Nutrients

Nutrients - especially nitrogen and phosphorus - are key water quality parameters. They have direct or indirect impacts on plant growth, oxygen concentrations, water clarity, and sedimentation rates, just to name a few. Nutrient concentrations vary according to surrounding land use, season, and geology. Plants require a number of nutrients - carbon, oxygen, nitrogen, phosphorus, magnesium, calcium, silica, potassium, iron, zinc, and copper—to grow, reproduce, and ward off disease. Of these nutrients, nitrogen and phosphorus are of particular concern for two reasons:

• they are two of the most important nutrients essential for the growth of aquatic and terrestrial plants; and
• the amount of these nutrients being delivered into streams has increased significantly.

A high concentration of nutrients in the water can cause Eutrophication. This leads to excessive algal blooms, which then deplete oxygen as they decompose (Fig.1). Eutrophication gives the water an unpleasant taste, smell and appearance. The organic production can also lead to sediment accumulation.

Fig.1:  Eutrophication. Estuary B receives more nutrient loads than Estuary A. As a result, Estuary B experiences more plant production and organic material accumulation. Dissolved oxygen levels are also lower in Estuary B, especially in deeper water (Adapted from [1])

Nitrogen and phosphorus enter streams from several natural and manmade sources. Natural sources of nitrogen and phosphorus include:

• fresh water that runs over geologic formations rich in phosphate or nitrate
• decomposing organic matter and wildlife waste, and
• the extraction of nitrogen gas from the atmosphere by some bacteria and blue-green algae (known as nitrogen fixation)

The three major manmade or anthropogenic sources of nutrients are:

• Atmospheric deposition, including fossil fuel burning by power plants and automobiles. Nutrients from these sources may fall to the land or estuary either directly or along with precipitation
• Surface water, inputs include point and nonpoint source discharges: effluent from wastewater treatment plants, urban stormwater runoff, lawn and agricultural fertilizer runoff, industrial discharges, and livestock wastes
• Groundwater, sources are primarily underwater seepage from agricultural fields and failing septic systems.

Nutrient concentrations are always in flux, responding to changes in:

• precipitation and amount of runoff
• fertilizer or manure application rates
• estuary flushing rates
• water temperature
• biological activity in the estuary; and/or
• the status of other water quality parameters

Nutrient concentrations are usually greatest during spring and early summer, when fertilizer use and water flow from tributaries and irrigation activities are high. High nutrient concentrations can also be detected during seasonal low-flow conditions. During winter low-flow periods, for example, the lack of land and aquatic plant uptake combined with contributions from groundwater can result in high nitrogen levels. Nutrient levels downstream from urban areas may also be high during low-flow periods. At these times, contributions from point sources can be greater relative to streamflow, and dilution is less. Nutrient levels also vary among watersheds. Natural features (e.g., geology and soils) and land management practices (e.g., drainage and irrigation) can affect the movement of nitrogen and phosphorus over land, creating local and regional effects on water quality.

 References [1], [2]

Nitrite (NO₂^-), Nitrate (NO₃^-), Ammonium (NH₄⁺)

Nitrogen is one of the most common air deposition pollutants, especially in the eastern United States. Since 1940, human activity has doubled the rate of nitrogen cycling through the global atmosphere, and the rate is accelerating (Vitousek et al., 1997). Depending on the waterbody and watershed being considered, it is estimated that roughly a quarter of the nitrogen in an estuary comes from air sources (Paerl and Whiteall, 1999). Each watershed is different, but extensive modeling and monitoring in the Chesapeake Bay watershed have produced estimates of the sources of nitrogen in that particular watershed.

The estimated sources of nitrogen in the Chesapeake Bay are:

• Waterborne point sources (e.g., industry, sewage treatment plants, etc.)
• Runoff from land (e.g., farms, lawns, city streets, golf courses, etc.)
• Air sources (e.g., electric power plants, vehicles, municipal waste combustors, etc.)

Nitrogen's primary role in organisms is protein and DNA synthesis; plants also use this substance in photosynthesis.

Although nitrogen makes up about 80 percent of the earth's atmosphere, it is inaccessible to most terrestrial and aquatic organisms. Some types of bacteria and blue-green algae, however, can "fix" nitrogen gas, converting it to an inorganic nitrogen form—thereby making it available to other organisms.

The quantity and form of nitrogen in the water can also closely relate to dissolved oxygen levels. Bacteria are able to convert nitrogen into different nitrogen species and gain energy from the process. Through nitrification, some bacteria transform ammonium into nitrite and then to nitrate. This biological process consumes oxygen. When nitrification is inhibited by low dissolved oxygen conditions, ammonia or nitrite forms of nitrogen may accumulate.

Through denitrification, bacteria convert nitrate to nitrite and then to nitrogen gas. This process occurs under anoxic conditions and helps rid the system of excess nitrogen.

At high concentrations, nitrates are toxic to eelgrass, and ammonia is toxic to fish.

References [1] [2]

Phosphate (PO₄)

For plants, phosphorus is critical for metabolic processes, which involve the transfer of energy.

Phosphorus exists in the water in several forms:

• orthophosphate (inorganic, dissolved phosphorus) comes from fertilizers and is the form commonly measured (PO₄-P e.g. HPO₄^2-, H₂PO₄^- or K₃PO₄)
• organic phosphate, results from plant and animal waste. Decomposition of dead plants and animals also adds organic phosphorus to the water
• total phosphorus (dissolved and particulate)
• polyphosphate (from detergents)

In general, excess phosphates can enter an estuary from agricultural fields, water treatment plants, sewage, animal feedlot operations, and lawns. Many phosphorus species attach to soil particles and are, therefore, transported to the water with eroded soil. Especially high phosphorus loads are often delivered during periods of high runoff from storms or irrigation activities. Under oxygenated conditions, phosphate will form chemical complexes with minerals such as iron, aluminum, and manganese and fall to the bottom sediments. In cases when this nutrient is found mostly in sediments, water column concentrations may not provide a full picture of nutrient loads and impacts. If the bottom water in an estuary has no oxygen (e.g. in case of eutrophication), phosphate bound to the sediments is released back into the water. This release can fuel yet another round of phytoplankton blooms.

Reference [1]