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

4.1 PHYSICAL PARAMETERSNear-Surface SalinityAnnual near-surface salinity patterns at the seven monitoring stations during wateryears 1994 – 2000 are summarized, in percentile form, in Appendix B.The most upstream stations in both river systems (CH-001 in the tidal Myakka and CH-029 in the tidal Peace) were located in oligohaline areas, with median salinities 20 ppt).Median near-surface salinities at the remaining stations were generally mesohaline,ranging between 10 and 25 ppt. Stations CH-002 (Myakka) and CH-004 (Peace)exhibited similar median values, of between 10 and 22 ppt. Stations CH-02B (Myakka)and CH-05B also exhibited similar median values, between 13 and 24 ppt. Highestmedian salinities (18–26 ppt) occurred at station CH-005, near the mouth of the PeaceRiver.The maximum salinity value shown in Appendix B (44.95 ppt), a near-surface valuereported from station CH-005, apparently reflects an instrument malfunction ortypographic error, and has been removed from the District database (C. Anastasiou,personal communication).Near-Surface ColorNatural water color, produced as a result of decomposition of leaf litter and otherrefractory organic matter, has been shown to be an important source of light attenuationin the Peace and Myakka river systems and portions of Charlotte Harbor (McPhersonand Miller 1987).Among the stations sampled during water years 1994-2000, median near-surface colorlevels varied inversely with median near-surface salinities. Color levels were quite highat the most upstream stations (CH-001 and CH-029), fell by about 50% between thosestations and the next downstream sampling locations (CH-002 and CH-004), anddeclined another 20-40% at the highest-salinity stations (CH-02B, CH-05B, and CH-005) (Appendix B).Color levels in estuaries are affected by a variety of physical and chemical processes.The simplest physical process is dilution, which occurs when highly colored river watermixes with sea water containing much lower concentrations of the dissolved andcolloidal humic acids and other compounds that contribute to water color. Humicmaterials are also removed from the water column through two physical-chemicalprocesses — flocculation and adsorption — whose rates increase when the materialsare transported from fresh to more saline waters. In estuaries, flocculation of humicacid colloids reportedly occurs preferentially at salinities between 0 and 5 ppt (Day et al.1989).52