Solutes in rivers and streams are influenced by interactions with the streambed, streambanks, and the land as they travel downstream. This ongoing process interests stream ecologists because some of those solutes are essential to the lives of organisms, most notably phosphorus and nitrogen.Their uptake, transformation, and release play an important role in determining the amount of life an ecosystem can support. As nutrient atoms are used in a stream they are displaced further and further downstream, leading to the term "nutrient spiraling" being used to describe the interdependent processes of nutrient cycling and downstream transport. These interdependencies can be looked at in two ways: (1) how nutrient supply affects biological productivity; and (2) how the stream ecosystem influences the supply of nutrients being transported downstream.

Basic Nutrient Cycles
It is a basic rule of ecology that chemical constituents of organisms are continually recycled between the biota and the environment. Carbon, nitrogen, phosphorus, and silicon are the elements most utilized by organisms as nutrients. Carbon, being readily available as dissolved carbon dioxide, is usually left out of consideration as one of nutrients considered to be most ecologically important. As a group nitrogen, phosphorus,and silicon are referred to as macronutrients.

Phosphorus

Phosphorus is often the nutrient in most limited supply and thus the nutrient most likely to limit the productivity of plants and other autotrophs in stream environments. Plants and microbes assimilate dissolved inorganic phosphorus (DIP) into cellular structures. This process transforms phosphorus into particulate organic phosphorus (POP). Dying cells may excrete or release particulate organic phosphorus as dissolved organic phosphorus (DOP). Dissolved organic phosphorus is then broken down to DIP by bacterial activity, making it available to autotrophs once again.

Major Forms of Phosphorus

• Dissolved Inorganic Phosphorus (DIP) (also known as orthophosphate or PO₄^-3)
• Dissolved Organic Phosphorus (DOP)
• Particulate Organic Phosphorus (POP)
• Particulate Inorganic Phosphorus (PIP)

General availability of phosphorus is not only shaped by its abundance in local rocks and soils, but also by physical-chemical transformations. Sorption of phosphate ions onto charged clay or organic particles occurs when dissolved inorganic phosphorus (DIP) concentrations are relatively high, while release or desorption is favored at low DIP concentrations. Additionally, DIP and DOP may complex with metal oxides and hydroxides to form insoluble precipitates which can be released under anaerobic conditions.

Silicon
Diatoms are the only lotic organism to which silicon is limiting. The silicon cycle begins with silicic acid being dissolved from the weathering of rocks and anthropogenic inputs (especially sewage). It is assimilated by diatoms and eventually released by chemical dissolution over time.

Nitrogen

The cycling of nitrogen within ecosystems is complex due to the many transformations in form it goes through as a result of microbial processing. The principal forms of dissolved organic nitrogen (DON) include urea, uric acid, and amino acids. Dissolved inorganic nitrogen (DIN) is present as ammonium (NH₄⁺), nitrate (NO₃^-), and nitrite (NO₂^-). Nitrogen is also present in gases as nitrogen gas (N2) and nitrous oxide (N₂O). The atmosphere, natural runoff, sewage, and agriculture all deliver various forms of nitrogen to receiving bodies of water such as streams and rivers.

1. Dissolved Inorganic Nitrogen (DIN)
   
   
• NO₃^-
• NO₂^-
• NH₄⁺
2. Dissolve Organic Nitrogen (DON)
3. Particulate Organic Nitrogen (PON)

In order for nitrogen to enter and move through an ecosystem it must be transformed by microbes a number of times. Initially, dissolved inorganic nitrogen (DIN - N₂,NH₃, NO₃^-,NH₄⁺) must be fixed or assimilated by organisms to make it available for inclusion in the synthesis of organic molecules. Autotrophs, bacteria, and fungi utilize NH₄⁺ preferentially over nitrate and nitrite, while bacteria and cyanobacteria fix nitrogen gas (N₂) in to NH₄⁺ for its inclusion in structural organic molecules. Following excretion and decomposition Ammonium (NH₄⁺) is converted to nitrite and then nitrate by nitrifying bacteria in aerobic conditions. If conditions are anaerobic, nitrate is converted to nitrite, and is eventually returned to the atmosphere as nitrogen gas by denitrifying bacteria. Denitrifying bacteria use ammonia as an energy source and nitrate as an oxidizing agent in the breakdown of organic matter under anaerobic conditions. In well-oxygenated streambeds, denitrification is of little consequence, but in deep sediments or in oxygen depleted streams, denitrification can be more important, occaisonally leading to an accumulation of ammonia.

