Understanding Nutrient Pollution
Nutrients of Concern
Although essential for life, excess nutrients can pollute waters and soils and fuel harmful algal blooms. Two nutrients of primary concern are phosphorus and nitrogen, derived from natural and human (anthropogenic) sources. Because soils in Sarasota County are naturally rich in phosphorus, nitrogen is the primary concern for limiting algal growth. In other communities in Florida, phosphorus is of greater concern. Understanding the local hydrogeologic conditions of the Sarasota County region and the site-specific occurrence of these nutrients is essential to developing a nutrient management program.
The nitrogen cycle converts nitrogen into various chemical forms as it moves through air, soil, water, and living organisms (Figure 1). Earth’s atmosphere is composed of 78% nitrogen gas. This gas must be “fixed” into a usable form for plant use. Although nitrogen gas can be fixed by lightning, most fixation occurs when bacteria combine nitrogen with hydrogen to produce ammonia. Ammonia can also be created by bacteria during the decomposition of plants and animals or their wastes. Other types of bacteria can “nitrify” ammonia to nitrite and nitrate, which can then be used by plants. Nitrates are highly soluble in water and easily transported by groundwater. Other types of bacteria can “denitrify” nitrates to nitrogen gas, completing the nitrogen cycle. Denitrification is the key to cycling nitrogen from soils and waters back into the atmosphere, where it is unavailable to feed algal blooms. Lengthening the cycle by locking up nitrogen in plant and animal tissue can also reduce nitrogen availability for algal blooms.
Figure 1. The nitrogen cycle in soils and atmosphere. Denitrification is the key to cycling nitrogen from soils and waters back into the atmosphere, where it is unavailable to feed algal blooms. Lengthening the cycle by locking up nitrogen in plant and animal tissue (bioassimilation) can also reduce nitrogen availability for algal blooms.
Human activities have altered the natural nitrogen cycle through fossil fuel combustion (see Chapter 5), creation of synthetic fertilizers (see Chapter 4), and wastewater treatment (see Chapter 1 and Chapter 2). For example, the industrial Haber-Bosch process is used to produce ammonia for fertilizers by “fixing” nitrogen gas and hydrogen from natural gas or petroleum under high pressure. Reactive nitrogen created by the synthetic Haber-Bosch process has dramatically increased globally since the 1950s to more than 150 tons per year. Synthetic fertilizer has fueled historic increases in food production worldwide, allowing global human population to soar above 7.8 billion people. Unfortunately, these fertilizers often find their way into natural waters, degrading water quality. Over-reliance on fertilizer has also degraded soils as less emphasis is placed on building and maintaining natural soil microbial health in urban and agricultural areas. Improving soils and reducing nitrogen-based fertilizer applications are important strategies for reducing nitrogen pollution (Chapter 4).
Conventional septic systems and older centralized wastewater treatment facilities with only secondary treatment technologies use bacteria to convert some of the nitrogen in wastes to nitrogen gas. Some of the remaining reactive nitrogen in septic drainfields leeches into surrounding soil and water. Advanced septic systems and advanced wastewater treatment technologies convert significantly more nitrogen from waste to nitrogen gas before releasing it to the environment (Chapter 1 and Chapter 2).
Aquatic systems in Sarasota County are sensitive to nitrogen inputs. Excess anthropogenic nitrogen loading in these systems can fuel micro- and macroalgal blooms that block sunlight to submerged aquatic vegetation, deplete oxygen, degrade water quality, and kill aquatic life (Figure 2).
Figure 2. Increasing nutrients feed algae that cloud the water and reduce light availability for seagrass. Without adequate light, seagrass meadows die off along with the fish and wildlife they support.
Some algae release toxins that directly kill aquatic life and can significantly threaten human health. Impacts to human recreation, economies, and fisheries can be profound.
Effective strategies to reduce excess anthropogenic nitrogen loading include:
- Improved wastewater treatment
- Restrictions on fossil fuel emissions
- Improved fertilizer management
- Improved biosolids treatment and management
Phosphorus occurs naturally in oxidized minerals, such as inorganic phosphate rocks. Humans mine phosphorus compounds from natural deposits in the earth to produce fertilizers, detergents, pesticides, and nerve agents. Organic sources of phosphorus such as livestock manure and wastewater biosolids have been used for centuries by humans as fertilizer.
