After several years of a general decrease in “concentrations” of various nutrients from managed watersheds, substantial increases in the concentrations of nutrients and soil particles were observed in streams during the summer of 2009 (Makarewicz and Lewis 2009). At Graywood Gully, for example, concentrations of soil (TSS), total phosphorus (TP), soluble reactive phosphorus (SRP), total Kjeldahl nitrogen (TKN), and nitrate increased in the stream water. At Cottonwood Gully, after a 5-year decrease, nitrate concentration (NO3+NO2) increased to levels not observed since 2003. Similar increases were observed in the Southwest, Sand Point, North Gully, Sutton Point and Long Point subwatersheds.
Several factors may have contributed to this observed increase in the concentration of dissolved and particulate material; some are natural (variation in rainfall amount and intensity); others are affected by human actions (changes in land use or management practices). Although the increases observed in all the monitored streams may be related to new or changing farming practices, it could not be ruled out that the significant rainfalls in the spring and early summer of 2009 are not the cause. A limitation of the approach taken in 2008 and 2009 was that discharge was not measured as it was in the USDA study. Concentration of analytes is a function of discharge from streams; that is, as discharge increases, concentrations increase as more material is washed from the land and more material is dissolved. The observed increases could simply be due to the higher than usual rainfalls in May and especially June. For example, the daily rate of precipitation in June was twice the rate for any other previous year since 2002. May precipitation was the highest since 2003. Also, a visual inspection of this watersheds in summer of 2009 ruled out any major changes in land use. The increase in nutrient loss from all of the USDA watersheds during the summer of 2009 suggests that the approach taken of using concentration data only to evaluate temporal trends may misinterpreted.
The three objectives of this summer’s work were:
1) To reevaluate the stream concentration approach to assessment of stream water by converting the data in the amount of an analyte lost from a subwatershed and to apply a statistical approach that account for discharge;
2) To monitor and nutrient and sediment input from selected watersheds; and,
3) To develop rating curves of discharge and evaluate nutrient loss from the Inlet and South McMillan Creek.
Makarewicz, Joseph C. and Lewis, Theodore W., "Conesus Lake Tributaries" (2010). Technical Reports. 35.