Home Introduction Study Area Methods & Procedures Estimation of Nutrient Loads & Water-Quality Analyses Summary & Conclusions References PDF Version

Introduction

A major concern in many coastal areas across the Nation is the ecological health of bays and estuaries. One common problem in many of these areas is nutrient enrichment as a result of agricultural and urban activities. 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 transported 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.

Figure 1. Location of the east coast canal sites and subregions in Miami-Dade County [larger image].

Biscayne Bay, a shallow subtropical estuary along the southeastern coast of Florida, provides an aquatic environment that is habitat to a diversity of plant and animal species. Increased nutrient loads in discharges from the east coast canals in southern Florida (fig. 1), as a result of agricultural and urban activities, are a potential threat to the health of Biscayne Bay. Dissolved-oxygen concentrations average about 5 mg/L (milligrams per liter) in Biscayne Bay, but hypoxic conditions along with nutrient enrichment exist in the east coast canals. Plans are being formulated by water-management officials to reestablish natural flow to Everglades National Park (ENP) by diverting water that now flows through the agricultural/urban corridor and then discharges by way of canals to Biscayne Bay.

An understanding of nutrient loading to Biscayne Bay is needed for both an assessment of the ecological health of the bay as well as an evaluation of the water-quality impact of the diverted water to ENP. The U.S. Geological Survey (USGS) began a study in April 1996 to: (1) develop methodology for the purpose of estimating nutrient loads in Biscayne Bay, and (2) determine whether point (grab) samples, historically collected at 0.5 and 1 m (meter) below the surface near the centroid of flow, accurately represent water quality in the stream cross section. The study was done as part of the South Florida Ecosystem Program, which is a collaborative effort by the USGS; various other Federal, State, and local agencies; and Indian Tribes to provide earth science information needed to resolve land-use and water issues in southern Florida.

For planning purposes, Biscayne Bay is divided into three subregions; namely, north bay, central bay, and south bay (Alleman, 1995). North bay extends from about 5 mi (miles) north of the Dade/Broward County boundary to the Miami shoreline, central bay extends from the Miami shoreline to the featherbed banks east of Model Land Canal, and south bay extends from the featherbed banks east of Model Land Canal to Barnes Sound (fig. 1). Certain areas of north bay exhibit severely degraded water quality that largely is attributed to heavy loads of nutrients and toxicants from the tributary canals, including Arch Creek, Snake Creek, Biscayne Canal, and Little River Canal. Hypoxic conditions exist in the canals and high concentrations of ammonia, total phosphorus, and trace elements are common. Water quality in the central bay region ranges from pristine to highly degraded. Miami Canal, flowing through a highly urbanized area, contributes heavy discharges of pollutants to Biscayne Bay. Tributary or coastal canals that drain into central bay, such as Miami Canal, Coral Gables Canal, and Snapper Creek Canal, tend to be nutrient enriched and contribute toxicants to Biscayne Bay. Canals draining into south bay receive nutrient enrichment from agricultural activities. Black Creek Canal contains high levels of ammonia as a result of leachate from the south Miami-Dade landfill.

The purpose of this report is to present methodology that can be used to estimate nutrient loads discharged from the east coast canals into Biscayne Bay in southeastern Florida. Graphical summaries have been created to compare medians and evaluate the distribution of nutrient concentrations in the east coast canal system based on land-use categories in the Biscayne Bay watershed. Vertical depth profiles of dissolved-oxygen, total phosphorus, and total nitrogen concentrations have been constructed to document horizontal and vertical variability within the canal system. A statistical approach is used to compare differences between point (grab) and depth-integrated samples (from the east coast canal sites) for total nitrogen and total phosphorus concentrations. A modeling technique is used for developing equations through simple linear regression analyses in order to estimate nutrient loads based on discharge. The nitrogen and phosphorus species described in this report include total organic nitrogen, ammonia, nitrite, nitrate, nitrite plus nitrate nitrogen, total phosphorus, and orthophosphate. Most of the data presented in this report were collected from the east coast canal sites during the 1996 and 1997 water years. Data from one site (S-26 along Miami Canal) were collected from 1966 to 1996.

