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

The primary manmade influences include the discharge of nutrients and organic wastesinto waterbodies, which can lead to increased eutrophication, higher rates ofphotosynthesis, respiration and decomposition, and larger daily and seasonalfluctuations in DO concentrations.The Florida standard for DO in fresh surface waters is 5 mg/L. For brackish and marinewaters, DO should not average less than 5 mg/L in a 24-hour period and should neverbe less than 4 mg/L. Laboratory studies have shown that DO concentrations below 4mg/L are stressful for most fish and many shellfish species, and concentrations below 2mg/L are highly stressful and potentially fatal for many organisms.During water years 1998 through 2001, DO concentrations below the 5 mg/L standardwere observed at least once at each monitoring station in the freshwater portions of thePeace and Myakka River watersheds. Many of these instances may have beenassociated with natural factors, such as high water temperatures. At several stations,however, low DO levels were a frequent occurrence, observed on 25% or more of thesampling dates. And for some stations the median annual DO concentration (the valuemeasured on at least half the sampling dates during a year) was less than the 5 mg/Lstandard. This latter group of stations included:• Peace Creek Canal near Wahneta (water years 1998, 1999, 2000, and 2001)• Saddle Creek at Structure P-11 (1999, 2001)• Peace River at Bartow (1998, 1999, 2000, 2001)• Shell Creek near Punta Gorda (1998)• Deer Prairie Slough near North Port (2000, 2001)• Big Slough Canal near North Port (2001).Chlorophyll-aChlorophyll-a concentrations, which are used to estimate the biomass of algae andother phytoplankton in the water column, are among the best indicators of awaterbody’s trophic state. Annual mean chlorophyll-a concentrations in Florida streamsare usually at or below 5.5 µg/L. FDEP considers stream water quality to be “impaired”if the annual mean is above 20 µg/L.Annual mean chlorophyll-a concentrations greater than 20 µg/L were observed at twomonitoring stations during the 1998-2001 water years:• Saddle Creek at Structure P-11 (all years)• Peace River at Bartow (1998, 1999).The elevated chlorophyll concentrations at these stations were apparently caused bydischarges from Lake Hancock, a hypereutrophic lake that experiences chronic, yearroundblooms of planktonic algae and cyanobacteria.Individual observations of chlorophyll-a concentrations greater than 20 µg/L, whichindicated the presence of occasional phytoplankton blooms but did not represent aviolation of state water quality standards, were observed at a number of additionalvii