Total phosphorus (TP) and total nitrogen (TN) are the two nutrients primarily responsible for algae growth. Measuring these nutrients provides an estimate of the potential for algal growth. In lakes, the concentrations of these nutrients are typically highest at the end with the inflowing stream and decrease toward the lower end of the lake. On May 7, we found a different result.
In the Little Niangua River concentrations of both nutrients were highest at the uppermost site (near Tunas). By the next site, 18 miles downstream, concentrations dropped significantly and remained fairly constant until the lake arm, where both nutrients increased considerably (more than 4-fold for phosphorus and 6-fold for nitrogen). Niangua River nutrient concentrations were also highest in the upper reaches and had approximately double the nutrients found in the upper Little Niangua. Unlike the Little Niangua, nutrients did not increase dramatically in the lake arm. Tributaries tended to have somewhat less phosphorus but similar amounts of nitrogen compared to the Niangua and Little Niangua Rivers.
The high nutrient levels at the upper sites of both the Niangua and Little Niangua probably reflect surface runoff following the rain that fell on the morning of the sampling day. The increased phosphorus in the lake arm of the Little Niangua is likely either backflow from the Niangua River or runoff from heavy rainfall event in mid-April.