Biscayne Bay, a shallow, oligotrophic, subtropical estuary along the southeastern coast of Florida, provides habitat for a wide range of plant and animal species. Increased nutrient loads in discharges from the east coast canals, as a result of agricultural and urban processes, are a potential threat to the health of Biscayne Bay. Dissolved-oxygen concentrations average about 5 mg/L in Biscayne Bay, but hypoxic conditions exist in the east coast canals along with nutrient enrichment. Plans are being formulated to reestablish natural flow to Everglades National Park by diverting water that now flows through the agricultural/urban corridor and then discharges by way of the canals to Biscayne Bay. An understanding of nutrient loading to Biscayne Bay is needed for the assessment of the ecological health of the Bay as well as an evaluation of the water-quality impact of the diverted water to Everglades National Park.

The U.S. Geological Survey is presently conducting a study to: (1) determine whether point samples (historically collected at 1 m below the surface near the centroid of flow) accurately represent water quality in the stream cross section, and (2) determine estimates of nitrogen and phosphorus loads discharging from the east coast canals to Biscayne Bay. This study uses a statistical approach to compare point with depth-integrated samples collected using the equal-width-increment method, and the use of regression models to develop relations between loads or concentrations and instantaneous discharge.

During the 1996 water year, water samples were collected at five canal sites (Biscayne Canal, Little River Canal, Miami Canal, Tamiami Canal and Snapper Creek Canal) to determine concentrations of total ammonia nitrogen, total organic nitrogen, total nitrate nitrogen, total nitrite nitrogen, total phosphorus and orthophosphate. Instantaneous discharge was determined from field data collected concurrently with each sampling event; discharge ratings were obtained from another South Florida Ecosystem Project. Preliminary analysis, as determined by the Wilcoxson signed rank test, indicates that statistical differences exist between point and depth-integrated samples. Regression equations were developed at one site using an ordinary least squares technique to relate loads of total nitrogen and total phosphorus with discharge. The coefficients of determinations were 0.86 and 0.57, respectively, indicating that 86 percent of the variation in the total nitrogen load and 57 percent of the variation in the total phosphorus load are accounted for by the variation in discharge.

Currently, water samples are being collected during periods of varying flow conditions upstream of the coastal structures at 10 sites in eastern Dade County. These sites are located in Snake Creek Canal, Arch Creek Canal, Comfort Canal, Coral Gables Canal, Cutler Drain Canal, Black Creek Canal, Princeton Canal, Military Canal, Mowry Canal and Model Land Canal. Similar analysis is being conducted on the water samples from these sites.

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