HABs (harmful algal blooms) are increasingly observed in rivers with high nutrient concentrations, low water clarity, and warm temperatures. The Ohio, Illinois, Kansas, and Mississippi Rivers are examples of the types of rivers that have such conditions. However, most analyses of algae in rivers often do not consider the movement of algae along the waterway. Not considering this suggests that biomass accumulates at a single location, possibly incorrectly indicating that all algal growth occurred at that location. 

For this study, USGS scientists researched the movement of algae along a 245-mile stretch of the Illinois River. The study authors found that most of the algae grew in the upstream section and moved downstream, potentially contributing to downstream turbidity, organic carbon, oxygen demand, and algal toxins. The computer model they developed clarified that there is not widespread growth. It showed that interpretation of data from a single location could lead to a double counting of the algal mass and a misidentification of the causes and timing. 

The results of this study highlight areas of the river systems that are more prone to algal blooms and also indicate areas where downstream effects may be important. More generally, this work is one of the first attempts to explicitly model the transported component of planktonic algae through a river network, which is a critical step in building predictive models, and thus is of interest to other HAB researchers and officials responsible for protecting surface and drinking waters.  

If scientists consider both local and upstream conditions, it may improve early warnings. It may also improve our understanding of potentially harmful algal blooms in river systems, especially downstream impacts, and on water availability.

Read the full publication to discover the findings of this study.