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

Secchi DepthSecchi depth — the average depth at which a standardized (20 cm diameter) white orblack-and-white disk disappears and reappears when lowered into the water columnfrom the deck of a boat or dock — is a simple measure of water transparency. Becausetransparency is affected by a number of factors, including water color, TSS andturbidity, Secchi depths can exhibit substantial temporal and spatial variability within andbetween water bodies. Median statewide Secchi depth values for Florida rivers andestuaries, reported by Friedemann and Hand (1989), are 0.8 m and 1.1 m, respectively.Median Secchi depth values for the tidal Peace and Myakka River stations are shown inAppendix B. The lowest median values were recorded at the most upstream stations(CH-001 and CH-029) in both river systems, presumably reflecting the high color andturbidity levels that occurred at those sites. In general median Secchi depths appearedto increase along the salinity gradient in both rivers, although reductions occurred atstations CH-02B and CH-05B. Both of these stations were located in shallow shorelineembayments away from the rivers’ main stems, where water transparency may havebeen affected by site-specific factors.Near-Surface pHpH is a measurement of the acidity of a water sample. Values 7 are considered basic or “alkaline.” Due to theirchemical characteristics, marine and estuarine waters tend to be more highly bufferedthan fresh water systems, usually exhibiting pH values in the range of 7.8 to 8.3(marine) or 7.5 to 8.8 (estuarine) standard units (Day et al. 1989).Highly acidic (ph < 4.5) or basic (pH > 9.5) conditions, which can occur due to a varietyof natural or man-made factors, can be harmful or fatal to aquatic organisms, eitherdirectly or due to increased mobilization or toxicity of other substances in theenvironment.pH values varied over a relatively narrow range at the tidal Peace-Myakka stations(Appendix B), with annual median values at each station ranging between 6.8 and 7.9units. Among the individual monthly readings the most acidic value (5.5) occurred atthe most upstream of the tidal Peace River stations, perhaps reflecting the high colorlevels (caused in part by humic acids) that occurred at that site. The most basic value(8.9) occurred at station CH-004, a site that experienced relatively frequent algal bloomswhich may have led to increased pH levels as a result of CO 2 uptake by algal cellsduring photosynthesis.Dissolved OxygenAnnual median dissolved oxygen concentrations observed at the tidal Peace-Myakkastations during water years 1994-2000 varied between 5.7 and 7.7 mg/L mg/L(Appendix B). During the period, concentrations in the “stressful” (< 4 mg/L) range were54