Excess nitrogen from agriculture may reduce variation of diatom species in streams

Researchers at the University of Oulu and the Finnish Environment Institute have addressed the question on how diatoms in rivers respond to different nutrient conditions. Interestingly, they found that high concentrations of dissolved inorganic nitrogen seem to reduce the species variability, probably because sensitive taxa disappear, while certain tolerant taxa can survive. Phosphorus is traditionally believed to be the limiting nutrient in freshwaters, but this paper joins newer studies with evidence that also nitrogen can have a highly important effect on freshwater ecosystems.

Dr. Kaisa-Leena Huttunen, who is main author of the paper, explains that the study was an in-depth look at temporal variation of diatom community variation along nutrient gradients. The team monitored one minimally disturbed and two agricultural boreal streams bi-weekly from May to October in 2014. In three riffle sections in each stream, they assessed temporal (among seasons) variation in algal biomass, species diversity and composition of diatom assemblages. They monitored the number of type-specific diatom taxa which is an index used in WFD assessment in Finland.

Graphical abstract of the paper. by Huttunen et al.

“Quantifying the degree of temporal variation is a prerequisite for insightful management decisions, and diatoms are a commonly used WFD quality element in streams,” explains Kaisa-Leena.

A key result was that temporal variation in community composition decreased with an increasing ratio between dissolved inorganic nitrogen (DIN) and total phosphorus (DIN:TotP). A further analysis on the result revealed that temporal variability was best explained by DIN, which suggests that excess of nitrogen reduces the temporal variability of diatom assemblages. The average levels of total nitrogen in the three study sites in the near-natural stream ranged from 493-664 ug/l, and between 730-962 ug/l in the six sites of the two agricultural streams. Average DIN varied between 14-65 ug/l in the near-natural stream and between 71-395 in the agricultural streams.

However, the variability was not only in time, but also in space: The monitoring revealed that sites within a stream did not always behave synchronously in time, resulting occasionally in differing assessment outcomes between sites. Hence, the findings also suggests that single sampling at a single site may be insufficient to reliably assess and monitor a complete stream water body.

Huttunen K-L, Muotka, T, Karjalainen SM, Laamanen T, Aroviita J 2020. Excess of nitrogen reduces temporal variability of stream diatom assemblages. Science of The Total Environment 713: 136630. https://doi.org/10.1016/j.scitotenv.2020.136630

Feature photo: Ripple section of a stream. Eva Skarbøvik.