Sources of nitrogen and phosphorus.
Phosphorus tends to attach to natural minerals within soil particles but may also seep into surface water bodies via groundwater flow. Soil erosion can significantly contribute to phosphorus in waterways. Bank erosion from fast-flowing flood waters can scour and transport phosphorus downstream to delta or floodplain areas where the phosphorus-rich sediments are deposited.
Sarasota County is located within the Bone Valley of the Hawthorn geological formation and is naturally rich in phosphorus. The richest deposits in the Bone Valley area are in Polk County where phosphate has been mined for decades.
The Bone Valley region of west central Florida extends into Manatee and Sarasota Counties with major hard rock deposits in northeastern Manatee County and southern Polk County. Sarasota County prohibits phosphate mining.
Over millennia, the Sarasota County region has adapted to these high natural phosphorus concentrations. As a result, growth of photosynthetic organisms is not typically phosphorus-limited. In many instances, lawns, urban landscapes, and golf courses in the County can thrive naturally without the addition of phosphorus fertilizers. Nevertheless, many aquatic systems are sensitive to excess phosphorus, leading to micro- and macroalgae blooms, oxygen depletion, and mortality of aquatic life. Effective strategies to reduce excess anthropogenic phosphorus include:
- Use of Advanced Wastewater Treatment
- Restrictions on phosphorus detergents
- Improved fertilizer management
- Improved biosolids treatment and management
Quantifying Nutrient Pollution
Nutrient Load Measurements
The annual land-based nutrient load to receiving waters like Sarasota Bay is the product of the annual runoff volume and the nutrient concentration in that water. Therefore, data on both water quantity (water flow) and water quality (nutrient concentrations) are needed. Longer periods of record, for which both water quality and water quantity monitoring data are available, yield greater precision for estimating variability from year to year and long-term averages.
Fortunately, Sarasota County has established an extensive monitoring program for the primary watersheds that contribute to its bays (Figure 3) (see Chapter 10.1). This monitoring program was initiated between 2004 and 2007, providing more than a decade of annual nutrient load determinations. There are also state (e.g., Southwest Florida Water Management District, Florida Department of Environmental Protection) and federal (e.g., U.S. Geological Survey) monitoring stations that provide long-term or short-term historical water quantity or quality data. For example, the USGS has operated a continuous streamflow gage on the Myakka River north of State Road 72 since 1939.
Figure 3. Location map and inventory of Sarasota County’s network of water quality and water flow sampling stations. Source: Sarasota County Water Atlas
Nutrient Load Estimation
Gross nutrient loads are typically estimated by applying land-use specific event mean concentrations (EMCs) to an estimated average annual runoff volume (hydraulic loading) for that land use. EMCs were established by the State of Florida by sampling runoff from various representative land uses. A second approach utilized by the South Florida Water Management District (SFWMD) applies nutrient budget coefficients for imports, exports, retainage, and discharge that are similarly specific to land uses. These nutrient budget coefficients were developed through a survey and interview program with land use managers in association with the Florida Department of Agriculture and Consumer Services (FDACS).
Both approaches require an estimation of the hydraulic loading (runoff volume) that is also dependent on the land use/cover. The state periodically produces the Florida Land Use, Cover and Classification System (FLUCCS) inventory and maps. The most current FLUCCS maps are based on 2017 cover (Figure 4). Land use/cover categories have been aggregated to align with the state’s adopted EMCs and/or the nutrient budget coefficients.
Figure 4. The 2017 land use and land cover map for Sarasota County. Source: Southwest Florida Water Management District
Gross loads assume an average annual hydraulic loading (runoff volume), understanding that the hydraulic and nutrient loadings will vary from year to year. Regardless, there is a fairly significant difference between the annual gross nutrient loads estimated by the two approaches. One or neither of these approaches may accurately estimate actual nutrient loads, therefore it is critical to reconcile any estimation approach or model to actual conditions through calibration. Once calibrated, variables such as land coverages or nutrient reduction strategies could be considered for future or scenario planning. It may also be possible to develop a “pre-development” or baseline land cover map by utilizing historical survey and soils information.