dramatically change the chemical characteristics of inland waters. I won’t analyze these sources here, because that’s a topic for a separate, uninteresting story. I prefer to focus on the effects. Interestingly, for the enrichment of water in mineral salts, there is no good Polish term to distinguish the process from the state. There is eutrophication (from trophic state), alkalization (from alkalinity), then maybe there should also be solinization or salinization (from salinity state). This sounds strange, so I will continue to use the term “salinization” to describe undesirable changes in the chemical composition of originally fresh water due to human activity (secondary salinization).

The level of abundance of salts in water is of great importance for its utility: as drinking water, for drinking purposes, for various industries or breeding aquatic organisms. Water salinity standards are most often discussed in the context of drinking water, since access to fresh water is a basic human need and right. The World Health Organization’s (WHO, 2022) drinking water quality guidelines cite 250 mg l-1 chloride and 200 mg l-1 sodium as the values at which a salty taste begins to be perceptible. Although this is, of course, a conventional value, since the perception of salty taste is determined by a great many factors, including physiological ones, as indicated by a myriad of clinical studies. In general, the taste of water deteriorates at the level of total salinity >600 mg l-1 TDS (total dissolved substances), and at the salinity >1000 mg l-1 the water becomes unpalatable. The same criteria for chloride and sodium are given by the EU Drinking Water Quality Directive, in terms of salinity, further introducing an electrolytic conductivity criterion of 2500 μS cm-1 at 20°C. Polish legislation introduces for conductivity a value of 1000 μS cm-1 as the limit for the suitability of water for drinking.

Another use of water that involves criteria, among others. for salinity, is to use them for irrigation. According to the Food and Drug Administration. According to the Food and Agriculture Organization (FAO), a conductivity value of 700 μS cm-1 is a minor to moderate limitation in irrigation applications, and values of >3000 μS cm-1 are a serious limitation. Other authors (including Zaman and co-authors, 2018 salinity assessment manual) define salinity levels <750 μS cm-1 (<500 mg l-1 TDS) as having no effect on the crop, values of 750 – 1500 μS cm-1 as likely to have a detrimental effect on sensitive crops, and levels of >1500 μS cm-1 (>1000 mg l-1 TDS) as having adverse effects on many crops. These criteria, or similar ones, are often used in studies of the impact of salinity, including in the EU’s 2022 report on the environmental disaster in the Oder River.

The criteria developed for assessing water utility are widely accepted and adopted in many national and regional laws and guidelines for assessing water salinity in the context of ecological status. And here the question arises whether these criteria are sufficient to ensure the proper structure and functioning of aquatic ecosystems. In other words, do they support a good ecological status.

How much salt for herring, and how much for barbel?

High salinity can slow growth or lead to the death of plants and animals due to the toxic effects of excess ions (ionic stress) or a shortage of water in cells (osmotic stress), or both. Aquatic organisms have developed a number of physiological and morphological adaptations and mechanisms that enable them to maintain osmotic and ionic balance in cells and tissues. A species’ tolerance to salinity, derived from its ability to regulate optimal internal osmotic concentrations relative to external gradients, varies widely among freshwater species.

Most freshwater macroinvertebrates maintain internal ion concentrations (usually at 1,000 – 15,000 mg l-1) through passive osmoregulatory mechanisms. When environmental concentrations rise above the physiological threshold, ion uptake increases and cells tend to lose water. This negatively affects their functioning and ultimately leads to the death of the body. At salt concentrations of >9000 mg l-1, osmoregulatory functions fail, although harmful effects of salinity have been observed at much lower concentrations. Most macroinvertebrates exhibit poor tolerance to increases in salinity, and negative effects are observed as low as 2,000 mg l-1 of dissolved salts. Unlike macroinvertebrates, which use passive mechanisms, fish control ion exchange through active transport against an osmotic gradient. Most freshwater fish tolerate salt concentrations in the range of 7,000 – 13,000 mg l-1, which corresponds to the range of their internal salt concentration. Despite considerable interspecies variation, among aquatic organisms it is fish that generally exhibit the greatest tolerance to salinity.

In the case of phytoplankton, different groups show different adaptations to a certain range of salinity, but a decrease in species diversity is observed at high salt content. Cyanobacteria show a higher tolerance to salinity (up to 17.50 g l-1, as in the case of Microcystis aeruginosa) than, for example, diatoms or green algae, which increases their competitiveness and can lead to an increase in the risk of cyanobacterial blooms. Elevated salinity promotes the emergence and development of invasive species in inland waters that prefer salty or brackish waters, such as Prymnesium parvum, which can cause toxic blooms. Although the specific conditions causing toxic P. parvum blooms are still not fully recognized, high electrolytic conductivity is widely considered a key factor, and a threshold of 1,500 µS cm-1 is considered a level of increased risk for this disastrous phenomenon.

