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

NEAR-SURFACE DINAnnual median near-surface DIN concentrations ranged between 0.02 and 0.52 mg N/Lat the tidal Myakka River stations, and between 0.03 and 0.40 mg N/L at the tidal PeaceRiver stations (Appendix B). Median concentrations appear to vary inversely withsalinity in both river systems.If 0.01 mg DIN/L — which was the effective detection limit for DIN forms in the Districtmonitoring program for DIN forms, and the mid-range of potential N limitation valuesdiscussed by Chapra 1997 — is used to estimate the threshold for N limitation inphytoplankton, it appears that N availability may have limited phytoplankton productivityat several of the tidal river stations sampled during 1993-2000. DIN concentrationsbelow the laboratory detection limit were observed during at least one sampling event ateach station. The 25 th percentile value was equal to the detection limit in one or morewater years at each station, indicating that DIN concentrations equal to or less than thedetection limit were observed on 25% of the sampling dates at the station during thatyear.4.3 CHLOROPHYLL-aChlorophyll-a concentrations, which provide estimates of phytoplankton biomass in thewater column, are helpful indicators of a waterbody’s trophic state. A summary of nearsurfacechlorophyll-a concentrations observed at the tidal Peace and Myakka Rivermonitoring stations is shown in Appendix B. Annual median values ranged between thelaboratory detection limit (1.0 µg/L) and 13 µg/L at the tidal Myakka stations andbetween 2.3 and 10.6 µg/L in the tidal Peace. The statewide median chlorophyll-aconcentration for Florida estuaries, as reported by Freidemann and Hand (1989), is 8.5µg/L.Algal blooms (indicated by chlorophyll-a concentrations > 20 µg/L) occurred at everystation at some time during the monitoring period. Pronounced blooms, with chlorophyllconcentrations > 50 µg/L, were observed at three sites (CH-002, CH-02B, and CH-004).Annual mean chlorophyll-a concentrations observed at several stations during wateryears 1994-2000 exceeded 11 µg/L, a value that has been proposed by the State ofFlorida as representative of potentially impaired water quality conditions in the State’sestuarine waters. Stations and years where the proposed impairment level wereexceeded include:• CH-002 (water years 1995, 1999)• CH-02B (1995, 1998, 1999)• CH-029 (1995, 1996)• CH-004 (1994, 1995, 1996, 1999)• CH-05B (1999)• CH-005 (1994).56