We've been getting a handful of questions this week by email and social media about the amount of algae in the Rocky River's Main Stem. This post to our Facebook page from our friends at Share the River is a good example:
Others have asked about nutrient levels in the river as well.
First, as of 2014, the lower Rocky River - for the first time since people started checking - is meeting its Clean Water Act goals for aquatic life. That doesn't mean that there's not elevated nutrient levels - just that they aren't elevated enough (generally) to negatively impact fish and aquatic bugs. So most of the time nutrients aren't a major concern in the main stem.
However, when the flow gets low - like it has been this past week - a large portion of the flow in the Rocky River is composed of wastewater treatment plant effluent (from North Olmsted, Lakewood, Medina, North Royalton, Strongsville and more). This treated wastewater tends to be higher in nitrogen than the groundwater and runoff that makes up the bulk of the volume during higher flow conditions. Add in the fact that the water will be much warmer during this time of year, and there's a good bet that you'll get some extra algae.
Anyway, since we'd been getting so many inquiries, I decided to go out and take a look on the afternoon of July 7, and to take a quick water sample. Water temperatures were in the upper 70s F. I did a field analysis of the sample for total phosphate and nitrate. The phosphate was 0.13 mg/L, which is reasonable, and below the 0.21 mg/L target established for the main stem in the Rocky River Watershed Action Plan. Nitrate levels were elevated, as anticipated, at 3.3 mg/L, which exceeds the 1.85 mg/L target for the main stem by 78%.
To sum up: The combination of low, slow flow mixed with warm temperatures and higher nitrate concentrations (due to a higher proportion of the flow being treated wastewater) creates an environment in the Main Stem of the Rocky River that promotes algae growth.
The good news is that these conditions rarely persist for more than a week or two at a time, and most of this excess algae will probably get flushed out of the system the next time we get a big storm.
-Jared Bartley, Rocky River Watershed Coordinator