A significant increase in salinity, therefore, inevitably leads to remodeling of the entire ecosystem. Freshwater organisms are no longer coping and are being displaced by species that tolerate higher salinity. Then we have the appearance of species alien to the habitat (sometimes local, but often alien to the region), most often with invasive characteristics, that is, displacing local species until they take over dominance. An example is the delicate soak(Elodea nuttallii), an aquatic plant that in experimental studies has shown a higher tolerance to water salinity (as well as reduced water clarity) than the native aquatic plant species of Eurasia.

There are probably as many ways to adapt and levels of tolerance to increased water salinity as there are aquatic species, and the literature on the subject is extensive. Here I have used only very general examples, taken mainly from reviews of the issue, done by, among others. by James and co-authors (2003) and Hintz and Relyea (2019).

That’s how much salt supports the good ecological status of inland waters?

The vast majority of information on the response of organisms to water salinity comes from experimental studies and ecotoxicological tests. The thresholds and tolerance ranges for individual species or groups of organisms determined on this basis do not reflect the complex effects of salinity on the ecosystem as a whole. Recognizing the impact of excessive salinity on the structure and functioning of the entire ecosystem is extremely difficult, on the one hand because of the wide variation in the tolerance of organisms to this pressure, and on the other because this pressure rarely occurs solo. Most often it is one of the elements of cumulative pressures, when there is nutrient enrichment in addition to salinity, and often chemical pollution of various types.

Regardless of the difficulty of setting precise environmental standards, a good approach in water protection is to adopt criteria that protect the weakest element. This is what the “worst decides” principle (the so-called “precautionary principle”), which is used in water assessments, serves. Criteria of good ecological status for salinity in Polish rivers and lakes have been developed in relation to the condition of aquatic organism communities in accordance with the 2018 EU guidelines. (Those interested in how to determine such limits are referred to the study). These criteria for salinity were set only for conductivity, on the assumption that if the standard for this indicator is not exceeded, it will not be exceeded for its components either (although these, in salinity-prone rivers, are monitored, only that they are not classified). In general, to ensure good ecological status, conductivity should not exceed 300 µS cm-1 for mountain streams, 500 µS cm-1 for upland rivers and streams, 700 µS cm-1 for small and medium lowland rivers, and 850 µS cm-1 for large lowland rivers. Only estuarine sections of rivers influenced by marine waters have an acceptable conductivity of 2,300 µS cm-1. In lakes, this limit is 600 µS cm-1, excluding the so-called “lakes”. Lobel lakes, for which it is <150 µS cm-1, and coastal lakes, for which salinity standards are not set.

With increasing pressure on aquatic ecosystems and climate change, which will surely exacerbate the problem of water salinity, are such standards feasible to maintain or achieve? I don’t know, but this is a question of administration, technology and environmental engineering, not ecology. Observations of catastrophic events in aquatic ecosystems should provide us with sufficient reasons to adopt minimum levels of ecological safety. An electrolytic conductivity value of 1,500 µS cm-1 seems a fairly liberal criterion by the standards supporting good ecological status, but it is the absolute minimum we must provide rivers to prevent undesirable phenomena and further decline in biodiversity. It also doesn’t change the fact that according to EU and national law, we are obliged to achieve the environmental goals, that is, also the salinity standards set for our waters, and, for the time being, nothing relieves us of this obligation.

In the article, I used, among others. From the works:

  • James K., Cant B., Ryan T., 2003. Responses of freshwater biota to rising salinity levels and implications for saline water management: a review. Australian Journal of Botany, 51, 703-713
  • Hinz W.D, Relyea R.A., 2019. A review of the species, community, and ecosystem impacts of road salt salinisation in fresh waters. Freshwater Biology, 64: 1081-1097
  • Szklarek S., Górecka A., Wojtal-Frankiewicz A., 2022. The effects of road salt on freshwater ecosystems and solutions for mitigating chloride pollution – A review. Science of the Total Environment 805 (2022) 150289
  • Tonk L., Bosch K., Visser P.M., Huisman J., 2007. Salt tolerance of the harmful cyanobacterium Microcystis aeruginosa. Aquat. Microb. Ecol. 46, 117-123 https://doi.org/10.3354/ame046117
  • Zaman M., Shahid S. A., Heng L., 2018. Guideline for Salinity Assessment, Mitigation and Adaptation Using Nuclear and Related Techniques. Springer

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