Nutrient Concentrations in Rivers and Streams
River chemistry is highly variable over time and space, making nutrient concentrations difficult to use as a predictor of biological activity and overall water quality. Where human influence is thought to be minimal, such as in small temperate streams or large tropical rivers, natural levels of nutrients can be estimated. Nutrient concentrations from these systems are very similar to levels found in rainfall, supporting the thought that they are unpolluted. Dissolved phosphorus levels are low, approximately 0.01 mg/L for orthophosphate and 0.025 mg/L for total dissolved phosphate. Levels of DIN are about 0.12 mg/L, nitrate (84%) the major contributor and ammonia (15%) and nitrite (1%) making lesser contributions. It is important to note that human activity can dramatically influence nutrient concentrations in rivers through industrial emissions, sewage and agricultural inputs, and other alterations to watersheds that facilitate the delivery of nutrients from the landscape. Non point nutrient pollution is currently a topic of major interest in addressing river water quality. For an introduction to this topic read "Nutrients in the Mississippi River" at http://water.usgs.gov and "Sediments and Nutrients in the Mississippi" at www.americanrivers.org.

It is normal to find higher nutrient concentrations at downstream sites along large rivers primarily due to increasing human influence. Nutrient concentrations also vary seasonally due to seasonal hydrologic regimes, the growing season, and seasonal variation in human inputs. If the input of a nutrient is relatively constant it is normal then to find that low flows will concentrate the nutrient and high flows will dilute it. Uptake by organisms can also influence nutrient concentrations and can, for example, explain why summer nutrient concentrations can be extremely low.

Land use has been shown to clearly influence nutrient levels in streams. For example, as land loses forest cover, nitrogen and phosphorus levels increase, with nitrogen levels increasing to a greater degree.

Nitrogen:Phosphorus Ratios
The ration of nitrogen to phosphorus in water indicates which nutrient is likely to limit algal growth. In algal tissues carbon, nitrogen, and phosphorus are found in a very consistent ratio of atomic weights, approximating 106:16:1. This indicates that if nitrogen to phosphorus ratios fall below 16:1 algae will become limited by low nitrogen availability. Conversely, if N:P ratios exceed 16:1 algae will be limited by phosphorus availability. When ratios are between 10:1 and 20:1, evidence indicates that limitation by both nutrients is occurring.

It is more common to find N:P ratios exceeding 16:1, indicating that nitrogen is less commonly a limiting nutrient than phosphorus. This varies regionally, however, with eastern streams more commonly showing high nitrogen levels and phosphorus limitation (high N:P values), and western streams showing nitrogen limitation and relatively higher phosphorus levels (low N:P values).

Nutrient Spiraling
As a nutrient molecule travels downstream it changes form, from being available as a dissolved nutrient, assimilated into living tissue, possibly passing through several links in the stream´s food web, being released via excretion or decomposition, and re-entering the pool of available, dissolved nutrients. As this process occurs in running waters it is important to keep in mind that transport downstream is simultaneously occurring - thus conceptually making it spiral-like in nature (Webster and Patten, 1979).

Understanding nutrient spiraling requires quantifying the distance that nutrient molecules travel within a river or stream. This is usually estimated by measuring the uptake rate of nutrient molecules and the distance traveled. The spiraling length (S) is the average distance a nutrient molecule travels downstream during one cycle. The cycle begins with the availability of the nutrient in the water column in inorganic form, and includes the distance traveled in the water (S_w) until its uptake (U) and assimilation by an organism where the nutrient becomes part of an organic molecule. Additional distance traveled as part of the biota (S_b) completes the distance as the nutrient atom is re mineralized and released (Newbold, 1992).

Nutrient isotopes such as ³²P are commonly used to track dissolved nutrients along their spiraling path.

Newbold, et al. (1981) found that ³²P traveled a total distance of 193 m in a small Tennessee woodland stream, 167 m of the total distance in the water column and 26 m within the biota. Spiraling length can be influenced by many things, some of which are abiotic, including; physical-chemical transformations, hydrologic regimes, and sediment characteristics. Other influences are biotic in nature, including: abundance of periphyton, abundance of heterotrophic microbes, uptake rates, and the composition of the animal community. Hydrologic influences may be seasonal or annual. Low flow conditions, especially when combined with a high ratio of streambed area to channel volume, favor retention of nutrients within a stream segment. High flow conditions favor export from a stream segment, increasing spiraling distance. Retention of nutrients can be enhanced (downwelling) or reduced (upwelling) by factors that favor interaction between the water column and streambed.

Animal communities play a variety of roles in nutrient cycling. Direct consumption of periphyton, microbes, and other animals reduces standing stock of such organisms and may serve to stimulate or reduce their productivity depending upon the severity of the reduction in standing stock. Movements and migrations of animals can influence nutrient spiraling as well. Insect emergence can reallocate nutrients within the stream or, perhaps, remove nutrients from a stream if the emerging insects move a great enough distance. Spawning migrations by fish can move significant amounts of nutrients, enough to demonstrate assimilation of phosphorus and carbon from the fish into periphyton, macroinvertebrates, and fish following the death and decomposition of the migratory, spawning fish (Kline, et al, 1990).