The ecological health of Biscayne Bay is dependent on nutrient loads. An understanding of the processes that control nutrient loads to the bay is needed to properly develop restoration efforts in southern Florida. Nutrients fall into three categories - macronutrients, micronutrients, and trace nutrients. The macronutrients (generally represented as nitrogen, phosphorus, and carbon species) are required in the greatest amounts and are major components in the cells of all organisms. Nitrogen and phosphorus compounds are important to free-floating type plants, such as algae (Hem, 1985, p.128). Micronutrients consist of trace elements such as manganese, copper, and zinc.

Nitrogen, phosphorus, and carbon species can exist in both organic and inorganic forms. The most common inorganic forms of nitrogen are ammonia, nitrite, and nitrate. Total organic nitrogen can exist as particulate or nonparticulate organic nitrogen. Most of the nitrogen compounds in surface-water bodies exist in an oxidized form as nitrate and nitrite. The U.S. Environmental Protection Agency (1986) has established water-quality standards or guidelines for nitrate, nitrite, nitrite plus nitrate nitrogen, and ammonia. Nitrate, nitrite, and ammonia in concentrations above established standards or guidelines can be detrimental to the health of humans, and ammonia is toxic to aquatic organisms.

Phosphorus exists inorganically as soluble reactive phosphorus and in the particulate inorganic and nonparticulate inorganic states. Soluble reactive phosphorus is commonly referred to as orthophosphate. Particulate inorganic phosphorus consists mostly of phosphate minerals, such as apatite. Nonparticulate inorganic phosphorus includes condensed phosphates, such as those found in detergents. Phosphorus exists organically as particulate organic and nonparticulate organic phosphorus. Particulate organic phosphorus exists in plants, animals, and organic detritus. Nonparticulate organic phosphorus comprises mainly dissolved or colloidal organic compounds (Chapra, 1997, p. 523).

Nutrients exist in water bodies as a result of both natural and anthropogenic (manmade) sources, although contributions from natural sources are minimal. Anthropogenic contributions might be significant, however, and can result from agricultural activities, domestic and animal waste, municipal wastewater, and byproducts of manufacturing processes. Phosphorus generally is lower in concentration than other nutrients because it does not naturally occur in the atmosphere nor is it abundant in the earth’s crust. Phosphorus also tends to strongly adsorb to sediments and fine-grained particles, causing its removal from the water column. Nitrogen differs from phosphorus in that it naturally occurs in the atmosphere and does not adsorb as strongly to particulate matter. Additionally, denitrification acts as a removal mechanism for nitrogen under anaerobic conditions.

The nitrogen cycle principally is a series of oxidation and reduction reactions catalyzed by bacteria, mostly the Nitrobacter and Nitrosomonas species (Lawrence, 1996, p. 9). Under aerobic and anaerobic conditions, total organic nitrogen catalyzed by bacteria is decomposed to amine groups in a process called deamination. This reaction continues under aerobic conditions to produce ammonia (ammonification) as well as nitrite and nitrate (nitrification). The process is expressed below:

Furthermore, the microbes use nitrogen under aerobic conditions by consuming electrons in the oxygen molecule. However, during reducing conditions (usually when dissolved-oxygen concentration is below 1.0 mg/L), the microbes use electrons from oxygen in the nitrate molecule. With this process, the nitrate ion is depleted of oxygen (denitrification), resulting in the formation of nitrous oxide or gaseous nitrogen (Lawrence, 1996, p. 10). The nitrogen atom eventually is reduced by reacting with hydrogen ions to form ammonia in a process called nitrate reduction. The process is expressed below:

Most nitrogen compounds in surface-water bodies exist as nitrate because of prevailing aerobic conditions.

