Ledford, S.H., et al. (2017). “Impact of seasonal changes in stream metabolism on nitrate concentrations in an urban stream.” Biogeochemistry 133: 317-331. https://doi.org/10.1007/s10533-017-0336-7
What does excess nutrient pollution do?
Excess nutrients flowing to estuaries, bays, and lakes are a large issue around the world. When we add extra nutrients to the water (mostly in the form of nitrogen, N, and phosphorus, P), we allow algae and plants to grow excessively. After these organisms die, bacteria break them down, removing oxygen from the water in the process because they respire just like we do. This results in low oxygen concentrations in water, which kills other life forms in the water that require oxygen to live, including fish and macroinvertebrates. This is the eutrophication process you may have heard about. One of the big goals for groups concerned about water quality, fishing, and ecosystem health, especially around areas like the Chesapeake Bay, is to keep excess nutrients from reaching the bay and stop eutrophication from happening. The most effective approach, of course, is to keep nutrients from entering streams at all. However, it is also important to figure out what happens when excess nutrients do get to streams, especially small streams. Are there natural processes that can help remove nutrient contamination, and if so, what are their pros and cons?
What do small streams have to do with it?
Headwater streams (the smallest streams in a river system) make up a majority of the total stream miles in the world1, and so are vitally important places. They are also unique because the total amount of stream bed is high in proportion to the water flowing through the channel. Think about licking an ice cream cone- when the ice cream is large, your tongue (the bed) interacts with less of the ice cream (the water)- this is what happens in large rivers. But, if the ice cream scoop is small, each lick covers more of it- these are headwater streams. This is important because one of the key spots in streams for nutrient processing and removal is where the water and bed interact.
What is happening in small, urban streams?
This study looked at nitrate, the most common form of nitrogen in freshwater streams, over a year in a small, urban stream in Syracuse, NY. The stream was a good case study because it had two different sections: the upper parts were highly urbanized (left panel below), and the lower reach was much more natural (right panel below). Urbanization showed itself through many ways in the first section: the stream runs in the middle of a two-lane road, the stream banks have been replaced with concrete, and the stream no longer meanders to-and-fro but has instead been straightened. In contrast, the natural section has lots of vegetation, natural stream banks, and has been allowed to meander back and forth.
What was the impact on the nitrate concentrations?
Sampling showed that nitrate concentrations in the urban section were low during the summer months, but increased as the stream flowed into the natural section. Measurements of algae mat size indicate that algae was removing nitrate in the urban section, and was able to grow as much as it wanted due to all of the direct sunlight it received, since there was no bank vegetation shading the stream. In fact, there was almost no algae in the natural section, despite having enough nitrate to grow, because the algae was missing its energy source- sunlight. During the winter, once the stream was cold, nitrate increased to a more uniform concentration along the entire stream length because algae couldn’t grow anywhere.
Well, it seems that the algae actually act like a scrubber to remove nitrate from the stream! However, there are caveats to this. First, it is only temporary, both in space and time. There is clearly some turnover as the water moves downstream because nitrate concentrations increase as the water moves into the natural section (space) and the nitrate is only removed when its warm and sunny enough for the algae to thrive (time). Secondly, algae growth like this is terrible for macroinvertebrates and fish. It may be ok in an urban stream like this, where they couldn’t live anyway, but to have a nice, balanced stream, you can’t have large algae mats like these. The moral of the story is that algae mats do scrub the water of nutrients in urban streams, but it isn’t pretty or good for the rest of the ecosystem, so it would be even better if the nutrients don’t reach the stream in the first place.