How do we science? / Science

From ocean acidification to freshwater alkalinization

By this point, most of our readers are probably pretty well versed in ocean acidification (just type ‘coral reef’ in the UNdertheC search box and you’ll come up with lots of interesting tidbits about ocean acidification). We know that increasing carbon dioxide in the atmosphere (from human activities, including the burning of fossil fuels) is increasing the amount of carbon dioxide in the ocean’s surface waters (thanks to a little thing called gas exchange) which in turn, results in decreasing pH (if you’re a chemistry nerd, then basically, the carbon dioxide interacts with water and carbonate ions and generates acid). Then, of course, this decrease in pH has all kinds of negative effects on ocean and coastal organisms, especially on corals and other organisms that produce calcium carbonate.

Ocean Acidification

The impacts of ocean acidification on calcium carbonate organisms (in this case, a pterapod shell). The upper left shell had a ‘normal’ pH with decreasing pH from the left to right. Image from here.

That’s all well and good, but a recent paper has brought to the limelight a counter part to ocean acidification: freshwater alkalinization. Okay, that was a lot of ‘A’ words with the suffix ‘-ation’ at the end. We just went over ocean acidification, which leaves freshwater alkalinization. Alkalinization is basically the opposite of acidification, where the pH of freshwater systems has actually been increasing over time. Meaning, that instead of getting more acidic, like the ocean and coastal waters, this paper shows that inland freshwater streams and rivers, are becoming more basic over time. The study found that in addition to increasing the pH of these streams and rivers, the salinity (or how much salt) is also increasing, meaning the freshwater in rivers has been getting saltier over time as well.

Stream

It can be hard to tell by just looking at a stream, but the chemistry of many streams and rivers in the US have been changing over the past 50+ years due to human activity in the watershed. Image from here.

But how did they figure this out? To look into this question of alkalinization and salinization of streams and rivers over time, the authors looked at salinity and pH data measured at hundreds of streams and rivers for the past 50+ years. By plotting this data across time, they were able to assess how salinity and alkalinity were changing. It turns out, for the majority of streams and rivers both salinity and pH have been increasing over time. Now the big question is: why?

The authors of the study offered a few hypotheses for why the salinity and pH of these freshwater systems is increasing, all of which stem from human activity in the watershed:

  1. Atmospheric weathering: Or, better known as acid rain. This was a big problem back in the mid-1900’s, but essentially, due to all types of things spewed from industrial smokestacks, the moisture in the atmosphere (as clouds and rain) became more acidic. As this rain falls onto the land, it leads to weathering and the dissolution of things like natural limestone and human made concrete (think about what happens when you put vinegar on baking soda: the baking soda dissolves). Both limestone and concrete are very basic, meaning they contain lots of salts and ions that, when washed away, increase the salinity and pH of freshwater systems.

    Weathering

    Ever seen a statue like this? The ‘fuzzy’ features are likely due to dissolution by acid rain. All of these salts that dissolve have to go somewhere (aka: rivers and streams). Image from here.

  2. Human inputs: Things like road salt, agriculture fertilizer, wastewater, and fracking wastes all tend to have a different salt content and alkalinity than natural waters. When these things are washed into freshwater systems, they can change the salinity and pH.
  3. ‘Biological alkalinization’: According to the study, this refers to changing alkalinity in freshwater systems due to natural processes that are being exacerbated by human inputs. To break it down: phytoplankton in rivers and coastal systems convert carbon dioxide dissolved in the water to organic biomass. When lots and lots of phytoplankton bloom at one time, they can actually remove enough carbon dioxide from the water to increase the pH. As humans put more nutrients into the water (things like run-off from fertilizer, nutrients in wastewater), then more phytoplankton grow. This in turn reduces the amount of carbon dioxide in the water, leading to increased pH.

This increasing salt and pH in freshwater is really important to the health and the use of these ecosystems. For one, many rivers and streams are used as drinking water sources. As salinity and pH increase, we may no longer be able to use these sources as drinking water or it may become more expensive to treat the water for drinking. Increasing salt and alkalinity can also alter how metals and other contaminants move through the environment. Under normal conditions, a lot of contaminants and metals are filtered out in the soil before being transported to streams and rivers. As salinity and pH change, these same contaminants and metals no longer stick to soils as well, and then end up in freshwater systems where they can harm the environment. And, of course, we all know that all water ends up in the ocean, so eventually this higher salinity, higher pH freshwater enters the ocean. One small benefit of the increasing pH in freshwater may be it’s ability to offset ocean acidification in coastal waters. But this is really a very small benefit (the ocean is still pretty big compared to all the rivers…).

I think what’s so interesting about this study is how it highlights the radically different ways humans impact the environment and further shows how much we don’t know about the impact humans are having on the total environment. From climate change to ocean acidification to now freshwater salinization and alkalinization, humans have to be more mindful about our impact on the environment in order to best preserve the Earth we have.

Some more info:

The study referenced throughout this blog post:

Kaushal, S.S., Likens, G.E., Pace, M.L., Utz, R.M., Haq, S., Gorman, J., and Grese, M. (2018). Freshwater salinization syndrome on a continental scale. PNAS, Early Ed.

A good resource for the chemistry of ocean acidification.

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