Water salinity promotes eutrophication of surface waters

Zasolenie wody

Water salinity is one of the basic parameters in monitoring. It is simple to measure and informs about the total amount of dissolved substances in the water. There are several sources of salinity. In addition to natural, we have anthropogenic sources, which include mine drainage, use of road salt, industrial effluents, agricultural fertilizers and climate change (droughts increase concentrations of dissolved contaminants in water).

Scientists in recent years have increasingly pointed to freshwater salinization syndrome [1], caused by the previously mentioned factors. In addition to the change in the aquatic environment itself, from freshwater to brackish or saltwater, increasing salinity can be another factor in increasing water eutrophication and the risk of phytoplankton blooms. Numerous scientific publications point to a number of effects caused by water salinity, related to salt release or reduced self-cleaning capacity of the resource. Two salt-prone areas of the aquatic environment are mentioned: abiotic (inanimate nature, including physical and chemical processes) and biotic (living organisms, including the biochemical processes they carry out).

Water salinity and abiotic processes

The negative impact on the aquatic environment begins already in the soil. Regular use of road salt causes calcium and magnesium ions to be more easily leached from the soil, replacing them with sodium or potassium. This leads to a change in soil pH, leaching of organic matter, biogens and heavy metals, as well as slowing down the mineralization of organic matter or reducing the soil’s retention capacity. Similar effects are observed in the sediments of reservoirs and rivers. Studies have shown that at a chloride concentration of 4,000 mg/l, 7.8 times more organic carbon and 13.3 times more dissolved Kjeldahl nitrogen are released than at zero chloride concentration [2].

Other studies have shown that the addition of 1 g NaCl releases an average of 0.07 mg of nitrogen (0.03 to 0.13 mg) in 9 of 12 sediment samples taken from different rivers (a statistically significant result), and the addition of 1 g NaCl releases an average of 2.34 µg of phosphorus (0.3 to 5.63 µg) in 7 of 12 sediment samples taken from different rivers (a statistically significant result) [3]. In the case of deeper water bodies, the inflow of saline water causes chemical stratification. Denser saline water sinks to the bottom, hindering the mixing of layers, including the cyclical spring and fall mixing of the entire volume of such reservoirs. As a result, not only does the oxygen content drop at the bottom, but also contaminants accumulate, both those reaching the tank from outside and those released from the sediments.

Fig. 1. Effect of salinity (chlorides) on abiotic processes in surface waters (own elaboration based on [6])

How water salinity affects organisms

Water salinity also affects organisms and even those living outside of reservoirs. In many cities, we have combined sanitary and storm sewers, making the winter application of road salt increase the salinity of wastewater. In addition, we are increasingly using salt in our homes, for example, for dishwashers. This negatively affects one of the pillars of wastewater treatment plant operation, which is activated sludge – a community of bacteria involved in the biological treatment of wastewater.

In inland waters, chloride influx also reduces self-purification processes. Denitrification is a bacterial process in which dissolved nitrogen in water (nitrate) is converted to gas – it occurs naturally in surface water and is used in wastewater treatment. However, increasing water salinity reduces the effectiveness of this process by more than 90 percent. At chloride concentrations of 2,500 mg/L [4]. The salinity of the bottom zone of reservoirs or rivers affects the entire structure of microorganisms and macroinvertebrates that are involved in the circulation of organic matter. In a saline environment, their diversity is lower, making them less resistant to negative external factors and less able to decompose matter.

Aquatic vegetation (macrophytes) is another important link in the self-purification processes of waters. In addition to building pollutants into tissues, they create favorable conditions for microorganisms in the root zone. Water salinity reduces the bioaccumulation capacity of nutrients and heavy metals. Studies have shown, among other things, that at chloride concentrations above 2,000 mg/l the bioremediation of copper and lead by three species: Juncus conglomeratus, Phalaris arundinacea and Carex panacea was lower by 20-40 times compared to the situation where the chloride concentration did not exceed 50 mg/L [5].

Water salinity also affects plankton communities. Cyanobacteria have a higher tolerance to salinity than diatoms and green algae. An increase in the concentration of biogens usually entails a more dynamic growth of phytoplankton (biogens are a fertilizer for them, so to speak), and this causes an increase in the abundance of zooplankton that feed on them.

The same relationship is observed in the case of periphyton (it overgrows the surface of the bottom, banks, stones and others) and the aquatic snails that feed on it. In both cases, already small increases in salt concentration (NaCl) cause zooplankton and snails to not develop as well, despite the increase in phytoplankton and periphyton biomass.

image 1
Fig. 2. Effect of salinity (chlorides) on biotic processes in surface waters (own elaboration based on [6]).

Adding up all of the above effects of water salinity, we have a body of evidence showing a link between it and increasing eutrophication and higher levels of certain pollutants. And as recent review publications on inland water salinity syndrome indicate, it still has many unexplored areas, from the micro scale of individual processes to the sum of all the effects at the catchment scale.

In the article, I used, among other things. From the works:

[1] Kaushal S.S., Likens G.E., Pace M.L. et al. Freshwater salinization syndrome: from emerging global problem to managing risks. Biogeochemistry 154, 255-292 (2021). https://doi.org/10.1007/s10533-021-00784-w

[2] Duan S., Kaushal S.S., 2015. Salinization alters fluxes of bioreactive elements fromstream ecosystems across land use. Biogeosciences 12, 7331-7347. https://doi.org/10.5194/bg-12-7331-2015

[3] Haq S., Kaushal S.S. & Duan S. Episodic salinization and freshwater salinization syndrome mobilize base cations, carbon, and nutrients to streams across urban regions. Biogeochemistry 141, 463-486 (2018). https://doi.org/10.1007/s10533-018-0514-2

[4] Lancaster N.A., Bushey J.T., Tobias C.R., Song B., Vadas T.M., 2016. Impact of chloride on denitrification potential in roadside wetlands. Environ. Pollut. 212, 216-223. https://doi.org/10.1016/j.envpol.2016.01.068

[5] Søberg L.C., Viklander M., Blecken G.T., 2017. Do salt and low temperature impair metal treatment in stormwater bioretention cells with or without a submerged zone? Sci. Total Environ. 579, 1588–1599. https://doi.org/10.1016/j.scitotenv.2016.11.179.

[6] 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, 150289. doi: 10.1016/j.scitotenv.2021.150289

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