THE IMPACTS OF STORMWATER DISCHARGES ON EUTROPHICATION IN THE FLORIDA KEYS

Brian E. Lapointe, Ph.D.
William R. Matzie, International Marine Research, Inc. Route 3, box 297A Big Pine Key, FL 33043 Phone (305) 872-2247

Estuaries, v19 n2B p422-435, June. 1996

EXECUTIVE SUMMARY
Storm water discharge is a major pathway by with land-based pollutants enter coastal waters. This project involved monitoring the effects of storm water inputs using four continuos-recording water quality monitoring instruments (Hydrolab Datasounde) along a nearfield transect extending from Big Pine Key to Looe Key National Marine Sanctuary (LKNMS); these instruments recorded dissolved oxygen salinity temperature, and tidal stage at 30 min. intervals. Water samples were also collected for nutrient determinations along this and another far-field transect (extending from Shark River on the southwest Florida shelf to LKNMS) to determine relationships between nutrient concentrations and salinity from both near-field and far-field watersheds.

Concentrations of nutrients, chlorophyll a, and turbidity increased along the near-field transect with increased freshwater inputs between the dry season (April-May, 1992) and the wet season (June-August, 1992). Freshwater discharges in early June resulted in anoxic conditions at the most inshore monitoring station (Port Pine Heights canal), a station directly impacted by septic tank leachate. High concentrations of total phosphorus correlated with freshwater inputs along both transects, indicating land-based runoff from the Keys and southwest Florida as nutrient sources. High concentrations (up to 21 uM) of ammonium followed rainfall events at the inshore stations; ammonium concentrations averaged > 1.0 uM along the entire near-field transect over the study period. Chlorophyll a and turbidity increased from the dry season into the wet season in response to the land-based storm water nutrient inputs.

A comparison of nutrient and chlorophyll-a concentrations in coastal waters of the Keys with threshold values for eutrophication on coral reefs suggests that waters of the Keys have entered a stage of critical eutrophication. Ammonium concentrations have increased several-fold at LKNMS over the past seven tears and total phosphorus, chlorophyll a and turbidity are currently 2-3 fold higher than on pristine coral reefs in the western Caribbean. The high levels of nutrients, chlorophyll a, and turbidity in coastal waters of the Keys demands that special precautions be exercised in the treatment and discharge of wastewaters and land-based runoff.