Peace and Myakka River Water Quality Summary 2002 - Southwest ...

stations:• Peace Creek Canal near Wahneta (1999)• Peace River at Fort Meade (1998, 1999, 2000)• Peace River at Zolfo Springs (1998, 1999, 2000)• Peace River at Arcadia (1998)• Horse Creek near Myakka Head (2000)• Horse Creek near Arcadia (1998)• Myakka River near Sarasota (1999, 2000, 2001).CONSTITUENT LOADS AND YIELDSEstimation of loads and yields of water and water quality constituents is a basiccomponent of most watershed assessment programs. ‘Load’ represents the mass of aconstituent (e.g., tons of nitrogen) passing a given point in a stream during a specifiedtime interval (e.g., month, day or year). The ‘yield’ of a constituent is the load divided bythe surface area of the contributing drainage basin, allowing loads produced by basinsof different size to be compared on a consistent, per-unit-area basis.In the present study the largest estimated yields of total nitrogen were observed at theSaddle Creek station during the 1998 water year, and at the Joshua Creek, Shell Creekand Big Slough-Myakka City stations during water year 2001. (It is likely that largeconstituent yields occurred at every station during the 1998 El Niño event, but monthlywater quality sampling was not conducted at many sites in that year.)Relatively large estimated yields of dissolved inorganic nitrogen (DIN) and nitrate-plusnitrite(NO 2+3 -N ) nitrogen occurred at the Joshua Creek-Nocatee site in all years. Avisible spike occurred at this site in water year 2001, when DIN and NO 2+3 -N yieldsexceeded 5 kg N/ha/yr. Other stations with estimated inorganic nitrogen yields greaterthan 1 kg N/ha/yr included:• Peace River at Zolfo Springs (1998)• Peace River at Arcadia (1998)• Big Slough Canal near Myakka City (2000, 2001)• Peace River at Fort Meade (1998).Substantial increases in the estimated yields of DIN (and PO 4 -P) occurred between theupstream and downstream stations located on Horse Creek, suggesting the potentialpresence of anthropogenic sources of those constituents at one or more locationsbetween the two stations.Estimated yields of inorganic and total phosphorus followed similar temporal and spatialpatterns, with the largest values occurring at gages on the main stem of the PeaceRiver (e.g., Fort Meade, Zolfo Springs, and Arcadia) during 1998. Relatively largeyields also occurred on the main stem of the Myakka River and at the Big Slough Canal-Myakka City station during 2001.viii