Eutrophication is defined as a natural or artificial addition of nutrients to a water body (Hackney and others, 1992, p. 693) and is the principal threat to lakes, estuaries, or streams that receive excessive total nitrogen and total phosphorus loads. Nutrient enrichment can result in inordinate algal production, known as cultural eutrophication, and produces an undesirable taste and odor problems to a water supply. Many other undesirable chemical effects also could be attributed to eutrophication. Algal dieoffs result in an increase in the amount of organic matter available for consumption by bacteria, resulting in oxygen depletion. If oxygen is depleted faster than it is replenished by photosynthesis or absorption from the atmosphere, hypoxic or anoxic conditions can ensue, thus adversely affecting aquatic species and ultimately resulting in fish kills. The consequences of oxygen depletion include changes in redox states, especially as they apply to certain trace elements. Sulfate reduction and methanogenesis can occur, iron sulfides can form in the sediments, and soluble iron can be transferred to the water column. Additionally, the release of phosphate to the water column from iron/phosphate complexes (because of changes in iron/sulfur oxidation states) could further accelerate the eutrophication process.

A considerable amount of historical surface-water-quality data exists from the east coast canal system in southern Florida, much of it having been collected intermittently for many years by the USGS for determination of concentrations of nutrients and major inorganic constituents. Since 1940, a voluminous amount of salinity monitoring data has been collected from the east coast canal sites. In 1974, the USGS began a program called the National Stream Quality Accounting Network (NASQAN), which was designed to determine long-term water-quality trends associated with major drainage basins throughout the Nation. Concentrations of major inorganic constituents and physical characteristics, nutrients, trace elements, suspended sediment, selected organic compounds, and bacteriological and biological constituents were determined at one NASQAN east coast canal site (S-26 along Miami Canal). Water samples from this site were collected monthly from October 1974 to September 1981 and quarterly from October 1981 to September 1994. Data that were collected under other programs since 1966 also exist. The Miami-Dade County Department of Environmental Resources Management (DERM) has collected surface-water-quality data from the east coast canals since 1979. Much of the same data, as determined under the NASQAN program, also were collected by DERM under a variety of programs. Most of the DERM data were collected as part of the Biscayne Bay Monitoring Program, General Canal Monitoring Program, or Intensive Canal Monitoring Program.

Previous studies involving the development of models based upon linear regression techniques are numerous. Smith and others (1982) studied trends in total phosphorus measurements at NASQAN stations based upon concentration/discharge models. Cohn and others (1989) studied the problem of retransformation bias, which can result when log-linear regression models are used to estimate constituent loads. Gilroy and others (1990) studied mean square errors of regression- based constituent transport estimates. Cohn and others (1992) evaluated the validity of a minimum variance unbiased estimator (MVUE) to estimate fluvial constituent loads. Watts (1993) and Ries (1994) used regression models to estimate monthly water-level changes in the Closed Basin Division of the San Luis Valley in Colorado and low-flow deviation discharges in Massachusetts, respectively. Belval and others (1994) estimated loads of water-quality constituents in the James and Rappahonnock Rivers in Virginia based upon multiple-regression techniques. Stoker and others (1995) used an ordinary least-squares technique to describe loading characteristics of McKay Bay, Delaney Creek, and East Bay in Tampa, Fla.

The author gratefully acknowledges the assistance and cooperation of the South Florida Water Management District (SFWMD) field offices in Miami and Homestead, with special thanks to Dawn Browning and Ron Dempsey, who gained access for the USGS to perform water sampling and collect hydrologic data at the control structures from the east coast canal sites in Miami-Dade County. Thanks also is extended to the National Oceanic and Atmospheric Administration for providing Miami-Dade County climatological data; to Frank McCune, environmental planner with Miami- Dade County, who supplied the land-use component of the Miami-Dade County Comprehensive Development Master Plan; and to Rick Alleman of the SFWMD for use of the map of Biscayne Bay showing subregions. The author gratefully acknowledges the assistance of hydrologic technicians Elizabeth Debiak and John Goebel and former civil engineering student trainee Frank Panellas of the USGS Miami Subdistrict office, all of whom collected much of the data for the study.