A highly controlled water-management system has evolved in south Florida as a result of a rapidly expanding population along the east coast and an increase in agricultural and urban activities. This managed system has led to an alteration in the hydrology and surface-water flow patterns of south Florida. Additionally, increased nutrient loads in discharges from the east coast canals, as a result of agricultural and urban processes, have had a deleterious effect on the water quality of Biscayne Bay, which is classified by the State as an Outstanding Florida Water. One issue which needs to be addressed, as part of the South Florida Ecosystem Program, is the quantification of nutrient loads discharged from the east coast canals to Biscayne Bay (fig. 1).

Figure 1 -- Sampling sites in eastern Dade County.

Of major concern in many coastal areas around the Nation is the ecological health of bays and estuaries. A common problem in many of these areas is increased nutrient loads as a result of agricultural, commercial, industrial, and urban processes. Biscayne Bay, a shallow subtropical estuary, has average depths of about 2 to 3 meters along the southeastern coast of Florida. The Biscayne Bay ecosystem provides an aquatic environment that is a habitat to a diverse array of plant and animal communities. Nutrients are essential compounds for the growth and maintenance of all organisms and especially for the productivity of aquatic environments. Nitrogen and phosphorus compounds are especially important to seagrass, macroalgae, and phytoplankton. However, heavy nutrient loads to bays and estuaries can result in conditions conducive to eutrophication and the attendant problems of algal blooms and high phytoplankton productivity. Additionally, reduced light penetration in the water column because of phytoplankton blooms can adversely affect seagrasses, which many commercial and sport fish rely on for their habitat. Providing reliable estimates of nonpoint source nutrient loads to Biscayne Bay is important to the development of nutrient budgets as well as input to eutrophication models. Understanding the effects of these nutrient loads is a necessary initial step in planning restoration of the ecological health of Biscayne Bay.

One objective of this project is to determine if point samples adequately represent the cross-sectional water quality of the stream. Additionally, models will be developed to provide reliable estimates of nitrogen and phosphorus loads based upon the relation between the concentration of these substances and instantaneous discharge at coastal sites.

Nutrient data have been collected from the east coast canals for many years by the Dade County Department of Environmental Resources Management. Much of the data, which have been collected, are from point samples collected about 1 meter below the surface from the centroid of flow. The degree to which these samples adequately represent nitrogen and phosphorus concentrations within the water column of the canals of south Florida is presently unknown and limits confidence in loading estimates. Furthermore, the relation between discharge and nutrient concentration that occurs in natural uncontrolled streams in other parts of the Nation may not apply to the artificially controlled canals of south Florida. Both of these issues need to be addressed to develop nutrient budgets and to plan effective restoration strategy now and in the future.

Water-quality samples will be collected upstream of discharge points of selected canals in eastern Dade County (fig. 1) during various seasons and flow conditions. Procedures will be applied to determine if any statistical differences exist between depth-integrated and point samples. Depth-integrated samples will be collected by the equal-width-increment method (fig. 2). This method consists of raising and lowering a sampler throughout several verticals in the stream at a constant rate so that the bottle is not overfilled when returned to the surface and then compositing the water samples in a churn splitter. To establish the number of verticals that need to be sampled and to determine horizontal and vertical variability within the water column, water-quality cross-sectional surveys will be made of temperature, specific conductance, pH, and dissolved oxygen. Point samples, obtained concurrently with the depth-integrated samples, will be collected at 1 meter below the surface in the centroid of flow. Additionally, the upstream and downstream stages and gates openings will be recorded during each sampling event to compute the instantaneous discharge from the rating curves. These rating curves have been developed using acoustic Doppler methodology as part of another South Florida Ecosystem Program project that is now in progress. Linear and nonlinear regression techniques will be employed to develop models for computing load based upon the relation between nitrogen and phosphorus concentrations and instantaneous discharge.

Figure 2 -- Depth-integrated samples collected using the equal-width-increment method.

• A comprehensive report is planned. This report will describe site locations, sampling methods and procedures, statistical comparisons between point and depth-integrated samples, and load models for nitrogen and phosphorus species for selected canals.
• Quarterly status reports are planned. These status reports will describe the progress of the project.

Tentative dates and the planned project activities are summarized below:

05/96 - 04/97: Conduct point and depth-integrated sampling at fives sites in eastern Dade County.
10/96 - 09/97: Conduct point and depth-integrated sampling at the remaining 11 sites in eastern Dade County.
05/97 - 04/98: Conduct an analysis of the data and prepare interpretive report.

For the duration of the project, close collaboration and exchange of technical information will be maintained with other projects of the South Florida Ecosystem Program as well with other Federal, State, and local agencies, including the South Florida Water Management District, and the Dade County Department of Environmental Resources Management. Additionally, close collaboration with the USGS South Florida National Water Quality Assessment Program will be maintained.

FS-129-96
July 1996
By Arthur C. Lietz

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For more information contact:

Arthur C. Lietz
U.S. Geological Survey
(Water Resources of Florida)
9100 N.W. 36th Street
Suite 107
Miami, FL 33178
(305) 526-2895
alietz@usgs.gov

Related information:

SOFIA Project: Determination of Nutrient Loads to East Coast Canals